I'm pretty new to manual mixture control, so forgive me if this is a rather dumb question.

I've noticed that at low altitude, having a rich mixture and an idling engine causes the engine to sputter, sounding similar to when the engines start to fail after a mechanical fault. This does not appear to occur above around 500-800 feet ASL. Is this damaging the engine?

Should I be leaning the mixture out at low altitude? I was under the impression that it's more common to lean it out at higher altitudes to keep the RPM up, but the engine certainly sounds displeased!

At low altitudes I always have the mixture fully rich, and as you said above I just lean it to get better revs at higher altitudes. I don't believe the engine will be damaged. Others will hopefully chime in with more thoughts.

@OP, might try leaning to about 85-90% even at ground level, if you find that it minimizes occasional belching from rotaries at idle throttle. An alternate solution is to keep the throttle at least at 30-40% on rotaries, while on the ground, and mixture at full rich -- but use the blip switch at such a setting (more historical that way too since most rotaries would conk out below about 40% or so throttle). For inline engines, if you experience the same problem while on the ground, again either lean mixture to about 85-90%, or, more simply, keep throttle at no less than about 10% on the in-lines.

Cheers & happy mixture/throttle tweaking,

For some reason, inline engines still splutter for me even if I lean the mixture at low altitude, but I have found about 15-20% throttle seems to get it to stop while often not quite being enough to start moving the aircraft. Thanks for your help!