Setting the Mixture Controls
on a Glow Engine
Many people have problems setting the mixture controls on their engines.
My belief is that this is because they do not fundamentally understand how a carburetor
Model aircraft engine carburetors are actually very simply in their operation.
This article will not discuss the inner workings of the carburetor.
However, if you do not understand how a carburetor works, then please read
How to disassemble a
Carburetor and How to assemble a
carburetor. These two articles should help you understand what's
inside and what the various parts do.
Once an engine is broken in and the carburetor is set properly you should not
have to make radical changes unless something else that affects engine
performance has changed. These things include changing fuel, propeller or exhaust
system or drastically different climactic conditions.
After you start your engine run it at 1/4 to 1/2 throttle for about 30
to warm it up. If the engine is new then you may have to make some
coarse adjustments just to keep it running, but let the engine run for a bit
before attempting to dial it in.
Setting the High-Speed Needle
Tip: When tuning a
conventional engine with the needle in the carb, you can turn the needle until
the engine is at the setting you want. Some engines with remote needle valves take a second or
two to react to changes in needle setting. Make small changes and then give the engine a chance to
react before making additional adjustments.
Most carburetors are designed to have the high-speed needle
set first and then the low-speed needle. After the engine is warm set the
high speed needle to near peak RPM. Do not lean the engine to the point
that it is screaming.
Set the high speed needle while holding the
airplane straight up.
In this position it is most difficult for the engine
to draw fuel and this is the condition under which you want to check the
Lean the needle to peak RPM and then back it off a
few clicks to richen the mixture slightly. If the engine is new and is not
engine then back off the high-speed a few more clicks so it is almost
breaking into a "4-cycle" setting.
This setting will give you
good overall performance without leaning out too much in climbs.
If you are having problems with your idle setting that did not exist before, it
is probably because you changed the high-speed setting since you last adjusted
the idle or the engine wasn't fully broken in. Make sure the high-speed is right and the engine is broken in
before attempting to set a good idle mixture.
Testing the Idle Setting
There are a couple ways to check the idle mixture. The most popular and
most accurate way is to use the pinch test. Pull the throttle back
to idle. Pinch the fuel line and hold it.
- If the engine begins to speed
up then the idle mixture is too rich.
- If the engine immediately slows down or tries to quit then it
is too lean.
- If the engine slowly speeds up or slows down then the
mixture is close to right.
When the idle is adjusted properly then when the fuel line is pinched the engine will run at the same rpm for
several seconds and then begin to slow down. The engine should transition
smoothly from idle to full throttle without burbling, loading up or dying.
The Wrong Way to Check the Idle Setting
The way a lot of people check their idle setting is to reduce the throttle to
idle and then immediately gun the engine. If it transitions ok then they
think everything is set properly. The problem with this techniques is that
it does not allow the engine to idle long enough to reveal a problem.
If the idle is close to being properly set then you have to let the engine idle
for several seconds to see if there is a problem. For example, if the idle
is ever so slightly too rich, then it may take several seconds for the engine to
load up with enough fuel at idle for it to be noticeable when the throttle is
If you really want to get the idle setting dialed in, then let the engine
idle for 15 to 30 seconds and then advance the throttle. If it still
transitions well, then the engine is set properly.
If the engine sputters and spits then the idle mixture is too rich. If
it slows down then it is too lean.
Setting an engine is not difficult. It is just a matter of going through
a set of procedures and being a little patient.