Bought a used 1988 YZ 250. The top end rattled bad. Never rode it before I riped it apart. Rebuilt the top end and replaced the power valve. It was bored .06" over and cleared the power valve for the bigger piston by a Yamaha dealer. I can't seem to get the power strong and clean. It seem to be very rich. It sounds and runs like the choke is on, but it not. It has a PC pipe. I started with the PC jetting which is also the stock jetting. 45 pilot jet, 370 main jet, clip in 3rd slot. This is like driving with a choke on. I'm now down to the next leaner needle jet (r-2) 45 or 40 pilot jet, clip in the 1st slot and a 300 main jet. It's been running better the leaner I go. It did ping a little when I closed the thottle at 1/2. I have the float set right and a new needle and seat and choke. It's not the plug either. Running one hotter plug. 40-1 HP-2 oil 93 octane. Does boring a 250 .06" over effect the jetting alot? It aways starts on the second kick. The bike also has alot of compression. Can't kick it with sneakers(ouch!). Should the head have been recut? Could this be the problem? Had the carb off alot and can't find a problem. The reeds are with in Yamaha specs.