EGT Temps

[TheDeltaFox 0]

Hey Everyone,

I finally got out to do some work on the plane and go flying today. Felt awesome to be in the air with my Avid again. The thing jumped off the ground about 1 second after full throttle and probably set a new stopping record at my airport when I landed a few feet short of the pavement.

I'm a two stroke noob and have been trying to get my egt's to show the same on both cyclinders. At the lower temps it runs about 75-100 degrees difference and at the high end its maybe 50 or less off. The thing is that when I'm at full throttle or running anywheres above roughly 5500 rpm the one egt creeps to 1200 or slightly higher while the other one is at 1150 ish. Normally its turning 6200 rpm at full throttle on the ground and tied to my truck. I turned both mixture needles (the little ones beside the big idle screws) fully in so it should be running at full rich; right? I'm wondering if one egt probe is off or something. I've ground run it a bunch and can't seem to figure it out. Should i raise the needle clips 1 notch and see if that helps?

Any advice would be super appreciated. I love flying this thing but don't want to cook the engine.

Thanks,

Darcy

[akflyer 1427]

Hey Everyone,

I finally got out to do some work on the plane and go flying today. Felt awesome to be in the air with my Avid again. The thing jumped off the ground about 1 second after full throttle and probably set a new stopping record at my airport when I landed a few feet short of the pavement.

I'm a two stroke noob and have been trying to get my egt's to show the same on both cyclinders. At the lower temps it runs about 75-100 degrees difference and at the high end its maybe 50 or less off. The thing is that when I'm at full throttle or running anywheres above roughly 5500 rpm the one egt creeps to 1200 or slightly higher while the other one is at 1150 ish. Normally its turning 6200 rpm at full throttle on the ground and tied to my truck. I turned both mixture needles (the little ones beside the big idle screws) fully in so it should be running at full rich; right? I'm wondering if one egt probe is off or something. I've ground run it a bunch and can't seem to figure it out. Should i raise the needle clips 1 notch and see if that helps?

Any advice would be super appreciated. I love flying this thing but don't want to cook the engine.

Thanks,

Darcy

YIKES! The screws on the side of the carb?? Those are the air bleeds for the idle, has NOTHING to do with the high speed. All the way in I am surprised it will idle at all! Those should be out 1 1/2 turns for a good starting point.

#1 Have your sync'ed your carbs? Make sure they are opening exactly the same, at the same time. This sounds like it could be your issue. http://www.avidfoxflyers.com/index.php?/topic/969-anatomy-of-a-carb-sync-how-do-you-know-which-one-to-adjust/

#2 You may have some crap in your carbs that is plugging them up.

#3 LOWER the needle clip, that will RAISE the needle and richen it up. I would also swap the probes and see if the trend follows the probe or the cylinder first. 25-50 degrees is pretty much normal for a difference, but they should not be out more than that.

[6320012s 12]

Once you follow akflyer's advice, you might want to consider in flight adjustable mixture control. My kitfox was set up with them when I bought it, and once I got it dialed in, it has been very nice. No need to change jets with the seasons.

Dave

[akflyer 1427]

Once you follow akflyer's advice, you might want to consider in flight adjustable mixture control. My kitfox was set up with them when I bought it, and once I got it dialed in, it has been very nice. No need to change jets with the seasons.

Dave

Dave,

Do you have the Arctic Sparrow mixture? If so, the main jet still has to be changed, but you can adjust the needles from the dash. If it is the HACMAN then you don't need to change jets.

On another note, I have to be in Anchorage next friday and Saturday. I can bring those Ski's that Bob has up with me for you to look at if you would like. Let me know!

[6320012s 12]

If you mean the 16th, that would be great. I plan to be in town that weekend. I'd love to pick your brain on the arctic sparrow mixture control. Once I got it set up right, Ive never had any trouble controlling EGT in both seasons. If you are around during lunch time, I'll buy.

Dave

[TheDeltaFox 0]

Thanks for the advice. I had a bit of a brain fart in regards to the screws on the carbs when i was at the airport yesterday. After i made that post and started to think about it, i started to realize that other than the main jets, there really is no control over the mixture at mid to full throttle. Unless a person installed a mixture control like you guys stated. Maybe you guys could clarify for me... The big screw with the spring is what the carb slide rests on at idle and sets the idle speed. Turning it in would increase the idle speed. The little brass needle with the rubber o-ring is for air by pass at idle and as per Rotax should be 1 1/2 turns out. Is this right?

I'm going to head the airport after work today and first turn those screws back out to 1 1/2 turns like I had them before. It idled nicley at 2000-2100 rpm until i monkeyed with it yesterday. Then i'll take the airfilters off and visually make sure that the slides are lifting at the same time and adjust the slack out of the cables to get it as close as possible. Then i'll hook up my homemade carb sync contraption and try to fine tune it. I'll also swap the EGT probes and see if it is one probe or the instrument in the dash or just an out of sync carb... If after all that its still hot, i'll LOWER the needle clip (raising the position number which is what i should have stated on the first post) and see how it runs. Anyone who ever said that these two strokes have fewer moving parts and are simpler than a 4 stroke have probably never worked with one LOL. I can't wait to have saved up enough coin for a 4 stroke like a 912.

[akflyer 1427]

Thanks for the advice. I had a bit of a brain fart in regards to the screws on the carbs when i was at the airport yesterday. After i made that post and started to think about it, i started to realize that other than the main jets, there really is no control over the mixture at mid to full throttle. Unless a person installed a mixture control like you guys stated. Maybe you guys could clarify for me... The big screw with the spring is what the carb slide rests on at idle and sets the idle speed. Turning it in would increase the idle speed. The little brass needle with the rubber o-ring is for air by pass at idle and as per Rotax should be 1 1/2 turns out. Is this right?

I'm going to head the airport after work today and first turn those screws back out to 1 1/2 turns like I had them before. It idled nicley at 2000-2100 rpm until i monkeyed with it yesterday. Then i'll take the airfilters off and visually make sure that the slides are lifting at the same time and adjust the slack out of the cables to get it as close as possible. Then i'll hook up my homemade carb sync contraption and try to fine tune it. I'll also swap the EGT probes and see if it is one probe or the instrument in the dash or just an out of sync carb... If after all that its still hot, i'll LOWER the needle clip (raising the position number which is what i should have stated on the first post) and see how it runs. Anyone who ever said that these two strokes have fewer moving parts and are simpler than a 4 stroke have probably never worked with one LOL. I can't wait to have saved up enough coin for a 4 stroke like a 912.

Do the changes one at a time so you know what it is that fixes your problem! Set the air screws back to 1 1/2 and sync the carbs, then do a run up and see how the EGT's look. If one is still off, with the carbs synced, then swap the probes and make sure that it is not a faulty probe or gauge. If the problem follows the probe, order a new one. If it does not follow the probe, then we get to dig deeper. When was the last time your carbs were rebuilt? Is the little plastic filter / strainer in the proper position in the float bowl. Are the floats adjusted right. Are the carbs LEVEL with each other? There are alot more moving parts with a 912, and just as much shit to tinker with on them.. Still have to sync the carbs, burp the oil, change the oil, dick with the sprauge clutch etc. They both have trade offs. There is no denying that ta 2stroke is MUCH more simple than a 4 stroke, once you learn them it super simple maint.!

