I have gone through 3 top end rebuilds in about 100 miles of riding. 1999 trailblazer 250 .20 over, ported, 35mm TMX carb 390 main jet with clip on 4th groove. K+N filter with outerware. Airbox removed. HSP pipe and clutch kit. All 3 times its the same thing, rings are stuck in groove and I only have about 80psi comp. 1st time had it bored 2nd and 3rd times the bore was fine. I even took off the shaved head and went back to stock. spark plug color looks good. If i go any bigger on the main jet it falls on its face so I think that I am not too lean.

 

What kind of oil are you using? And are you premixing or oil injection?

 

Get rid of that carb, your lean in needle throttle range, I've spent a great deal of time with a 38tmx on a modded 400. I had to reshape the needle to get enough fuel through the midrange throttle settings, 3 pistons later and a egt gauge I cured the piston melting by retapering the needle. It's PIA but if you've plenty of time and patience plus a hand full of needles it can be done but not recommended.

 

I am running klotz, oil injected. but I had problem while running premix with the injection when breaking in. As far as the carb, it is the one HSP sells on there site for the trailblazer so I would hope it would work with my setup.

 

Run caster 927 pre mix klots will stick the rings every time