01 CR125 cylinder

Hi all, this past weekend during my sons moto the bike stalled mid air and would not refire and had zero compression. I was bummed because I had just completed a top and bottom end rebuild but oh well what can you do. I took the bike apart today and during dissasembly I noticed one of the expansion chamber springs was missing. So immediately I figured it sucked air from the exhaust port and burnt the piston. After I got the head off I saw the piston was in fact burnt and melted away down to the ring right in front of the exhaust port.

My question is this, right at the top of the stroke on the cylinder wall there is a spot where the metal from the piston melted and fused to the cylinder wall. It was a cast OEM piston, how do I get that stuff off the cylender wall. Thanks a ton for any replys, Kris :thumbsup:

would a missing exhaust spring really cause this type of malfunction? i only run 1 on my bike, same year and everything

it's the only thing that makes sense. My son jumps pretty high and far and I could see that the pipe allows air in if it gets cocked one way breaking the seal.

i dont think the leaky exhaust would cause the melt down , iv had various 2 strokes with pipes hanging off in worst cases i rode my old cr 125 1984 model 30 mins home 1 day with the pipe cracked all the way round and hanging off...just more noise

was the stator installed correctly when re-built & aligned with the marks on the casing, this could of changed the timing, making it run a lot hotter, causing a seizure

must be something similar


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