[TheDeltaFox 0]

Thanks for the advice, i'm off to the airport to troubleshoot. This forum is worth its weight in gold. Especially the advice from all of you that have been through stuff like this before. I'll try to remember to take a better camera than just my cell phone and get some picks if I get it all sorted and go test flying. We lost the little snow fall we had and i'm chomping at the bit to try the wheel skis out. There's a few frozen lakes that I noticed last time i was up and once she's running smooth and cool i'm going to go practice some ice landings.

[Dave B 0]

This article may help....

http://www.n566aj.com/cgi-bin/bbs/archive.cgi?read=27862

Tuning the 2 stroke.

Dave

[akflyer 1427]

This article may help....

http://www.n566aj.co....cgi?read=27862

Tuning the 2 stroke.

Dave

Dave, that is a great article! Thanks for posting that one up! I am going to archive that one!

[TheDeltaFox 0]

Thats a good article! I printed it and added it to my two stroke library. I may need a small filing cabinet soon

I made it out to the airport and adjusted the carbs and synched them mechanically last night. I didn't have time to fly but it now idles smooth at 2100 rpm and is running cooler at mid range. I didn't have a chance to do a full power runup to see what the temps are like because the one probe started acting up again. For some reason the probe works but when i give a shot of power, the one EGT needle drops in tune with the rise of the rpm guage. It was also not reading at all at first which meant a lot of cable wiggling and stuff. I'm thinking that the probe cables need to be run better and away from any other cables/metal that might be affecting them. Either that or the guage/probe is faulty and it might be a good idea to just replace the probes and guage. The wires look rather worn out where they run into the probes. For the bit it was working the EGT's were with 50 diff which made my day.

I've got that feeling like a kid at Christmas that just wants to tear into his presents, but instead I want to go tear up the sky. This work thing sure is getting in the way of my flying..

[Dave B 0]

Dave, that is a great article! Thanks for posting that one up! I am going to archive that one!

Great! So where do you post all these archived jewels so we can find them later?

Glad this one is of use.

Dave

[Dave B 0]

Great! So where do you post all these archived jewels so we can find them later?

Glad this one is of use.

Dave

Here's another handy one. The link dosen't seem to work, so I'm pasting it here....

Engine and Prop Adjustments JAN 2010

Picking the right combination of engine, reduction unit and prop is

discussed in a different article. This article tells you how to adjust the

engine and prop correctly after they are installed. Its also a good guide

to things you should check on a used plane and before you first start a

new engine.

PRELIMINARY CHECKS AND ADJUSTMENTS

A lot of problems can be traced back to inaccurate instruments. It is very

common for tachs to have large errors. Borrow a Tiny Tach or an optical

prop tach and compare readings to check your tach over the whole range.

It is impossible to correctly adjust the prop with a bad tach. Be certain

that your EGT and CHT gauges read correctly. Useing the wrong type of

thermocouples or wire for them is a common mistake. It's also real easy

to enter wrong numbers or settings for electronic displays. See my article

on thermocouples for more information.

Check for water or crap in the carb bowls. Make sure carbs are mounted

square with the engine since a tilted carb can cause one cylinder to run

hotter than the other on some engines. Cracks in the carb mounting

sockets can cause lean mixtures so replace them if they are brittle.

Check for adaquate fuel flow to the carbs. Any time you notice higher

than normal EGT or if the plane won't reach normal full throttle RPM

install a new fuel filter before you waste a lot of time looking for other

problems. See my article on fuel systems for more info.

Each carb has an idle jet, a needle jet with a jet needle and a main jet.

All have tiny numbers on them which tell you the size. Check to be sure

they are the factory recommended jets and the jets are the same in both

carbs because its common to find that a previous owner has installed

different jets. The factory recomendations are almost never wrong so

its very important that the factory jets are used during testing.

Check that all throttle cables are routed to drain out any water inside the

cable outer jacket. Water collecting in a low spot can cause corrosion

or freeze in the winter to lock the cable in one position.

CABLE ADJUSTMENT FOR A SINGLE CARB

Pull the throttle all the way back against the lower throttle stop. Where

the cable enters the top of the carb there is an adjustment that is covered

by a rubber boot. Slide the rubber boot up the cable to get it out of the

way then gently pull upwards on the outer jacket of the cable. There

should be a tiny amount of free movement before you feel the inner cable

start to move the slide upwards. That tiny amount of free play is critical

to

be sure that the cable is not holding the slide up off of the idle speed

adjustment screw. If you have trouble feeling it you can remove the air

filter and look inside the carb throat to watch the slide move. Loosen the

lock nut and turn the adjustment screw until you have that tiny amount of

free play if needed. Lock the adjustment and work the throttle back and

forth a few times to be sure the cable outer jacket ends are fully seated

in their sockets correctly then pull the throttle all the way back against

the

lower throttle stop. Double check that the amount of free play is still

correct at the top of the carb before you reinstall the rubber boot.

Push the throttle all the way forward against the upper throttle stop and

look in the carb throat. The slide should go up far enough so that it does

not block any of the carb throat opening. Its okay if it goes up just a

little

extra but if it goes up too much it will bind and put a strain on the

throttle

cable. You should install some sort of upper throttle stop to prevent that.

CABLE ADJUSTMENT FOR DUAL CARBS

Dual carbs must be mechanically matched or one cylinder will have a

higher EGT than the other and/or you can not get a smooth idle. Pull the

throttle back against the lower throttle stop. Where the cables enter the

tops of the carbs there are adjustments that are covered by rubber boots.

Slide both boots up the cables to get them out of the way. Loosen the

locknuts and turn both adjusment screws down two turns. That makes

sure that the cables are not holding the slides up off of the idle speed

adjusting screws.

Remove the air filters and use the smooth ends of drill bits as round

feeler gages to check the clearance between the bottom of the slide and

the bottom of the carb throat on each carb. Pick a drill bit that will

barely

slide into the smaller of the two openings and use it to adjust the idle

speed screws on both carbs until both openings are the same. Your

carbs are now mechanically matched. To keep them matched you must

ALWAYS turn the idle speed screw on both carbs the same amount

when makeing idle speed adjustments. NEVER adjust just one idle

speed screw.

Make sure the throttle is pulled all the way back then go back to the cable

adjusters on top of the carbs. Screw them out to take out ALMOST all of

the free play. You check that by feeling how much you can lift the outer

jacket before you feel resistance. It takes a delicate touch but you need

just a tiny amount of free play to make sure that the slides are not held up

by the cables when the throttle is pulled all the way back. If the free

play

is not the same on both carbs then one slide will start to rise before the

other so this adjustment is critical. Tighten the locknuts and work the

throttle back and forth a few times to be sure that all the cable outer

jacket ends are fully seated in their sockets correctly. Double check

that the free play is still correct on both carbs then slide the rubber

boots

over the adjustments. If you have done it correctly both slides will start

to move upwards at the same time and the bottom of both slides will be

flush with the top of the carb throat just before you reach full throttle.

Set the throttle wide open and check that both slides go up far enough

so that they don't block any of the carb throat openings. Its okay if they

go up just a little extra but if they go up too much they will bind and put

a strain on the throttle cables. You should install some sort of upper

throttle stop to prevent that.

INITAL CARB ADJUSTMENTS

Because the jet RPM ranges overlap you should make the idle speed

adjustments first and work up to higher RPMs. If any large adjustments

are made it will be necessary to repeat the idle adjustment procedure to

fine tune all adjustments. Make sure a clean air filter is installed before

makeing adjustments.

