So this one is killing me! My 05 kx250 is all stock. It starts making this noise about half way through the motos. It is a rattly noise, sounds like its coming from the left side engine case. Plugs look good. I run 50/50 trick race fuel and good synthetic oil. I thought it was my chain guides (which were worn into the swingarm) I changed them, noise is still there. I went fatter on the main (from 160 to 162) but have the needle in the lowest position (clip at the top). One more thing, I have broken 2 return springs on the power valve assembly. I'm on the 3rd one, and would bet it is now broken too. Maybe the power valve is rattling? It never makes the noise at idle or even medium speeds. I have to be pinning it for a few laps, and then as I rev out the gears, in 3rd and 4th, it makes this godawful sound. It is worse than pinging, but could be pinging I guess. I replaced the top end 5 days ago. The old piston looked so good I feld dumb for replacing it. I think it's either pinging, the chain rattling somehow, or the powervalve. Anyone have a clue?
