What aircraft is it?
Maule MX7 180a with lycoming 0-360 with 125 hours since overhaul and complete aircraft rebuild with new mags etc, annua 4 lmonths ago.
Perhaps i did not give it enough opportunity to burn off any fouling on the plug, the CHT gauge was not too far in the green when i did the 2000rpm lean out for 30 seconds, tomorrow i will go and let it warm right up, run it for 10 15 minutes then do a high rpm lean out for 1 or 2 minutes