There is an AIR MIXTURE adjustment screw which will fine tune the

amount of air at idle speeds only. The IDLE SPEED screw limits how

far the slide can come down to close off the air flow. If you aren't sure

which screw is which you can see the end of the idle speed screw

sticking out inside the carb air inlet if you remove the air filter. A

spring

above the slide pushes the slide down against that screw.

To prevent engine shake and gearbox chatter our engines need an idle

speed of around 2000 RPM so warm up the engine and adjust the idle

speed screw for about 2000 RPM. Remember to turn BOTH idle speed

screws the same amount to keep the carbs matched if the engine has

dual carbs.

Adjust each air mixture screw to get the highest RPM at that idle speed

setting. 1/8 of a turn on adjustment screws can make a big difference so

keep the adjustments small. You will have to go back and forth between

dual carbs a couple of times to get the best possible idle because they

interact.

As the mixture gets better the idle speed will increase so adjust both idle

speed screws the same amount then fine tune the mixture screws again.

Keep at it until the engine idles smoothly at the lowest speed which has

minimum engine shake. Check the cable adjuster on top of both carbs

after you finish to make sure there is still the same tiny amount of free

play on each carb.

Be aware that it's easy to get an arm cut off by the prop and anything you

drop may go through the prop. I usually tie a short safety rope around my

waist and the base of the wing spars so that I have to stretch way out to

reach the adjusting screws. That way I can't forget and move toward the

prop. Fortunately you won't have to do this again until you notice an idle

problem.

INITAL PROP ADJUSTMENTS

Now you need to know how the factory recommends that your engine be

used. For Rotax engines download the operators manual at

http://www.kodiakbs.com/tiintro.htm

The figures used below are for the Rotax 503 so look at page 10-1. If

you have a different engine you need to change those figures to match

your engine manufacturers recommendations. If you have a fixed pitch

prop all you can do is hope that it is the correct prop but all the other

info

below will still apply.

The Rotax manual lists a "takeoff speed" of 6800 RPM for no more than

5 minutes and a "cruising speed" of 6500 RPM with no time limit.

Its common practice for aviation engines to have a normal operations

limit and a higher takeoff limit so what Rotax should have said was "The

engine can be safely operated at 6800 RPM for five minutes but you

must reduce the throttle to 6500 RPM or less after that."

The RPM that a given engine/prop combination can reach is greatly

influenced by the air speed and any disturbance of the air flow into the

prop. During discussions you will hear several terms used so you need

to understand those terms.

During static testing the plane is tied down. There is no airflow into the

prop so the engine can not reach full RPM.

During the actual takeoff the plane does not reach full speed. There is

reduced airflow into the prop so the engine can not reach full RPM.

During climbout the plane does not reach full speed. There is still

reduced airflow into the prop so the engine still can not reach full RPM.

In addition the use of flaps or even the planes design may block or

disturb at least part of the airflow into a pusher prop.

If you set the prop so that the engine turns 6800 RPM during the actual

takeoff your engine will seriously over rev once you build up more speed

and/or finish the climbout. To prevent that you should adjust the prop to

allow 6800 RPM at full throttle during straight and level flight. This

gives

you the maximum airspeed and keeps your engine below the maximum

factory recommended RPM during all phases of your flight.

Rotax describes 6500 RPM as "cruise speed" but if you actually cruise

at 6500 RPM your engine won't last very long. Its a general rule of thumb

to cruise at the RPM where your engines power curve produces 75%

power for best life. For the 503 thats around 5200 RPM.

Engines are expensive and pilots are cautious so many of them try to

adjust their prop to give lower full throttle RPMs thinking that will be

easier on the engine. That actually works an engine harder because

the prop becomes a bigger load than the engine was designed to

handle. The ignition timing, port timing and carbs are all designed to

work best with the engine loaded just enough to reach the max full

throttle RPM specified by the manufacturer. If you really want to be

easier on your engine simply give it less throttle after the prop has

been adjusted correctly. Loading the engine down so it can't reach

the maximum RPM at full throttle is called "over prop" or "lugging".

It puts an extra strain on all engine and reduction unit moving parts

which will shorten their life.

The first step is to adjust the prop to a pitch where you will have a safe

first test flight. We know that the prop load will be reduced once air is

flowing into the prop at high speeds so the engine will turn faster in

flight.

The amount depends on the type of prop, diameter, pitch, etc. To

prevent exceeding the max RPM in the air we need to first adjust the

prop for about 6300 RPM at full throttle while the plane is tied down.

The exact final adjustment will be determined by flight tests later.

Tie the plane down and monitor the EGT and CHT temperatures as you

slowly apply throttle. If the RPMs exceed 6300 before you reach full

throttle, stop and add more pitch to the prop. If you reach full throttle

and

RPMs stay below 6300 stop and reduce pitch on the prop. If the EGT

exceeds 1150 stop and find out why before continuing tests. If the CHT

exceeds 430 stop and let the engine cool down before continuing tests.

Maybe you need to think about adding some more cooling capacity.

Note that the operators manual lists maximum EGT as 1200 and

maximum CHT as 480 on page 10-1 but we want to stay below those

figures during our early tests.

This is also a good time to watch for surprises like leaks, fuel starvation,

and overheating problems. It's better to find them now than find them in

the air.

TEST FLIGHTS

Prop adjustments and carb adjustments will affect each other so read

this whole section before any test flights and make all the adjustments

in the sequence given below.

During the first phase of first test flights the pilot needs to determine

the

full throttle engine RPM that is reached during straight and level flight.

He

should also monitor EGT and CHT closely the entire flight because they

may try to exceed the maximum allowed until the plane has been fully

checked out and the prop is correctly adjusted. After each flight make

small adjustments on the prop until the engine turns very near 6800 RPM

at full throttle during straight and level flight.

During the second phase of flight tests the pilot needs to check the

needle jet and jet needle operation by flying for a couple of minutes at

each of several engine speeds between about 3000 RPM and 5500 RPM.

There will be variations but all EGT readings must be in the normal range.

If most of those engine speeds show a high EGT try moving the clip

down on the needle to provide more fuel for the whole mid range.

Repeat test flights until you find the clip position which works best over

the whole mid range.

You may find a small range of engine speeds which have a high EGT

while all the other mid range speeds are normal. Avoid running in that

small range of engine speeds until all other adjustments are completed.

An incorrect prop or prop adjustment is almost always the reason.

During the third phase of flight tests the pilot needs to check the main

jet operation by watching the EGT during full throttle operation in straight

and level flight. If the EGT gets too high reduce throttle until it cools

down. You MAY need a larger main jet but if the engine could not turn

near 6800 RPM during the test the real problem was the prop pitch.

Adjust the prop and try another test flight.

You may have to tinker a little more with the needle clip to get the EGT

correct over the entire mid range after you have fine tuned your prop.

That may mean some minor compromises but try to keep the EGT as

close to ideal as possible over the most used RPM ranges. Double

check that the final adjustments will not let the EGT run above 1150 or

below 1000 at any RPM.

Remember that air temperature and altitude will have an effect too. I

have to adjust my needle one notch every spring and fall to keep mine

set correctly but four thousand feet of altitude change doesn't change

my EGT enough to matter.

JET CHANGES

There are four paths fuel can follow to get through the carb. The first is

the choke. Our choke is not an air flow restriction like on cars. Our

choke

is a manual "on" or "off" lever which opens a path for a tiny amount of

extra fuel to flow into the throat of the carb to enrichen the mixture for

better starting. Always turn it off after the engine warms up enough to run

smooth.

