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It's impossible to "overlean" the engine on the ground. Lycoming has long recommended aggressive leaning for all operations below 75% power. That shouldn't be a problem for most ground ops. Unless you're on long, wet grass... going uphill... against the wind... over the chocks you forgot to move... Leaning during ground ops can be very benificial as you've already been told. Besided avoiding the strange noises, spark plug fouling can be greatly mitigated by aggressive leaning for ground operations.
The technique I've used with great success for many years (flying a span can) is thus: 1. As soon as the engine idles smoothly following start set 1200 RPM, then slowly lean until you start to lose RPM (1200 RPM should be enough to get you rolling on most level surfaces). What you're shooting for is a mixture setting that won't allow you to increase RPM above 1200 RPM. I don't know if there's a technical term for this, but I call it "ground lean."
2. Limit engine operation below 1000 RPM. 1000 RPM should be more than adequate to keep you rolling on most level surfaces. Lycoming says that below 1000 RPM the spark isn't hot enough to activate the anti-leading agent in the fuel.
3. You'll need to enrichen for the runup (shouldn't get more than 1200 RPM, remember?). After the runup, follow the same procedure as before to attain "ground lean."
3. Following landing, as soon as clear of the runway, again set to "ground lean."
4. When parked, set the RPM to 1800 (you'll need to enrichen the mixture a bit to get 1800) for 15 - 20 seconds, then smoothly back to 1200, then mixture cutoff. 1800 RPM should help clear any lead build-ups and the quick shutdown will ensure you put anything else there before the engine stops. The throttle is alread set at 1200 RPM for your next engine start (assuming you don't move it in the mean time).
Of course, good judgement should always prevail over procedure in the face of unusual circumstance. Should you fid a need for more RPM during taxi, simply enrichen the mixture. Operation below 1000 RPM is fine if you're trying not to hit something. BTW, it's impossible to harm your engine by failing to set the mixture before takeoff. If you've properly set the mixture prior to taxi, you won't be able to get more than 1200 RPM--you won't even get thru the runup without resetting the mixture.
As always, YMMV...
Good luck,
Mark Sletten
Legacy FG N828LM
web.hometel.com/~legacyfgkit
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