Originally Posted by Dirtdobber

Got a question. Ran my FMS again today with the 8mm venturi. I started it up with a slightly richened needle from last run. The engine would not clear out and would backfire at wot. I leaned high speed about two hours and engine started running great. After about half tank of fuel I richened the low speed needle about one hour and did not change the high speed needle. My engine again started sounding loaded up and backfiring. I closed the low speed needle back and again engine started running good. Would this be caused by a bad glow plug? Also, is there a rule of thumb on which glow plug to use? I'm in Alabama where the temps are 90-100 and the humidity is always 50% or more. Maybe suggestions going from day time to evening. I use 25% nitro w/ .7mm shimming. Have been using 97t or 77t odonnell plug. Thanks