There is some interaction and overlap between the three main fuel paths

but generally speaking, the IDLE JET will control all of the fuel flow at

idle

speeds. The JET NEEDLE is attached to the carb slide and it moves

into and out of the NEEDLE JET. That combination controls almost all

of the fuel flow in the lower midrange, most of the fuel flow in the upper

midrange and some of the fuel flow above that. The MAIN JET provides

the additional fuel needed from the upper midrange to full throttle.

Way too often people jump in and start changeing jets attempting to cure

some problem that isn't really caused by the jets at all. The factory isn't

stupid ... they choose the jet sizes for a good reason and it is very rare

for them to be wrong. Before you even think about any jet changes you

should double check that the prop is adjusted as described above to

provide the correct load on the engine. Maybe your tach reads wrong.

Many jet changes are done by misguided people attempting to get more

speed by adding pitch to the prop. You might get away with a little bit of

extra pitch but if you get weird EGT temperatures or weird fluxuations in

RPM in a narrow RPM range you have gone too far with it and jet changes

won't help much because the engine is overloaded.

If you were able to get a significant speed increase with too much pitch

you will be much better off to use a slightly shorter prop with that pitch

so

your engine can reach the correct RPM.

Some other things to keep in mind before you tinker with jets.

IDLE JET If you were ever able to correctly adjust the idle mixture then

there is no reason at all to change the jet size. Blow the trash out of it

or

replace it with the same size.

NEEDLE JET If the midrange EGT temperatures were ever correct

there is no reason at all to change the jet size. It might have the hole

enlarged from wear against the jet needle. Blow the trash out of it or

replace it with the same size.

JET NEEDLE If the midrange EGT temperatures were ever correct

there is no reason at all to change the needle size. It might get bent, it

might get worn in the taper area, it might get worn in the clip grooves on

top or the clip may need adjusting. Replace it with the same size.

NEEDLE CLIP If the mid range EGT temperatures were ever correct

the most it will need is adjustment one notch up or down to allow for

seasonal changes in air density.

MAIN JET If the full throttle EGT temperatures were ever correct there

is no reason at all to change the jet size. Blow the trash out of it or

replace it with the same size.

AFTER THE ENGINE HAS ABOUT 20 HOURS ON IT

Go over all the adjustments above because parts wear together and

cables stretch during their early life. After you get the engine broken in

good it will produce more power and that might mean changes.

After you are more familar with the plane you might consider this trick.

The prop is still producing a certain amount of thrust at idle speed. That

means more "float" distance and longer roll outs which can really matter

in short field landings. After correct adjustment as described above I

back the idle speed screw out a little for a very slow idle. I use a little

bit

of throttle to bring the engine speed back up so the engine doesn't shake.

On final I can pull the throttle all the way back for a steeper but slower

glide. The forward speed of the plane helps the prop windmill fast

enough to prevent engine shake and the prop has a large amount of drag

instead of a small amount of thrust. As the plane slows down on rollout

I add just enough throttle to prevent engine shake. It can make a big

difference in short field landings and more experienced pilots may prefer

it.

[Dave B 0]

And another. Back to basics, but great info....

Fuel Systems Jan 2010

Actual engine failures are rare. Almost all so called "engine failures" are

returned to service without ever working on the engine itself. It's the

systems which support the engine that usually fail and fuel systems fail

more often than all other things combined. Most of those failures can be

easily prevented and this article will show you how.

FUEL

RON and MON are two slightly different methods used to determine gas

octane. European countries use the RON method only. Our American

pumps have a label on them which shows that the advertised octane was

calculated as RON plus MON divided by two. If your engine manual says

to use 91 RON then you can use American 87 octane gas since its the

same thing as 91 RON.

Aviation fuel is often called 100LL. It is 100 octane but the low lead part

is misleading. It does have less lead than aviation fuel once had years

ago but it still has much more lead than any automotive gas. That lead is

hard on our type engines because it fouls plugs and causes rings to

stick if we use it on a regular basis. A few gallons of aviation fuel once

in a while doesn't really matter much but I still avoid it as much as I can.

Some "experts" claim that the volatiles evaporate out of gas so fuel

goes "stale" fast. There is a tiny bit of truth in that but it certainly

doesn't

happen very fast. How long does gas have to sit in your car or lawn

mower before you notice any difference? In my plane 3 or 4 weeks

certainly doesn't cause any problems and I have gone twice that long

with only a very minor difference in performance ... and I even premix

the oil with my fuel. Don't get too paranoid about "stale" gas.

Gas picks up small flakes of rust and dirt as it is pumped hundreds of

miles through underground pipe lines. Slugs of water are inserted into

the pipeline between different shipments and some of that gets mixed

with the gas as it travels through the pipe. Large storage tanks at your

local pipeline terminal always have some water and crap in them. Even

the truck that delivers gas to your local gas station may contribute and

of course water and crap in the gas station tank gets stirred up when the

tank is filled. Gas companies try to filter the gas and most even have a

filter right at the gas pump nozzle but some water and crap ALWAYS

gets to our tanks. A lot of fuel problems happen shortly after refueling

from a new source and even airports can have contaminated fuel.

Some pilots filter the gas going into the tanks on the plane with chamois

cloth, felt or a Mr. Funnel. It doesn't matter what you use or how careful

you are to avoid condensation in the tank sooner or later there will be

water and crap in your tank so you should plan for your fuel system to be

able to handle some of it.

ELECTRIC FUEL PUMPS

The Mikuni pulse pumps are designed to suck fuel up to the engine and

can easily handle the 3 or 4 feet of vertical lift needed with our engine

and tank locations. Faucet recommends mounting electric pumps below

the fuel tank because they can NOT produce much suction.

Many people think that an electric fuel pump will provide an extra margin

of safety. That's okay but make sure that you understand what you are

doing or you can wind up LESS safe. Dual pumps can be used safely

but you need to do it correctly.

If you use pumps in series each pump must provide enough fuel

pressure by itself in case the other pump fails. The second pump can

boost the pressure of the first pump so high that the carb inlet needle

valve can't close and then fuel will squirt out of the float chamber vent.

To

prevent that you need a pressure regulator ... but then you depend on

that ONE regulator to stay in the air. Wasn't your orginial concern that

you depended on ONE fuel pump? Pumps in series usually add more

potential problems than they remove. I do NOT recommend series use.

For parallel use we usually use the Facet 40104 or 40105 4PSI pump

when the pump is in the wings and the Facet 40106 6PSI pump when

the pump is mounted low in the plane so that the carb sees the correct

pressure. NONE of those models have a check valve built in so any

time there is more pressure at the outlet port than is being created by

the electric pump fuel will be forced backwards through the pump. If

the electric pump gets turned off then a large part of the fuel flow from

the Mikuni pump will flow backwards through the electric pump.

With engines mounted high and fuel pumps mounted low even with no

pumps running there is one PSI pressure on the pump outlet port for

every 37 inches in height just from the head pressure of the fuel in the

line up to the carb. For this discussion call that 1 to 2 PSI head pressure

at the electric pump outlet when all pumps are off.

Fire up the engine and the Mikuni will add 4 to 7 PSI on top of that head

pressure to give about 5 to 9 PSI total at the electric pump outlet. The

electric pump can only pump 6 PSI so fuel from the Mikuni can flow

backwards through the electric pump at high engine speeds EVEN

WHILE THE ELECTRIC PUMP IS TURNED ON.

