I have a 1991 KDX200 that has a new top end, pipe mod per Canadian Dave, boysen reeds, air box mods, FMF silencer/spark arrestor and it seems to want to run only at half to full throttle. I cannot get idle or the enriching circut to work. The air screw and idle screw have no effect on the motor and I am seeing bubbles in the coolant tank when I rev it. I have tried several jet/needle combos including 155 main, 48 pilot and the needle clip moved down 1 from center. In this setup it would run at 1/4 to full throttle although it seemed a little down on power overall. There don't seem to be any air leaks around the carb and the reeds are new. I have taken the pipe off and the power valve is in time and clean. I should mention here that the cylinder was sleeved and a stock piston fit by Wiseco last summer. I am starting to think that I have a cracked head or bad headgasket. Any ideas what else to look for as I tear this thing apart one moe time?