I have a 2005 Trailblazer 250 that I purchased last year. Configuration at time of purchase: 0.020 Namura cast piston high stall clutch hot seat exhaust oil injection disconnected (running 32:1 pre-mix) .140 main jet (installed by repair shop to correct "low bottom end power" complaint) Drove for one season. At end of season, after a 2 hour ride, my son rode it hard for about 15 minutes at which time it stalled and wouldn't restart. Teardown inspection showed a melted piston. I've since bored it to .040 over and installed a new Wieseco forged piston. Crankcase air pressure leak tests were negative. I can't seem to find any information on what the factory jet size is. However, the previous owner did give me the jet that was in the carb prior to replacement (.150). Prior to and after repair, When cruising 20-25 MPH, slightly opening (tip-in) the throttle results in what sounds like pre-ignition (spark knock). I relly hope to avoid rebuilding the top end again, so any answers to the following questions and/or other suggestions would be greatly appreciated: Is the .150 (OE?) jet large enough for this configuration? Should I install larger jet and/or raise the main jet needle? Thanks!