Electric pumps can force fuel backwards through the Mikuni pump too if

it has grit in the internal flapper valves. Reverse flow can cause fuel

starvation and other problems like unexpected fuel transfer to a different

tank depending on how the pumps are connected.

McMaster-Carr sells a good check valve to prevent reverse flow. Part

# 7775K51 only takes 1/3 of a pound of pressure to open and it has a

Viton seat which is rated for gasoline but you need two # 44555K131

barbed adapters to install one in our fuel line. I strongly recommend a

check valve on each pump outlet anytime there is more than one pump

in the fuel system. I also recommend a fuel filter on each pump inlet so

if one filter stops up the other pump can still provide fuel.

If you provide a seperate fuel tank dip tube and fuel filter for each pump

you can have two completely independent fuel systems as long as the

two systems don't connect together except on the carb side of the check

valves. There you can use a 4 port manifold, a cross fitting or two tees to

provide for one line from the electric pump check valve, one line from the

Mikuni pump check valve and two lines to the carbs.

MIKUNI PULSE OPERATED FUEL PUMPS

Two stroke engines use the piston to suck the air/fuel mixture from the

carb through the intake manifold into the crankcase each time the piston

moves toward the combustion chamber. As the piston moves away

from the combustion chamber the mixture in the crankcase is pushed

from the crankcase into the combustion chamber through the transfer

port. This creates a small vacuum and then a small pressure inside the

crankcase which we call a PULSE. A connection through the crankcase

wall is connected to a chamber inside the fuel pump by a piece of

tubing so the fuel pump diaphram can be moved by these pulses.

Condensate forms when warm fuel/air vapor hits the cooler pulse line.

If that condensate can not drain back into the engine it will collect in the

pulse line or the fuel pump pulse chamber. Fluid trapped in a pulse line

will weaken the pulse at the pump. Fluid trapped in the pump restricts

the diaphram movement. Either one means less fuel gets pumped.

Rotax drills a tiny drain hole in the Mikuni pumps which they supply with

their engines. That hole is located at the pulse port connection to drain

both the pump pulse chamber and the pulse line if the pump is mounted

right side up and lower than the engine pulse connection. Click on the

link below to see how to drill that hole. Some models have the hole in a

90 degree brass fitting at that location.

http://www3.sympatico.ca/murray.j/mikuni_pump.htm

I don't do that because the hole weakens the pulse and the hole may

get plugged with crap. I mount the pump right side up but above the

engine pulse connection so that any fluid will drain out into the pulse line

and the pulse line will drain back into the engine. Do NOT bolt the pump

solid to the engine. Use mounting which isolates the pump from vibration

and engine heat.

Long pulse lines, leaks in the pulse line and thin wall tubing which can

expand and contract with the pulses also weaken the pulses. Weak

pulses move the diaphram less so less fuel is pumped. Pulse lines

must be less than 18 inches long so the pump must be located near the

engine pulse connection. You can use the engine pulse connection on

the other cylinder to help keep the pulse line short on Hirths but Rotax

only has one engine connection.

It's not easy to do all that so some plane manufacturers take the easy

way out and then problems develop over time after condensate builds

up in the fuel pump or pulse line.

I recommend the large round Mikuni pump since it can pump up to nine

gallons (35 liters) an hour ... but that is only when they are installed

correctly. Remember that they will pump less if they have installation

problems. Both outlets come from the same internal chamber so cap

off one outlet or tee the two outlets together if you only need one outlet.

Its VERY rare to hear of any problem with a correctly installed Mikuni

pump. They have been used on many brands of ATVs, cycles, jet skis

and snomobiles for many years. They don't use electrical power and

they have few moving parts. Those parts are replaced during a low cost

rebuild which is so easy that anyone can do it.

I think they are more reliable than electric fuel pumps so I use a Mikuni

which I replace or rebuild every two or three years and I don't have any

other pump on my plane. If I was concerned I would install a second

Mikuni pulse pump for a backup. Hirth engines have a pulse connection

for each cylinder and there is no reason that you can't use a seperate

pulse pump on each connection. Rotax engines are limited to one pulse

pump since they have only one pulse connection. Do NOT try to use two

pulse pumps on the same engine pulse connection. That will cause both

pumps to recieve weaker pulses and any pulse line problem will affect

both pumps.

FUEL TANKS AND THEIR CONNECTIONS

Be a little leary of fiberglass and other composite fuel tanks. Every once

in a while we hear about a tank which partially disolved because it could

not handle ethanol or some other fuel additive. I suspect the wrong resin

was used to make those tanks.

I don't care what you use to filter your fuel or how careful you are there

WILL be crap and water in your fuel tank. Your first line of defense is to

leave a little room at the bottom of the tank where the crap can settle out

without getting into the fuel lines.

I don't like holes anywhere in a fuel tank except at the top. Sooner or

later holes in the sides or bottom will leak no matter what you do. The

dip tubes described below use a hole in the top of the tank so there are

no leaks. If you already have a hole in the side or bottom of your tank

you should use part #FTG-12240 from Wicks Aircraft. Those grommets

are made of Viton instead of black rubber so they will stand gasoline

service much better.

With no drain on the bottom, you need some other way to get the water

and crap out of the tank. A siphon hose will do that but I use a piece of

1/4"OD stainless or aluminium tubing long enough to reach the bottom

of the tank through the filler opening with enough extra for me to hold as

I guide the probe to the low spots in the tank. 1/4"ID Tygothane tubing

goes over the probe and connects to a squeeze bulb so I can suck out

trash and water. I don't do it everytime I add fuel but I do make it part

of

my regular maintaince routine. Do it after the crap and water has had

time to settle to the low spots.

Only planes capable of inverted flight need flexible lines inside the tank.

Flexible lines can be a real pain in the butt and may collapse from the

vacuum needed to suck fuel out of the tank. Do NOT try to use them.

Do NOT use screens or filters inside the tank. They will get clogged

sooner or later and you will have a hard time cleaning or replaceing them.

A clear Fram G1 filter at the pump inlet will protect everything plus it's

cheap, eazy to inspect and easy to replace. If you install it with the

inlet

on the bottom and the outlet on the top it even tells you when its getting

clogged up. When its new fuel fills the bottom part but the top part stays

full of air. As the bottom part of the filter element gets clogged the fuel

automatically rises to get through the unrestricted partt. Replace it when

the fuel covers the entire element.

The fuel line connection to the tank should be a metal dip tube which

goes through the top of the tank and ends about 1/2" above the bottom

of the tank to keep it above any water and trash in the tank. Do NOT let

dip tubes lay against the bottom or sides of the tank. That can seal off

the open end or let vibration rub a hole in the tank. Fuel will corrode

copper or even brass but aluminium or stainless steel will work fine.

Very large engines might need as large as 3/8 OD X .035 tubing for

dip tubes but 1/4 OD X .032 will easily handle 5 or 6 gallons per hour.

If the tank doesn't have any connections on top install a stainless steel

panel mount coupling through the top of the tank. McMaster-Carr

#5182K375 fits 1/4 OD tubing. It is like a normal tubing coupling except

it has a threaded section in the middle so it can be inserted through a

hole and mounted solidly in place by a nut. If you install a 7/16 viton,

nylon, or PTFE washer as a gasket outside the tank it will seal fumes.

