Is it normal to lean the mixture for ground ops, takeoff?

QUESTION: I’m operating a C-172 with a Lycoming O-360 engine at an approximately sea level airport. Is it normal to have to lean the mixture for ground operations and takeoffs? I have to or the engine runs a bit rough. Perhaps the mixture settings on the carb are not adjusted correctly, although some say it is normal to lean even at sea level. Should one use continuous carb heat for landing? Some people say it’s not necessary.

John Thomas

Port Orange, Fla.

ANSWER: John, I think it’s time to get your engine looked at and the proper adjustments made by a qualified maintenance technician.

What really got my attention was your comment regarding leaning for takeoff.

This is one thing that should never be done unless you’re in Denver or some other high altitude airport where the engine isn’t making power due to density altitude. I really think you need to have this looked at soon, especially if you are, in fact, having to lean on takeoff.

This may be oversimplified regarding having to lean while on the ground, but my first guess would be that your idle mixture needs adjusted. If you set your idle rpm around 650-700 (at normal engine operating temps) and have your maintenance facility adjust the mixture knob on the carburetor in the lean direction (there is an arrow and the letter “”R”” for rich marked on the knob) I think it should improve the roughness you’ve been experiencing. As the mixture is turned lean you may see an increase in rpm, which can be adjusted by readjusting the idle rpm screw on the carburetor.

When it comes to applying carb heat, I’m going to defer that question to the airframe POH since there are so many different induction systems used on various aircraft and the individual airframe manufacturers would know best for their particular installation when to apply carb heat and under which circumstances. However, if you feel your engine runs rich continuously regardless of the rpm, it may be that you are sucking hot air into the induction system, which will cause the engine to run rich. Aging aircraft sometimes suffer from this due to the carb heat box not fully closing or being forced open in flight by high power settings and/or ram air coming into the inlet. Make certain the carb heat door is closing properly and staying closed when carb heat is off.

One other thing that may be contributing to a rich mixture is a leaking primer system. I’d suggest you completely block off the primer system to see if this has any impact on performance, especially in flight. This change may not be noticeable on the ground at low rpms, but at higher power settings it may be siphoning fuel through the primer, causing a richer mixture and a slight drop in rpm.

Either of these situations could give you the same condition as pulling on carb heat in flight, which you notice causes a loss of rpm.

John, I really don’t think you’ve got a serious problem here, but one that needs a little looking after. This kind of thing is probably more common on aging aircraft and I’m certain it can be corrected with a little commonsense troubleshooting.

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

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