A few guys here in Edmonton, Alberta bought S421B engines for our buggies but are having a bitch of a time keeping the motors running. They seem fine for the first 5 minutes and then flame out at idle. We are running 30% race blend from Sidewinder (10% oil) but it doesn't really seem to matter what percentage of nitro is used...motor does the same thing. Had a couple of guys playing with head shims...bumping up 1mm-2mm for the high nitro content but nothing seems to work.

Is this motor a dud or does someone have some tuning magic for this mill?
Thanks,
John Hughes