MOPED ARMY

Did you upjet?

I bet you do.

Anyone have a legit answer to this problem?

Could be rings snagging the ports, did you camfer?

Is the piston hitting the head? Take it off and look. You need to measure and set the squish. You do this by making a piece of solder that reaches across the top of the piston and abuts the cyl wall. You do this in 2 places opposite the wrist pin. You hand turn the the piston up crushing the solder. Measure the solder. I don't know what your engine should be but I bet it needs to be 15 thou or so. When the engine is running the piston goes higher due to bearing clearance, rod stretch and lengthening of the piston because it's aluminum and hotter than the cly. You need clearance for all these things so the piston never touches the head. A proper squish band is a real horse power maker. More complex and not an answer for this question

The looser a piston fits, the more the piston rocks in the stroke. The more it rocks the less well the ring fits flat to the cylinder wall. As it rocks, it needs to be not-round to contact everywhere. So a rocking piston reduces compression.

Youre absolutely right. Took everything apart and found the piston rings were degraded at the point where they join, and the piston at this point was deeply pitted almost as if the metal had vaporized, also leaving deep scratches inside the cylinder wall. This is probably why I could never rev up and at high revs had no power and what actually sounded like a combination between pinging and sparking sounds. When I initially installed the 70cc kit I broke the piston ring and use the ring from another similarly sized piston, and this is what you get when you take these shortcuts lol appreciate your responses!

When you took it apart were the head nuts loose?

Yes we all live for carnage.

But sorry for your loss. (edited)

dang, even got hot enough to melt the intake pin side,ohhh

I know the post is old but was the pin gone or still in the piston? It's hard to tell with all the damage BUT since the ring is still pretty much where it should be it seems to have snagged a port like stated above by pat splat :) Dang

Is that a Ford Capri? They had them in the states?

Ok I hate to be back here again but trying to prevent similar issues lol I got a new kit chamfered the ring and rejetted and dialed in timing. Ended up needing a new clutch as well. the kit probably has 200 miles on it and ive been running it a bit rich the entire time while breaking in. It runs really well now, decent takeoff and great acceleration, hits 46 with 16/45 gearing...but i still hear this sparking sound that is hard to describe. it kinda sounds like if youve ever sparked two wires together but a bit lower pitched. It only happens when under heavy load like going up a large hill or at WOT, but its a way less severe, less frequent sound than before and doesnt come with any performance loss. I took everything apart today to look in the cylinder for markings, look at the piston and ring and they were all clean and clear, any idea what is going on? Is this just normal?

Did you gap your Piston rings?

Interesting, adding some octane booster has gotten rid of it, thanks. I already use high octane non ethanol stuff so maybe ill try a colder plug, right now using an NGK B7HS ill try a B8HS, really appreciate all the responses ive learned so much