Idle mixture adjust

[M20 Ogler]

My engine minimum idle is at 750 rpm. In order to get the 50 rpm rise the O-360 manual calls for when pulling the mixture out I had to back the mixture adjust screw out 4 turns. But now my WOT static rpm is only 2650 when full rich. I have to lean with the mixture knob to get 2700. 
 

4 turns seems like a lot to me. What is usually required?

[EricJ]

The idle mixture adjustment shouldn't affect static WOT RPM unless something else is wrong.   You might try decreasing idle rpm a bit, and it'll help reduce float on landing as well.   You should check idle mixture adjustment again if you do change idle rpm.

My full-rich idle rpm is about 550 rpm.   Landings got easier when I cranked it down.

 

[MB65E]

Gov could be adjusted up a 1/2 turn. Other than that 4turns out on the mixture seems about normal. Idle mixture checks should be done at 900 RPM. However, I didn’t realize the O-360 had an idle mixture setting test. It can’t really be adjusted otherwise. only the IO- fuel servos have the adjustment. 
-Matt

[DXB]

55 minutes ago, M20 Ogler said:

My engine minimum idle is at 750 rpm. In order to get the 50 rpm rise the O-360 manual calls for when pulling the mixture out I had to back the mixture adjust screw out 4 turns. But now my WOT static rpm is only 2650 when full rich. I have to lean with the mixture knob to get 2700. 
 

4 turns seems like a lot to me. What is usually required?

Not sure, but carb idle mixture has negligible contribution to mixture at WOT so I doubt that caused it.  Static WOT rpm of 2650 is pretty normal - governor will typically go to 2700 once you get going and the prop gets unloaded a bit.   

[PT20J]

750 rpm sounds high; I set mine for 650.

Four turns is a lot. If it used to be right and now you have to richen it that much, you might have an induction leak somewhere.

Adjusting the idle mixture usually affects the idle speed. It usually takes a few iterations to get both speed and mixture right.

Are you sure that the static rpm changed or are you just noticing it for the first time? During a full power run up, the prop should be sitting on the low pitch stops and the governor should be out of the picture. It should turn a bit less than 2700 rpm. As you begin the takeoff roll the increased airflow will want to speed up the prop and is should come up to 2700 and then the governor should keep it there. If there is any significant wind during a run up power check, the airplane should be positioned 90-degrees to the wind so that it doesn't affect the prop load.

Skip 

[Guest]

8 hours ago, M20 Ogler said:

My engine minimum idle is at 750 rpm. In order to get the 50 rpm rise the O-360 manual calls for when pulling the mixture out I had to back the mixture adjust screw out 4 turns. But now my WOT static rpm is only 2650 when full rich. I have to lean with the mixture knob to get 2700. 
 

4 turns seems like a lot to me. What is usually required?

If everything was normal before, four turns of the idle mixture screw seems odd.  I’d look for an induction leak.  Verify your tachometer accuracy before worrying about the prop and governor.

Clarence

[M20 Ogler]

Will check for induction leaks. 

We also worked on the fuel lines to remove and splice some corroded sections of fuel tube. So maybe a clogged fuel strainer? 

Or maybe the old carb air box and induction boot were so leaky that it was making my mixture rich and now that those are new and sealed the mixture is suddenly to lean?

maybe my carburettor needs service?

I not going to fly the plane yet because my No. 2 cylinder EGT at full rich idle reads hotter than it used to leaned to cruise at 100 degrees ROP. The others are hotter as well 

Edited November 17, 2021 by M20 Ogler

[M20 Ogler]

It was an induction leak. The IA on field inspected and said the intake flange on the No 2 Cyl was warped and he machined it flat. Now that Cyl egt is much closer to the other ones. He says that happens sometimes doesn’t know why 

[carusoam]

Great follow up Ogler!

-a-