RE: 50 sx
open up the carb to .330, raise the exhaust by .010 and put a angle on the exhaust port pointing at the plug, widen the exhaust at the top of the exhaust port by .020, radius the case under the carb and square off the crank under the carb, and the hardest part i to lower the sleeve in the case by .015 to lower the transfer ports. You want to get around .120 blowdown. Should run a 10x7APC at 16,000-16,500 with a Jett red muffler.