First of all, all ground ops should be done at a barely running mixture setting, get it running and lean until the RPM peaks and starts to fall again, stop there. You run the risk of leaning just a bit and then forgetting to go back to FR for takeoff. If you lean to a RPM rise and fall, you will not be able to takeoff without knowing.....it will try to quit rather than run when you advance the throttle.
Do your mag check using the leanest possible mixture, as described above, then forgetting the RPM drop as it may well be more than 200, lean mixtures or LOP mixtures will do that. What you are looking for is for all the EGT's to rise, none to fall. If one falls then that is the faulty plug.
The most diagnostic mag checks are done in order of diagnostic value;
1. High power LOP (typically at just prior to TOD)
2. High power ROP
3. Low power on the ground LOP
4. Low power ROP - almost worthless, except when bad ground leaning practises have fouled a plug, or post maintenance.
I generally do a quick one while taxiing at a very lean mixture just to prove the plugs are all clear, but it does not find faulty plugs. In fact many a plug that is faulty or becoming so will fail a high power LOP mag check, but pass the workshop plug tester. I have seen this many times.
Next is the magneto and harness.....these faults can often be found at high power and with temperature effect, and never be found on the ground. So sure, do a quick one, but do all ground ops leaned right out, and start doing a TOD mag check which is where it is most diagnotically beneficial, and you can get the bug fixed before next flight rather than discover it at the runup bay. This is the SOP from when airlines ran piston engines, and they did it for a very good reason.
My mixture lever never hits full rich ever except for the takeoff roll or during the go around. (OK..except when I prime on a cold start) Not even during the actual start.....it gets about half way up. From 1000' AGL I start leaning, and it never gets to the FR part of the quadrant again until the next takeoff or prime.
Hope that helps.
PS....What Walt said!
And note my investment advice below, you have possibly just spent the entire cost of an APS class on this one incident.
