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How to Set the Mixture Controls on a Model Airplane Engine

May 05, 2015

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Airfield Models ( 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 works.

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 seconds 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 high-speed needle.  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 an ABC 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 advanced.

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.



Starting and Running a Model Aircraft Engine
Breaking-In a New Model Airplane Engine

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Copyright 2003 Paul K. Johnson