Another idle mixture question

[rag]

Well, we have had low ceilings and miserable visibility for over a week now.... so time to over think all things aviation. This is a question I have been thinking about: It has been suggested and recommended by many much smarter than me to lean aggressively during idle (Lycoming type carbureted engines). I believe this is to reduce lead deposits? When I review the diagrams of the Marvel Schebler carbs, the mixture control metering sleeve controls the flow to the main jet and the idle jet circuits. Trying to control the mixture during idle is very difficult to do with any accuracy due to the very low fuel flow. It either kills the engine or it is so rich the idle circuit is controlling the flow - not the mixture control. My question is - why not set the idle mixture to the lean side (no RPM rise during idle cut off) and forgo trying to lean the idle mixture with the mixture control? I would think this would be more accurate and consistent, would allow full throttle control during taxi operations and one less item to forget when on the takeoff roll. Thoughts??? Forecast is for low ceilings for another 4 days.... maybe more dumb questions coming.....

[mahlon_r]

Engine will not accelerate well out of idle and will not run very well when cold.
Good Luck,
Mahlon

[lr172]

You are correct that it is very difficult to lean an idle mixture with the red knob and not really practical or necessary. A properly set idle mixture will provide an optimal fuel level and shouldn't be further leaned. Simply set the idle mixture properly and there is no reason to lean it with the red knob. The reason for leaning on the ground is reallly for taxi operations above 1000 RPM, where the main circuit is coming into play and total mixture will be richer than desired/optimal with the red knob all the way in. The main circuit tends to be quite rich with red knob all the way in and the only way to adjust it is via the red knob. Idle has it's own mixture setting, as optimal mixture is key to keeping the engine running at low RPMs, as well as to provide proper off idle transitions. The main circuits just don't have enough air flow to work properly at idle level RPMs.

As Mahlon mentioned, you need your Idle mixture to be at optimal in order to ensure normal operation. Again, no need to lean from optimal. Only need to lean from overly rich conditoins, often seen on the main circuit.

If you truly leaned your idle level mixture with the red knob, you would not have enough fuel flow available to accelerate more than a few hundred RPM without a severely lean and missing engine. Better to lean for the 1200 RPM setting. You can experiment with this. At idle, lean until your EGT peaks. Then go ahead and see how much RPM increase you can get before the engine dies.

I personally don't bother with this and have never had an issue. Others do and have. You will find different, varying opinions on this subject. Clearly not all of them are correct. Encourage you to do you own investigation and develop your own conclusion. Clearly you understand how carbs work.

Larry