What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Idle mixture and RPM rise

rjcthree

Well Known Member
When setting idle mixture, the requirement is something like "set idle mixture such that when moving mixture to ICO, there is to be about a 50 RPM increase before RPM falls off".

How fast does one move the mixture knob for this test? Like you're trying to shut the engine down by going to ICO (fast) or incrementally, or like you're trying to find the RPM number (slow)? I've not been able to find a definitive answer. Today, if I move it fast, I hit the the 50 RPM number, but I have to look at the EMS data to see it. If I roll out mixture turn by turn slowly, I can get 250 RPM rise consistently and with steady running, hot or cold.

My perception (low EGT, smell, slight miss at 1000 RPM in flight that is smoothed by leaning) is that I'm rich at idle. I just don't want to chase this any more than I have to.

Thoughts?
 
Last edited:
Pull the mixture slooooowly when checking idle mixture. You should have a max rise of 50 rpm or 1 inch of mp decrease when checking and things are set in the ball park. After getting the 50 rpm rise or 1 inch mp decrease test acceleration and if good you could try leaning a bit to 25 rise and 1/2 inch mp drop. If acceleration is good leave it there... if it is now poor enrichen until it accelerates smoothly. Some engines like to be on the richer side to get smooth acceleration. Most notably 200 hp slant valves like more than 50 rpm and 1 inch mp to accelerate well.
Good Luck,
Mahlon
 
Moh. has it here. +1

When setting idle mixture, the requirement is something like "set idle mixture such that when moving mixture to ICO, there is to be about a 50 RPM increase before RPM falls off".

How fast does one move the mixture knob for this test? Like you're trying to shut the engine down by going to ICO (fast) or incrementally, or like you're trying to find the RPM number (slow)? I've not been able to find a definitive answer. Today, if I move it fast, I hit the the 50 RPM number, but I have to look at the EMS data to see it. If I roll out mixture turn by turn slowly, I can get 250 RPM rise consistently and with steady running, hot or cold.

My perception (low EGT, smell, slight miss at 1000 RPM in flight that is smoothed by leaning) is that I'm rich at idle. I just don't want to chase this any more than I have to.

Thoughts?

This is one of those little adjustments you will be making every annual or two. You will get used to it. As Moh. has set out, pulling back nice and smooth to see where it starts to die you will find a nice rise if it is adjusted OK. Each engine and it's set up will be different and some will feel wright at 30 RPMs rise and some will feel wright at 75-90. I have seen two C-172's that came out of the factory one after the other set a little different. The thing you are looking for is to be a little rich at ideal so when you push the throttle open (smoothly) that the engine does not be so lean that it starts to die with the mixture fully open to its stop. That 50 number is a good rule of thumb but it is not a definite set point. You will get used to your engine and its caricature, they all have their own. Hope this helps. Yours, R.E.A. III # 80888
 
Back
Top