Melted Piston?

[jski]

I have a 1987 CR-500 that has been set-up for hill climbing that recently lost compression and upon tearing it down I found the exhaust side of the piston to be melted 1/4 inch back. The jetting was correct and the plug condition was a light brown although it had specs of piston after the meltdown. I was running a 50/50 mix of Sunoco and premium with Super M at 32:1 at the time. I have used Super M in my 00 CR-500 from new with no problems and when I talked to the previous owner of the 87 he informed me he was running Klotz concentrate race gas mixed with premium and 40:1 with Klotz oil. Could this have been caused by detonation because of the fuel I used or? Also, this cylinder has been ported for top end and has a thick sleeve and a 1mm bore with a Wiesco piston that had from what I was told 6 hours on it but when I looked at the condition of the piston and the wear marks on the skirt it seems to have more then 6 hours.

Thanks,
Jim

 

[EricGorr]

Sounds like an air leak, possibly at the left crank seal or the exhaust manifold.