McMaster-Carr #90295A186 is nylon. Use a metal flatwasher between

the gasket and mounting nut to protect the gasket. The tubing nut and

ferrule inside the tank are not used so that the dip tube can be removed

later. The tubing nut and ferrule outside the tank will hold the dip tube

in

place, seal around it and allow removal of the dip tube. There is a small

shoulder inside the coupling which needs to be drilled out with a 1/4 inch

bit before the tubing will slide all the way through.

Most tanks have a small filler opening so here are some tricks to help

install connections. Large drill bits make a sloppy hole in soft plastic so

drill a small hole then use a tapered hand reamer to enlarge the hole to

the correct size. Don't forget to deburr the hole. Drop a string down

through the diptube hole then fish the string out through the filler

opening.

Remove everything from the coupling body then push the string through

the coupling. Tie a weight on the end so the string can't pull out of the

coupling and drop the coupling in the filler opening. The string guides

the coupling to the diptube hole as you pull it out. Install a viton, nylon

or

PTFE gasket with a stainless flat washer on top to protect the gasket.

Use Locktite when you tighten the mounting nut. Put both pieces of the

ferrule and the tubing nut loosely on the top of the coupling and slide the

dip tube through the coupling to the bottom of the tank. Pull it back up at

least 1/2 inch before you tighten the tubing nut.

A hand pump primer connected to the fuel pump suction line can leak

air into the fuel system if the primer goes bad. I use a seperate 1/4"OD

dip tube just for my primer to avoid that.

A cheap fitting to use for vents in the top of tanks is a metal valve stem

for tubeless truck tires from an automotive parts store. They have a nut

for solid mounting in the top of the tank but throw the rubber seal away

and use a viton, nylon or PTFE washer as a gasket. Remove and

discard the valve core before you slide Tygothane tubing over the fitting.

If you route that vent line below the bottom of the tank fuel will not leak

out if you flip the plane over. If you extend the vent line out the bottom

of the plane it will keep fumes and any slosh overflow out of the cockpit.

I stick a fuel filter on the open end. It keeps bugs out and its my spare

fuel filter on cross country flights. Any vent built into the filler cap

can

be sealed off to prevent leakage and fumes when the tank is vented

this way.

New tanks and lines usually have construction crap left in them which

often causes problems on the first flight. Clean and flush them out

before you trust them.

HEADER TANKS

Some installations with more than one wing tank use a smaller header

tank installed lower than the wing tanks. Fuel drains out of the wing tanks

into the header tank which is then used as the main fuel tank. Header

tanks often have serious problems with leaks so I recommend only an

all metal tank with threaded connections welded in place for every

opening needed. It should also include a welded connection for a drain

valve to drain the water and trash which will collect there. Since the fuel

uses only gravity to drain down into the header tank each wing tank must

have a connection on the bottom and that connection often leaks. You

should be aware that the vent used on the header tank must extend up

as high as the vents on the wing tanks to prevent overflow. Without that

vent, fuel may not drain into the header tank because air trapped in the

header tank will try to work its way up to the wing tanks through the wing

tank drain lines.

FUEL, VENT AND PULSE LINES

Saint-Gobain is a very large company that includes several subsidary

companies, factories and/or divisions. Combined they manufacture

many different kinds of plastic tubing known by several brand names

includeing Chemfluor, Fluran, Norprene, Pharmed, Tygon, Tygothane,

Versilic and others.

Tygon is probably the best known brand name but remember that there

are many different types of Tygon tubing. Most types are NOT suitable

for our application so be sure to check the formulation number. There

are also other companies and they each have their own brand names of

tubing made with different chemical compounds.

Years ago some manufacturers tried to identify their products with

different colors so that buyers could tell a supplier to "Give me 30 feet

of the green stuff". When the "orange stuff" proved to be a better

product or more popular competitors quickly colored their brand orange

too. It quickly got to the point where color tells you absolutely nothing

about what the tubing can safely handle.

Today there are hundreds of different types of tubing available so we

need to understand and correctly use some industry terms. Do NOT

tell a manufacturer that that his product will be used with "gasoline",

"fuel"

or "ethanol" because those mean three completely different things to him

and none of them accurately describe what we really use to him. We use

a 10% ethanol 90% gasoline mixture. Be certain that he understands that

so he can recommend the correct product.

We need a product that lets us easily see air bubbles inside the tubing.

You will see several words used to describe how well you can see

through the tubing. "Translucent" means that you can see light on the

other side but you probably won't see details like air bubbles. "Opaque"

means you can't even see light through it. We need "transparent" or

"clear". Sometimes a color is opaque and sometimes its like the tinting

on your windshield. Its there but you can still see through it fairly well.

We need a product thats flexible. If the minimum bending radius is

more than four or five times the tubing OD then it may get kinked shut

in our application.

We need a product thats soft enough to seal easily on some of our

fittings so it needs to be less than about 90 on the Shore "A" durometer

scale. It also needs to withstand hose clamp pressure so it needs to be

more than about 60 on the same scale.

We need a product thats able to stand a strong vaccum. Mikuni fuel

pumps can suck hard to get fuel up to them through clogged filters.

We need a product that remains stable, resists UV well and lasts at

least 4 or 5 years without getting brittle enough to crack or break with

age and vibration. Most get brittle and crack after a year or two.

My choice is clear Tygothane which is Tygon formulation C-210-A. I

left some on my plane for over 5 years with no problems and it was still

as flexible as when new. I finally replaced it only because the additives

in gasoline had stained it brown on the inside so it was getting hard to

see air bubbles. Tygothane is sold by Lockwood Aviation, Mark Smiths

Tri-State Kite Sales and McMaster-Carr sells it as Part #5549K47 for

1/4ID X 7/16OD. Part #5549K31 for 1/8ID X 1/4OD. They also have

other sizes.

A friend has had simular good results for 3 years with Tygon formulation

F-4040-A but it has a couple of drawbacks. Its tinted yellow so its not

quite as clear. Its too soft for metal hose clamps or twisted safety wire.

It doesn't handle vacuum quite as well and its not made in a heavy wall

size so its not as good for pulse line or pump suction line.

McMaster-Carr sells it as Part #5552K25 for 1/4ID X 3/8OD and

#5552K23 fot 1/8ID X 1/4OD.

Heavy wall tubing should be used for the impulse line and on the suction

side of the fuel pump or vacuum may cause those lines to collapse.

The lines on the outlet side of the pump and the vent lines can be a

thinner wall thickness if you want.

Suction lines should maintain a steep slope all the way from the fuel

tank to the fuel pump so route the tubing to avoid haveing high spots

and low spots. Tiny amounts of water in the fuel can accumulate to

fill low spots and small air bubbles can accumulate into one very large

air bubble to fill high spots. A large slug of water or a large air bubble

can cause problems.

Automotive type worm gear hose clamps often let air leak into suction

line connections because the small size we need doesn't form a circle

very well. McMaster-Carr sells nylon double snap grip clamps which

work well. Part #9579K63 is typical but get the right size. They also

sell spring steel wire formed into a circle as "constant tension spring"

clamps. Part #5324K61 is typical but get the right size. They also sell

a band tightened by a nut and bolt. Part #5412K48 is typical but get

the right size.

If all else fails wrap two turns of safety wire around the tubing and twist

it tight but don't cut into the tubing.

FUEL FILTERS

There is a lot of discussion about fuel filters. Some Cessna pilot

"experts" claim that any small stuff which goes through the gasolator

screens they use is so small that it goes through their fuel pumps, carbs

and engines with no problem. Obviously that has worked for them for a

long time but you need to remember the differences in planes and

engines.

