912 UL Engine Vibrations - Kitfox IV

[Av8r3400 1178]

Since this summer, I've been having an issue with vibrations in the 2000-3000 rpm range.

 

There are airframe issues and there are engine issues.

 

The 912 engine mount for the Kitfox I-IV mounts the oil tank above and behind the engine.  This requires the intake manifolds to be reversed making a wider gap between the carbs.  This wider mounting exaggerates any vibration by shaking the carbs making the motor sputter making the vibration worse, shaking the carbs more making the engine sputter making the vibration worse, shaking the carbs more …..

 

Kitfox has a service bulletin to tighten the engine mount dampers due to this known issue.  #SB 54A

 

I tried all of the obvious things:

1. Synced Carbs
2. Disassembled, cleaned, adjusted and re-synced carbs
3. disassembled, cleaned, installed rebuild kits, adjusted and re-synced carbs
4. Serviced engine mounts per #SB 54A
5. Removed, replaced springs, cleaned and re-assembled gearbox
6. Removed, cleaned, re-shimmed, lubricated and re-assembled gearbox

During the gearbox work, I noticed that there was some spauling on the drive dog and driven gear.

 

 

 

This spauling bothered me.  The techs at LEAF agreed with me.  At light loading the drive will move back and forth and if this movement isn't smooth, a vibration will be caused.

 

Through the research I did, I also found that these parts have been redesigned to allow for a longer service life.  OEM, the gearbox is supposed to be serviced every 300 hours.  This service is new springs and inspection for damage on the dogs.  My motor has 450 hours.  Ahem...

 

I got my new gearbox parts today...

 

 

Hopefully, sometime in the near future it will get above 0º here and I can rebuild my gearbox again.

[akflyer 1427]

ohoh... a 4 stroke having issues?? sumbitch, call the wife and tell her the world is ending and you wont be home for dinner!    Just busting your chops Larry.  I am interested to see what you find out and if this takes care of the issue.  Can you mount the tank different and flop the carbs back around

 

[Av8r3400 1178]

It is possible, yes, but that would require redoing the complete firewall forward including the engine mount and cowling.  The Kitfox V and newer use an engine mount that pushes the engine about 6" forward and puts the oil tank on the right side, just below and behind the cylinders.  This was done because that airframe was designed to mount the boat anchor weight certified motors.  Moving weight forward was needed to keep in balance.

 

The 1-4 Kitfoxes were designed for 75# two stroke motors under the cowl.  When the  912 came along and people were finally sick of fouled plugs, smoke, stink, high file burn and the general unreliability associated with two strokes ( ), Kitfox did what they had to do in order to make them work.  These early 912 mounts kept the motor as compact and rearward as possible.  This is the reason for the odd style oil tank mounting.  It is still far better than the oil tank in the trunk method that Avid used.

 

I don't really like the setup, but it is far more trouble than worth to reengineer the whole thing.  I thought a long time about redoing this on my project plane.  I finally decided that it still wasn't worth the time and expense to rework the whole FWF.

[C5Engineer 1170]

A required service at 300 hrs!!!! Ludicrious I tell you!

[akflyer 1427]

Ahhh, sorry mr.. Guess the sweet smell of two stroke blue smoke doesn't do the same thing to your wife as it does to mine if it did you would be running twin 582's on that mangy bird hahahaha

[Bandit 286]

I love the sweet smell of the 2 strokers!

[Av8r3400 1178]

Two stroke smoke...

 

[herman pahls 9]

The die hard or is it jealous 2 strokers are sure having a good time with any weakness's in the 912.

So is the 912 vibration due to carb mounting a common issue on the Model 4 since I am hoping to own one in the future?

[Av8r3400 1178]

Not so much in the 80 hp as the 100. The 100 is more violent a shake at start and shut down.

Mine is defiantly an anomaly. Stemming from a very easily 912 (1992-3). The newer ones have a different gearbox design. My project planes motor is a 1995-6 and has the newer style gears.

[akflyer 1427]

Herman, I think Larry knows I am just pulling his chain... If I had a 912 laying around you can bet I would have it stuffed into the nose of my Avid.  I just hate the fact that Rotax is bending us over just because the military is using the 912 in the drones and therefore, selling them to the general public means little to nothing to Rotax.  The price for that engine is STUPID!  But, once again, if I had one you can bet I would use the hell out of it

 

I am getting over the price of Rotax shit now that I am looking at big bore Continental and Cessna prices...

