Ok, long story short. The 02 YZ125 i recently purchased in a box has some issues. I put it back together and everything seemed fine - until i ran it. Has no bottom end response and erratic idle. It was initially running very lean with a 430 main. Went through the engine (air leaks) and carb with a fine tooth comb - no fault found. I did have to go up to a 460 main jet to get the plug a nice brown color. Jetting didn't help with response/power. The conclusion that i came to is that i have a mis-matched cylinder and/or piston (port timing) to the bottom end. I did some pictoral research online and i now believe i have a 98-01 bottom end style based on the intake manifold design (also takes a larger diameter wrist pin bearing). I would have to assume my cylinder and head are factory 2002. I have confirmed that i have a 03-04 piston (is a smooth round piston without charge ports on the sides). Does anyone know which combination of cylinder/piston can be ran on this? OR better yet is there any way of positively identifying the year of the cylinder/head?