Their engines are much larger so they use larger jets in their carbs while

our smaller jets are easier to clog up. The main problem is that our fuel

pumps depend on internal flapper valves being able to seal properly up

to 100 times a second. Grit that gets through screens can keep those

flapper valves from sealing shut properly. Most of us install a fuel

filter

just before the fuel goes into the fuel pump to protect the pump and carb.

Purolator (and others) make a filter with a glass sediment bowl that you

can take apart to replace the filter and clean out the bowl. It works fine

on the pump output and there is no problem with them there.

Unfortunately we need a filter on the pump intake and there is suction

there. You may have already noticed that suction is harder to seal than

pressure if you had a hard time keeping air bubbles from leaking into

your suction line connections. The seal at that filter bowl was NOT

designed to handle suction and if you try to use it on the suction side of

fuel pumps it WILL suck air into your fuel system. Many UL pilots have

known this for years and avoid them but sometimes a newbie will try to

use one then post that he has a problem with bubbles in the suction line.

Some internet "experts" claim that filters should not be used on the

suction side of pumps and use those posts as "proof". You should

remember that many of the "experts" use wing tanks to gravity feed

fuel to a lower engine. Their fuel doesn't need to be sucked up to the

engine so they are not familar with our fuel pumps or the suction needed.

They claim that the bubbles are "vaporizeing" out of the fuel because a

high vacuum is needed to suck fuel through a filter.

It is true that a high vacuum in fuel lines or high temperatures can cause

"vapor lock" when fuel vaporizes but that is NOT what is happening here.

The vacuum used normally isn't much more than we generate when we

suck on a hose to siphon gas and the bubbles are present even when

the temperature is below freezeing. Its real easy to prove that those

bubbles are simply air leaking into the suction line. Replace the glass

bowl filter with any different type of filter and there are no more air

bubbles in the suction line.

These same "experts" have claimed that our other filters leak air into

suction lines. Thats not true. The ONLY reports of air leaks are those

glass bowl filters and tubing connections which use the wrong clamps.

I did see one report where vibration unscrewed one brand of so called

"high performance" fuel filter. They are a metal cylinder which screws

apart so you can replace the very small filter element. I do NOT

recommend them because the elements are too small.

Some fuel filters use a sintered bronze element. That element has a

very small surface so it doesn't take long for them to get cloged up.

I do NOT recommend them.

It's hard to beat the plain old pleated paper type fuel filters which have

been used in cars, boats, cycles and many other applications for years.

I highly recommend them. The Fram G1 is large and clear with

connections that fit our 1/4"ID tubing. Get them from Mark Smiths

Tri-state Kite Sales or from Lockwood Aviation as part #414-3636 if

you can't find them locally.

Some "experts" claim that ethanol in the fuel absobs water and that

water will clog up paper filters. It is true that ethanol absorbs water but

neither gasoline, ethanol, water or any combination of them will clog up

paper filters. Anyone with a paper filter and a squeeze bulb can easily

suck any or all of it through the filter. Think about the millions of paper

fuel filters used in cars for years.

You should trap excess water and trash in the fuel tank before it ever

gets to the filter anyway. Cessna "experts" should think of our fuel tank

as our big gascolator where water and trash settle out. We just use a

filter at our fuel pump intake instead of a screen inside our "gascolator".

Some "experts" claim that its better to use a gascolator because filters

will clog up. The reason filters clog up is because they are doing their

job by keeping stuff out of our pumps and carbs. Rather than clean a

screen we replace filters. As long as we replace filters when they need

replacement there is never a problem.

SQUEEZE BULBS, CHOKES AND PRIMERS

A squeeze bulb will only pump fuel to refill the carb bowls and lines

which have drained down while the plane was sitting. It will NOT squirt

fuel into the intake manifold to help the engine start like a hand pump

primer. It will NOT enrichen the mixture to help cold engines run

smoother like a choke. Some people have had so much trouble with

squeeze bulbs that they refuse to use them. Squeeze bulbs should be

replaced once a year to minimize problems like splitting, leaking,

shedding rubber particles into the fuel or failure of their built in check

valves. I do NOT recommend them.

Our "choke" is actually just a seperate fuel path inside the carb which

allows fuel to flow through a seperate jet. That enrichens the mixture to

help cold engines run smoother but it doesn't "prime" the engine or fill

the carb bowls and fuel lines. I don't use mine at all.

A hand primer pump squirts a little fuel into the intake manifold every

time you pump it. Proper use eliminates any need for a squeeze bulb

or the choke. It does take a little experience to learn how many times

to pump it before starting the engine and when to pump it to keep the

engine running while the carb bowls and fuel lines refill and the engine

warms up a little. I recommend them highly.

I mounted my primer near my right shoulder when I'm seated in the

plane to keep all fuel behind the pilot. That keeps the primer lines

short and lets me reach across and pump it with my left hand to use

it as an emergency fuel pump. I can also reach it from outside the

plane to prime the engine as needed when starting the engine.

To start my plane I lean in to make sure the throttle is all the way back

and the kill switches are in the "run" position. I position myself so that

my left leg is against the front of the right main tire. If the engine were

to rev up the plane may run over a chock but it can't run up my leg

before I can kill the engine. I look over the wing to watch fuel move

through the primer line as I pump the primer. Once it reaches the carb

I give it a couple more pumps to squirt fuel into the engine. The exact

number of pumps depends on my past experience with the plane and

the current temperature. I look to be certain that the area near the

pusher prop and the area in the prop blast are clear then yell "Clear

prop". Last I lean in and hit the starter. It may take another pump or two

to keep the engine running until it warms up. It doesn't take long to learn

when to pump it.

FUEL PRESSURE GAUGE OR LIGHT

I'm a firm believer in KISS (Keep It Simple Stupid) so I won't put one

on my plane but some people like to have a fuel pressure gauge or a

light that comes on when fuel pressure drops below 2 PSI. Thats okay

as long as you understand what you are really seeing.

You need to see the pressure right at the carb inlet. If you just tee into

the fuel line there and run a line down to a gauge mounted on the panel

you will see the head pressure of the fuel in that line added to the actual

pressure at the carb. You need a 10 PSI gauge but normal gauges are

not very accurate near the ends of their scales and can not survive

vibration very long. You have also added the potential problem of fuel

leaks and fire in the cockpit. Electronic sensors for an EIS or a

pressure switch for an indicator light installed near the carb inlet solve

those problems but are they really useful?

If trash blocks the carb inlet needle valve or the main jet the engine will

starve for fuel but the light will never even flicker because there will

still

be normal pressure at the carb inlet.

If air is leaking into the pump suction line the pump will pump air bubbles

into the fuel lines. The light will never even flicker as small bubbles

pass

through the carb inlet valve and get vented because the pressure in the

the fuel line stays normal. If there is enough air in the fuel line to

change

the pressure in the fuel line then you won't get much advance warning

from the light before the engine quits.

The most common problem is crud building up in the fuel filter. How

fast that happens depends on how much crud is in the fuel and how

much filter surface area the fuel filter has. Refueling with heavily

contaminated fuel can clog a small brand new filter very shortly after

takeoff. Normal fuel usually takes over 100 hours of flying before

problems ever start. Once the fuel filter can not pass as much fuel as

the engine is useing the fuel level in the carb bowl starts to drop which

opens the float needle valve more. The PRESSURE in the fuel line

starts to drop as the needle valve opens more because the fuel FLOW

is restricted by the fuel filter instead of the float needle valve.

