Well, if you know the tuning is fine and you tested the fuel tank then it could be an internal thing but, that is really hard to diagnose. One time i had just an impossible time trying to get an OS .15 cva to start, checked everything 3 times and still no hope. If i got it to start it would just die when i add throttle. With experience with rc car engines i decided to take the motor apart and see if the sleeve was in correctly. Looking at the sleeve ended up the bypass port on the sleeve wasn't milled enough. It was to small. Contacted OS and they sent me a new sleeve and works wonders.

I do have a question with my saito. I cant seem to keep the exhuast to stay tight. The header keeps on backing out of the block. Anyone had that problem. I was thinking of some lock-tite but I dont think it would work with the temps these mufflers see.