no compression in mx80

[Mark4883]

i just bought a cheap yamaha MX80 from a friend to mess around on. The thing is mid 1970s. It was running fine until i hit a jump and came down and the carb stuck going wide open. I ended up bailing off of it, and it was running full throttle for around 20 seconds. It doesnt run anymore. Its getting gas, has good spark, but has barely any compression whatsoever. Can anybody tell me what the hell happened?

[Kane]

Likely cooked the top-end. Unfortunately, those old bikes would maintain compression even with rings as thin as wire due to the very thick height of them. It could be that they got so thin they lost their tension from high heat or you could have broken/folded one. You'll only find out after pulling the cylinder off.

[Banshee]

Do those old bikes have reeds? Might have broken the reeds. But I think Kane is right the motor probably had a melt down