582 Jetting guide by Green Sky adventures

[C5Engineer 1170]

By Jerry Olenik of Green Sky Adventures

 

As promised, I am going to go through some things that may help with
properly tuning the 2-stroke aircraft engine.

No matter who makes the engine, the first thing I do is lean the idle
mixture. Even if you think you have a smooth running engine at idle, you may still get problems at other power settings from excessive fuel that loads up or even harmful long term effects like excessive carbon build up. Often people go changing main jets or needle clip positions because of fowling spark plugs or whatever when the problem is really coming from a rich idle.

Most of the time with a 2-stroke, you can not lean to peak efficiency like
in a Cessna or the like because the engine will seize before you get there.
However, at low engine speed you CAN lean to peak efficiency without
damaging the engine. To do this, I would suggest securing the airplane so
it can't roll away, because as you lean the mixture, your idle speed is
going to increase and may increase enough to make the plane roll away.

Once appropriate measures are taken to secure the plane, start the engine
and listen carefully at idle. Listed for occasional bumps or misses.
Listen to the general tone of the engine. Feel the airframe for vibration.
This is sort of like tuning a guitar by ear. Then shut it down. The 2SI
engine really is usually a smooth running engine at idle even when it's too
rich, so you may have to go by RPM more than the smoothness. RPM will
increase as you get closer to peak efficiency. With the Bing 84 carb or
Bing 54 carb, start turning the idle air screw out (counter-clockwise) about
1/2 turn (180 degrees). This would be the smaller screw that is recessed
into the carb. Turning it out allows in more air and effectively leans the
mixture in the idle RPM range. As you turn it out, keep track of how far
you have gone. After each half turn, start up the engine and listen and
feel for bumps, misses, roughness and the general tone of the engine.
Generally a deeper tone will be richer and a higher pitched tone will be
leaner. Continue to go through this until the engine seems to be running
completely smooth. As smooth as glass. Note where you are on the air screw
adjustment at that point.

One thing you may run into is that you could run out of adjustment on the
airscrew. If you turn it out far enough to see the o-ring that seals it,
you need to go to the next leaner idler jet. If you change to a leaner
idler jet, start out with the airscrew turned out about 1/2 turn and
continue with the process. Even if it seems like it is smoothed out, note
that setting and continue. You will eventually get to a point where the
engine will decrease in RPM and/or die or not want to run. Note that point
as well. I would then set your idle air screw about mid way between where
it seemed to totally smooth out and where it was too lean to run. That way
you should not need more adjustments with changes in the weather or density
altitude unless those changes are severe.

Now that you have your idle set, you are ready to try your mid range. NOTE:
leaning the idle can sometimes have some effect on the midrange and you may
want to start out in the richest (lowest clip position) needle setting until
you know where your EGT is going to be. With the engine properly warmed up,
increase the throttle into the 4000 - 5000 RPM range. Make movements with
the throttle watching your EGT for the hottest spot and just make a mental
note of where your engine has it's hottest EGT. Be cautious about staying
within limits on all your temperatures because now you are running at engine
speeds that can cause damage if mistakes are made.

For reliability and engine longevity reasons, I take exactly the opposite
approach to mixture at higher power settings as I do for idle mixture
settings. What I mean is that I like to make the mixture rich until it runs
a little bit rough, then lean it out a little from there. This is just the
opposite that you would do a in Cessna or other GA type aircraft. While you
are searching for that hottest spot in your mid range, keep your ears open
for any little misses or bumps, and feel for any momentary waves of
vibration in the airframe. Those are all signs that the mixture is
bordering on being too rich. The reason you are looking for the hottest
spot in your EGT is so that you don't make an adjustment that would put that
over limits. Each position on the jet needle will change the mid range EGT
approximately 50 degrees + or -. If you don't notice any roughness or other
signs of being too rich in the mid range. Make it a little richer. If you
are already in the richest position on the jet need and well within temp
limits, you might just leave it alone here. Otherwise you would need to go
to a different needle jet to make the mixture richer. If the mid range is
rough and acts like it is rich and you are well within limits on your EGT,
you can move the clip to the next leanest position. When leaning the
mixture always make small incremental changes because too lean will cause
damage often without warning. So if your EGT is 900F or 1000F and the
engine is running rough in the mid range, try moving to the next leaner
needle position. Do this until either the engine smoothes out in that range
or you come within 50F of your upper EGT limit. If your EGT limit is 1200F
and you are at 1150F, you probably do not want to make another adjustment
any leaner.

Once the mid range is set, with the engine properly warmed up, you can do a
static run up to full power. Make a note of the maximum RPM and the EGT.
If your engine makes its rated power at 6000 RPM, you maximum static RPM
should usually be a little less than that. How much less really depends on
how fast the aircraft is because that max RPM will increase during flight.
So if you have a relatively slow aircraft with an engine that makes it's
rated power at 6000 RPM, you might expect to set your maximum static RPM to
5800 or so. If you have an engine that makes it's rated power at 6500 RPM,
you might set your maximum static RPM at 6300 or so. So if you are not
close to that, you should make a propeller pitch adjustment to load the
engine properly. If you do have to make a large change in propeller pitch,
it will also have an effect on your fuel-air mixture throughout the entire
RPM range of the engine. So if you were loaded to 6300 and had to make it
5800, your mixture is going to get richer. If you were loaded to 5800 and
you need to change it to 6300, your mixture is going to get leaner. So
after adjusting your pitch, it is a good idea to go through the carb
adjustment process again.

If your full power EGT is fairly low, like in the 800 range, that may also
be having an effect on your full power output. Generally, you want the full
power range to have a lower EGT than the mid range for added protection
under the higher stress conditions, but if it is too much lower, the engine
will not perform. If your EGT seems to be very low at full power, and the
engine seems to bog down, you may need to change to a leaner main jet. The
main jet is the only part of the carburetor that has control over the full
power mixture. At full power, the jet needle is all the way out of the jet
and a needle adjustment will not change this setting. The main jet,
however, WILL have some effect on the mid range mixture. So if you change
to a leaner main jet, it is always a good idea to change the needle clip to
the richest position on the needle for the first run up. You can always
make the mixture leaner in the mid range if its too rich, but if it's too
lean, you might need a new piston.

Since mixture effects the power output of the engine and propeller loading
effects mixture, this process is sort of like hitting a moving target in
some cases. If you find that you have to make a large propeller adjustment,
you may have to go back and forth through this a couple of times. Once set,
and set properly, however, you should be able to enjoy many hours of
enjoyable, trouble free use. If you don't set the engine, carbs, and
propeller up properly, you will likely have many hours of problems,
frustration, and may even cause damage to the engine and/or airframe.

These are the exact same steps I use when setting up an engine on a
customer's aircraft weather the engine is new or used. My customers are
usually very pleased with the results since they no longer have to put up
with the vibrations and throttle response problems that they just took as
normal before. Hopefully you guys can get the same results.

As always, let me know if you have any questions.

[Brodstol 41]

Have you gone through this on your new Avid? What kind of results did it yield?

[C5Engineer 1170]

Have you gone through this on your new Avid? What kind of results did it yield?

absolutely! That’s step one with a two stroke and key to hours of trouble free flying. I’m running 45 idle Jets with the clutch because it’s idling at 1600 rpm