Hello KX Riders,
Just picked up a 2004 KX125 for my son who outgrew his TTR125. The bike is in relatively decent shape and has fresh top end but something isn't right about the power delivery.
At low RPMs the bike is just dog tired and sluggish, almost no power at low RPMs at all, to the point where you have to really rev it up just to get it rolling.
At high RPMs this thing just about rips the grips out of your hands. It will gain RPM slowly and consistently and then BAM, loads of power. It almost feels like a nitrous hit.
My theory here is that the power valves might be stuck in the open position. I have the carb tweaked about as best I can get it with a 410 main and 37 pilot, 2 turns out but until I track down this other issue, I can't tune it any further.

Has anyone ever seen this happen or am I missing something here?
Thank you

 

Turns out the previous owner installed the piston backwards. Fun fact, the 2004 KX125 has a port in the piston skirt which should face the intake side but will run (Like crap) if pointed at the exhaust side.