How can I fine-tune idle mixture?

How about a discussion of the scalloped wheel on the injector pump for an IO540-C4B5 engine? My engine will “”load up”” at idle and I have to run the rpm up to about 1,200 quite often during taxi. Also the engine will “”talk to you”” on final with the throttle closed. I have tried the idle cut-off procedure looking for rpm rise, but this doesn’t seem to help.


Decaturville, Tenn.

Lou, I was a little confused when you mentioned the “”scalloped wheel”” on the injector pump, but then my mind returned to me and I know exactly what you’re talking about. We’ve always referred to this as the “”star wheel,”” but I guess it doesn’t matter what we call it as long as we’re talking about the same thing.

From your description I’d guess your mixture is running on the rich side, which I believe can be corrected without too much trouble.

This wheel is only put there to do one thing: adjust the idle mixture setting of the fuel injector throttle body. Typically, after the engine has been replaced following overhaul or a replacement engine has been installed in an aircraft, we want to fine-tune the idle setting on the engine for both speed and mixture. We accomplish this by running the engine and bringing it up to near normal operating temperatures and then setting the idle rpm at the set screw on the fuel injector throttle body on the opposite side from the star wheel. Ideally, the rpm should be set around 650 to 700. At this range you want to slowly bring the mixture control on the quadrant towards the idle cut-off position, being careful to observe the rpm on the tachometer. You should notice a slight rise in rpm just before the engine shuts down. The rise we’re looking for should be between 25 and 50 rpm before the engine quits. By adjusting the star wheel either rich or lean you should be able to accomplish this desired rpm rise.

Here’s a little hint: You mentioned your engine loads up when the throttle is at a lower rpm during taxi or when the throttle is reduced when making an approach. My guess is that your idle mixture is slightly on the rich side, so I suggest looking closely at the star wheel and take notice of the two blocks that hold the spring and the star wheel. On one of them you should find the letter “”R”” with an arrow. This indicates the direction you want to turn the star wheel to make the mixture richer. In your case, you’ll want to move the wheel in the opposite direction in order to lean out the mixture. By turning the wheel just a couple of clicks at a time, you should notice a change in the manifold pressure as well as the idle rpm. By working back and forth between the star wheel and the idle rpm adjustment screw on the opposite side of the body from the star wheel, you should be able to bring the mixture to a point where the manifold pressure will be around 10 to 12 inches and the idle rpm between 650 and 700 rpm. This applies only to normally aspirated engines like your IO-540-C4B5 and other non-turbo-charged models. On turbo-charged engine models the same basic tweaking is done, but the manifold pressure is normally a bit higher at idle.

Please take note that during the time you are making these fine-tuning adjustments, you may have to increase the engine rpm to 1,200 or a bit higher just to clear the engine out.

Just in case there may be someone out there who is experiencing the opposite of the situation you mentioned, I’ll go over what steps they should take to resolve the issue. This may be a bit repetitive, but I feel it may remove any confusion for those who may need to attempt this procedure.

If the manifold pressure is high, say around 14 or 15 inches, then I’d guess the mixture is a little lean. Therefore you’d want to enrich the mixture by turning the star wheel in the rich direction. By just turning the wheel a couple of clicks on the spring towards rich, you should notice the manifold pressure coming down. You may have to increase the engine rpm to “”clear out”” the engine a little and then bring it back to the idle position and see what you’ve got. You also may have to adjust the idle rpm at this time too, in order to keep it in the 650 to 700 rpm range. By patiently adjusting both the wheel and the idle set screw you will soon have the idle mixture and idle rpm right where they need to be.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:


  1. jon fay says

    my 310 navajo is rich at idle i cant see the arrow but the star wheel is adjusted to the most rearward position and it is still rich what do i do? this is a new overhauled engine. all is ok at cruise

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