Modern Vespa

Opened up a pinasco 177 last night to find 4 nice evenly spaced scores on the piston from top to bottom...rings intact....barrel has minor marks hich will hone out

On installation I was careful on ring gap. went through 5 heat cycles prior to first ride. ie started for 2 mins..stop.cool down and repeat times 5. Jetting was way over rich on purpose(Im at work so not sure of numbers)..I upped the main, the throttle slide and the atomiser.....careful running in......soft seized 20 minutes into first break in ride...saved by clutch

Im suspecting poor piston tolerance....I did nt check it (doh)and bolted it on out of the box after chamfering the port edges 1mm.

plug was black....no marks on top of piston (so timing is good at 18)

what else can cause this type of sieze?(no pics)
shaneo

overheated piston. Bogging it too slow, going up a long hill and running it in too high of gear, timing not right, jetting too lean ( but you said you were running rich, so this one is out).

Lack of oil

Running it hard before it's fully warmed up will also cause 4 point seize. It's the result of more heat sink by the cylinder studs causing it to warm up and expand a little more slowly right there.

I had it pretty bad on my cylinder when I had a highway run 1/2 mile into my commute. I'd leave the house and be WOT after two minutes.

Heat is the cause.

How do you know it was running rich? was it bogging at full throttle? what throttle position were you running at when it seized?