Any updates? Saw a thread you started that you managed to get the tune right on your engine.. A need a little help because I may have been experiencing the same issue on an O.S. 12TZ 5 port with an RP6 or P6 plug, with NO carb reducer/restrictor. Tried tuning with Cosmo 20% and Tornado 25% with both shims on. No air leaks since I've sealed the carb and the crank.