 

[akflyer 1427]

hmmm... redneck video.. the dude has a monster PTO winch on the bed of the rig, yet has a Nissan truck trying to pull him out..  Wonder how many beers was involve in that decision making process

 

[C5Engineer 1170]

Two STROKE!!

 

 

[C5Engineer 1170]

Sorry Larry for the thread drift...I have thousands of miles on two stroke dirt bikes and still own one. My 582 is treated like a gem compared to that abuse my bikes have taken over the years and gotten me home.

 

[EDMO 1078]

And if you think Rotax prices are going to come down - Just look at what the Contisaurus and Lycosaurus engines are selling for, and that is 1930's design - BTW:  With the blessing of the FAA who "Ordained" these old engines!

Somehow, we need to find a junkyard, low cost, low weight, auto engine to turn the fan!

EDMO

[Av8r3400 1178]

You stole my thunder, Ed. Just go out and price a new O200 lightweight or Lycoming 233. That Rotax is a bargain.

[C5Engineer 1170]

You stole my thunder, Ed. Just go out and price a new O200 lightweight or Lycoming 233. That Rotax is a bargain.

 

 

I am about to pull the trigger on a Ford Racing 306 c.i. 340HP crate engine for my 64 1/2 Mustang.... Ready to run with all the accessories it costs quite a bit less than a new 582!! Mind numbing the cost of the aircraft engines!! Wonder what a turbo'd IO-540 costs

[SuberAvid 617]

hmmmmmSubaruhmmmmm

[herman pahls 9]

Joey

I just noticed your thread creep and I enjoyed the video.

I ride a KTM 2 stroke and technical single track with water crossings is the kind of riding I go out of my way for.

I just came to the realization that my dirt bike and Kitfox have similarities.

I am considering 4 strokes for my next dirt bike and airplane engine.

Both have been modified with centrifugal clutches.

Neither have ever failed me.

Herman

[Chewie 19]

You'd be amazed how many drones UAVs are flying in theater on hobby-grade 2-stroke engines!

 

...and in Alaska, I might add.

Edited 23 Dec 2013 by Chewie

[EDMO 1078]

Check out the importers from Japan - The Japanese have to replace their auto engines or overhaul them every 30,000 miles.  Easier to just replace than overhaul, I guess.  These engines are being imported into the US - I have an EA-81 Soob spare with 31,000 miles since new sitting in my shed that cost me about $1000 including shipping to Alaska. 

The twin carbs are scarce, but out there I guess.  A single carb, auto tranny, 94 station wagon is probably the best for planes.  A redrive is $2500 to $4000, depending on who made it.  I saw one with redrive sell on ebay about a year ago for $1500 with about 300 hours on it.

But, then again, if you are looking for light weight - the 2-stroke cant be beat.

No telling tho, when a light 4-stroke may come up again in some kind of car?

EDMO

Edited 23 Dec 2013 by Ed In Missouri

[Av8r3400 1178]

… and you still have an auto-conversion that is nearly impossible to insure and will sell for less than 1/2 the price of a Rotax powered plane with the same equipment.  I'm not knocking the Subies.  They are a good motor and the guys who have them, like them.  In the end, you still have single ignition, a home-made re-drive, and little if any support from the aircraft company for FWF equipment or propellers.  If you enjoy being an aircraft designer and aeronautical engineer, more power to you and best of luck.  Most people don't.

 

Personally, I'll take an engine designed and manufactured for aircraft use.  The problem (that this thread has drifted so far away from I'm not even going to bother posting further) that I am having is an anomaly.  Definitely very rare.  I now have the parts in hand to fix it and the full engineering support from both Kitfox and Rotax.

[akflyer 1427]

screw the thread drift, post up your repairs and the outcome... Passions rise up when it comes to engines.  We can agree to disagree on some things, and bust each others balls a little... have another toddy n post up some pics Mr.

 

[EDMO 1078]

Seems like we generally have a tendency to get off of the main subject on these posts. mostly on engines.

For my part, you have my apology.

Maybe what you can post on a new - same - topic about your fixes will help someone else.

Merry Christmas,

EDMO

Edited 25 Dec 2013 by Ed In Missouri

[BryceKat 125]

Dang you Joey and those KTM videos😜â—ï¸ If you don't quit posting those I'm going to end up with one and a divorce!

[akflyer 1427]

OK Larry, ya got me wondering on this one.. What was the outcome?  Did you get the vibration figured out?