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.