RCU Forums - Engine Problem

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Engine and Carb tuning 101 (2 Stroke)

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Engine Tuning 101
written by Mathew Kirsch

Remember that in all tests, you do want the engine to run SLIGHTLY rich on the ground. There is less resistance on the propellor when the plane is in the air, so the engine will run faster and will need a little more fuel to keep it from overheating.

Basically, with any test you perform, there are two possibilities:
1. Engine sputters and/or runs slightly faster - too rich.
2. Engine dies - too lean.

Tests:
Pinch test - Pinch the fuel line to the carb momentarily. If it runs
slightly faster, you're a little rich. If it doesn't change, you're
right on. If it stalls, you're running too lean.

Vertical test - At full throttle, hold the plane vertical. If the engine
dies, you're running too lean. If the RPMs don't change, you're right
on. If it speeds up it's a little rich.

Idle test - Let the engine idle for a minute on the ground. Quickly open
the engine to full throttle. If it sputters, the idle mixture is too
rich. With a low-speed needle carb, turn it clockwise to lean the
mixture by reducing the amount of fuel. With an air-bleed carb, turn the
air bleed screw counter-clockwise to increase the amount of air. Never
move it more than 1/8 of a turn at a time. Your instructions will tell
you which is which.

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Carburetor Adjusting 101
written by Tim Mills

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Here's a scenario: Pilot takes off, plane's engine sounds
great. After several minutes of flying, engine seems to
lose power, sounds kinda "thin", pilot keeps flying. Engine
continues to sag, now full throttle is very weak, pilot now
understands that maybe this isn't gonna clear up. Engine
dies (what a shock! ), pilot calls for deadstick landing
overshoots, tears off landing gear, etc. Never seen this at
your field, right?
Here's the way that *I* set mixture on non-airbleed carbed
engines (90% of the engines out there fit this category, but
the theory is similar for air-bleed carbs). First of all,
understand that the high speed needle has its main effect
from 3/4 to full throttle, and the low speed needle controls
everything from idle up to 3/4 throttle. It thus makes
sense to me to spend the biggest majority of my tuning time
adjusting the needle that controls the largest portion of
engine running, right? Also, remember that there is a
proper air to fuel ratio (mixture) that allows the engine to
run properly. Too much fuel is rich, and too little fuel is
lean. We "richen" the mixture by adding more fuel (turning
the needle out, or counter-clockwise), and we "lean the
mixture out" by decreasing the fuel (turning the needle
valve in, or clockwise).

I start the engine give it full throttle, and lean it to
it's highest rpm (peak), then richen it by maybe a quarter
turn. Then with the glow plug igniter still attached, I
slowly close the throttle to an idle rpm. At the lowest rpm
that the engine will still reliably run, I then remove the
glow igniter. If the engine dies immediately, I know it's
too rich, and I then lean out the LOW SPEED NEEDLE by 1/8th
of a turn (don't touch the high speed needle). Start the
engine again, (and this is important) give FULL throttle
briefly to clear out excess fuel, then slowly close the
throttle again. Remove the glow igniter, and this time it
may run a little longer before it dies, so lean the low
speed another 1/8th turn. Re-fire the engine, give a burst
of full throttle to clear it out, and slowly close the
throttle again. remove the glow igniter and now notice that
the rpm DROPPED a bit when you removed the glow igniter, but
the engine kept running. We're getting there. It's still
too rich, and you'll prove that by opening up the throttle
and hearing the engine "blubber" then die. That's because
excess fuel has collected in the crankcase during the rich
idle, and when you opened up the throttle, the excess was
pulled into the cylinder, making it WAY too rich. Supposed
you were on a landing approach, and decided to go around,
you throttle up but the engine "blubbers" and then dies
(another thing we haven't seen, right?). Yep, the LOW SPEED
needle was still too rich, allowing excess fuel to collect
in the crankcase, just WAITING on you to try to go around so
it could "LOAD UP", blubber, and die!

Keep leaning the low speed needle down until it idles well,
but now, when you open up the throttle, it HESITATES instead
of BLUBBERS. When this happens, you've lean it down too
far, so richen it up 1/16th of a turn and try again. You
know you've got the LOW SPEED needle right when you can fire
it up, remove the the glow igniter, and the rpm doesn't
change AT ALL, and you can open the throttle up, and it
doesn't blubber or hesitate, it just runs!

The final thing you do is re-adjust the HIGH SPEED NEEDLE,
leaning it to it's highest rpm (peak) and then richening it
up maybe 1/8th turn to give it a slightly rich mixture. We
also know that the fuel mixture will change in flight when
you point the nose up (harder for fuel to travel uphill) and
also as the fuel level in the tank changes. In both cases,
a leaner mixture results, so we actually need to set the
mixture a bit further on the rich side to account for this.
While the engine is running at full throttle, CAREFULLY pick
the model up and raise the nose to at least a 45 degree
angle while listening to the engine. If the engine sags a
bit, then you'll need to richen up the high speed needle
1/16th turn. Try it again, and when you can point the nose
up and the engine doesn't sag, but maybe shows a slight GAIN
in rpm, you know you've got it right.

Now the engine will be happy, and chances are will reward
you with reliable running. If you've got one of the few
engines with an air bleed adjustment for low speed
adjustment, the theory is the same, just refer to your
manual to see how to richen and lean the low speed mixture.

Gotta go rest my typing fingers!
Tim
A BAD day at the field STILL beats a GREAT day at work!! [link=http://www.********.net/pages/tmills]Tim's homepage [/link]