Eventually

the fuel PRESSURE drops below 2 PSI and the light comes on. Notice

that the engine is still getting some fuel and that fuel FLOW may be

enough for the engine to run for hours. The light tells you that the fuel

PRESSURE has dropped below 2 PSI but thats all it can tell you. You

have no way of knowing why. You have no way of knowing how long the

engine can continue to run. You need more information before you can

make a decision about what to do.

NEVER base a decision to make an emergency landing on an idiot

light alone. That light should only be used to tell you to check the EGT

gauge. The EGT will tell you if your engine is getting enough fuel or not

so base your decisions on the EGT instead of an idiot light.

Heres why. As previously explained when the fuel system can't deliver

as much fuel FLOW as the engine is useing the level in the float bowl

starts dropping. As that level drops the jets deliver less fuel so the

mixture starts getting lean and the EGT starts to climb. As long as the

EGT is normal you have no reason to make an emergency landing. If

the EGT is high you should look for a landing site but also reduce

throttle to see if the EGT will drop to normal when the engine uses less

fuel. If the EGT drops to normal you can limp a long way to a better

landing site at reduced throttle so you don't really need to make an

emergency landing right away.

If the float level drops too far the engine will start surgeing. That is a

short increase in RPM and power when the mixture gets very lean as

the engine runs out of fuel. Once the mixture gets too lean to ignite the

engine coasts to a stop unless it gets another shot of fuel to run or

surge again. The prop windmills for some time after the fuel is gone.

That keeps the fuel pump working so its common for the engine to

surge several times before it quits completely. Now you have a real

emergency but you may still be able to limp to a better landing spot if

you can use the hand primer to help the fuel system deliver enough

fuel and/or reduce the throttle so the engine doesn't use as much fuel.

Most fuel problems show up first at full throttle because the fuel system

works the hardest then. Get in the habit of checking for high EGT during

all full throttle operations such as climbout. When you start to see that

happen, its time to replace your fuel filter and check the rest of your fuel

system.

The pressure gauge, idiot light or EGT will not give any warning of a

sudden serious fuel system failure so you should fly like the engine

could quit at any time and practice emergency landings so you will be

ready for a real one.

FUEL INJECTION CONSIDERATIONS

Fuel injection requires a high pressure fuel pump. Most of those pumps

need a return line to be run from the pump back to the fuel tank because

they use a built in pressure relief valve as a form of pressure regulator

for the outlet port. When the pressure at the outlet port rises higher than

the set point of the pressure relief valve the pressure relief valve opens

to divert fuel from the outlet port to the return port until the pressure at

the outlet port drops back down to the set point.

That means that the fuel flow into the fuel pump is much greater than the

fuel flow required by the engine alone. The return line from the pump

back to the tank and the suction line from the tank to the fuel pump inlet

port must be big enough to handle the maximum fuel flow that the pump

can produce. Note that the fuel filter in the suction line must be able to

handle the larger flow also. The line from the fuel pump outlet port to the

fuel injection can be much smaller since that flow is only the amount

actually used by the engine but that line must be able to handle the high

pressure that is at the fuel pump outlet port.

THE BOTTOM LINE

You should NEVER attempt the first takeoff until you are absolutely

certain that your new fuel system will provide enough fuel under all

conditions. Tie the plane down and run it at full throttle to be certain

that

it won't starve for gas during takeoff and check closely for leaks. Always

be prepared for the engine to quit on takeoff.

[Dave B 0]

I use this pdf to cross check my limits, both aircooled and liquid cooled. Resize, print, and tape to the panel....

2STref.pdf

Dave

[herman pahls 9]

Dave B

I appreciate your aircraft fuel system advice.

Did you mention elsewhere that you fly on 26 inch airstreaks on the Columbia river?

I am from the Southern Oregon coast and would like to get your contact info.

Thanks Herman Pahls Coquille, OR 541 404 6464 hpahls@gmail.com

[bdavies 0]

Hey Everyone,

I finally got out to do some work on the plane and go flying today. Felt awesome to be in the air with my Avid again. The thing jumped off the ground about 1 second after full throttle and probably set a new stopping record at my airport when I landed a few feet short of the pavement.

I'm a two stroke noob and have been trying to get my egt's to show the same on both cyclinders. At the lower temps it runs about 75-100 degrees difference and at the high end its maybe 50 or less off. The thing is that when I'm at full throttle or running anywheres above roughly 5500 rpm the one egt creeps to 1200 or slightly higher while the other one is at 1150 ish. Normally its turning 6200 rpm at full throttle on the ground and tied to my truck. I turned both mixture needles (the little ones beside the big idle screws) fully in so it should be running at full rich; right? I'm wondering if one egt probe is off or something. I've ground run it a bunch and can't seem to figure it out. Should i raise the needle clips 1 notch and see if that helps?

Any advice would be super appreciated. I love flying this thing but don't want to cook the engine.

Thanks,

Darcy

Check your spark plugs and see if there is any difference between cylinders. EGTs are not all that accurate, the only true indication of how your temps are doing is by reading the plugs. You might also check your carb synchronization.

[TheDeltaFox 0]

Bill,

I see your from Fox Creek, AB. I used to drive by there on the way to Fort St John, BC. Nice little strip right along the highway. Always saw what looked like an RV4 or T18 sitting in one of the open hangars.

Do you have an Avid there?

[ChrisBolkan 130]

Dave B

I appreciate your aircraft fuel system advice.

Did you mention elsewhere that you fly on 26 inch airstreaks on the Columbia river?

I am from the Southern Oregon coast and would like to get your contact info.

Thanks Herman Pahls Coquille, OR 541 404 6464 hpahls@gmail.com

Hi Herman,

I stumbled on this post while looking around. I am NOT DaveB My name is ChrisB. I do not know if DaveB lives on the Columbia River or not, but I do. I thought you might have names confused. I have Airstreaks on my Avid. I thought I might be the person you meant. I live in the Tri Cities, Wa, (Richland, Pasco and Kennewick). Where is Coquille?

Chris Bolkan

[herman pahls 9]

Chris

Sorry for the name confusion.

Tell me everything I need to know about Airstreaks.

My sons Pacer has 31 Bushwheels and I need help deciding between 26 and 29 Airstreaks.

I would like to land more of the places he does.

We live on the Southern Oregon Coast and land on soft sand and rocky gravel bars.

I fly a 500 pound model 2 Kitfox with a 582.

I currently have 22 inch ATV tires with 8 inch diameter Matco wheels and Brakes.

Matco said it was simple to switch to 6 inch wheels for the Airstreaks.

We know from the Pacer that bigger Bush wheels cost more, weigh more, drag more and help smooth out the gravel bars.

Tell me about your Avid and your performance experience with the Airstreaks.

What tailwheel are you using?

Thanks Herman Pahls 541 404 6464

[akflyer 1427]

Herman,

Stick with the 26" lightweight ABW. Randy flys the hell out of his avid off airport using them and loves them. 29s are just adding weight and drag and unless you are planning on landing in boulder piles, totally not needed. If I could justfy the cost, I would have a baby bushwheel on the tail on my Avid. I just cant do it, although I do love it on the Pacer. The Matco 8" wide is a great tailwheel and will give you pretty damn good flotation on the beach!

[cloudtoper 0]

I had a probe that was botoming out and tuching the inside.I like to naver figured it out,when I moved it back a little that fixed it.