A couple years ago I was running my son's 86CC 4-stroke down the road, WOT for a good while. Something suddenly let go and it lost about 30% power, and you could audibly hear a change in the exhaust tone, if I remember right, it got a little louder. I don't remember if it was trying to stall when I let off the throttle, but I think it was being difficult. I was easily able to ride it the rest of the way home and parked it. I noticed a couple drops of oil on the floor, may or may not be related. I forgot about it for a couple years, recently decided to pull it apart and see what happened. The only thing I found was that the intake valve was leaking air when blowing air into the spark plug hole. I adjusted the valve clearance on both valves and that seemed to fix the leaking valve. I disassembled down to the piston, which looked fine, and no slop was found. I put it all back together, waiting on a new carb to try and start it, but I'm sitting here questioning what could have happened and why I found nothing major wrong. Any ideas?

 

May be a carb issue or, if exhaust did get louder, a baffle turned, partly blocking the exhaust.

 

Turned out to be a valve issue. There was no clearance, works fine after adjusting them.

 

They usually get very bad to start when that happens, had one last week with both inlet and exhaust valves closed up.

 

I still wonder what happened with mine that it did it at WOT, how would the valves smash down and remove the valve clearance like a switch? I was running WOT for quite a while.