Upgrade from 49cc to 72cc x Issues with Pistion and Cylinder Walls

[pdwald]

Recently I ordered an upgrade in order to upgrade my bike from 49cc to 72cc; it came with cylinder bore, piston, etc. I installed it properly (pegs on the grooves of the piston perfectly aligned with gaps of rings, etc). At the very first begininng it was running great and in order to perform a stress test I left it idling for a long time and it seemed to work flawlessly. Once I went with it for a ride it was working great at the begining, but after a little while and at higher rpms (like 30mph) I heard what seemed to be soft seizures: a rash brittle metallic sound coming from the engine and engine coming to a halt.

I really can not be 100% sure of it, but for me it seems very unlikely to be something else besides the geometry of the cylinder walls or the piston. The carb is more on the rich side (not lean; clip is almost all the way down), and even if it is not an ideal mix (since I still have to rejet the carb) it could not have caused such dramatic effect in such very short space of time. Lubrication could not be an issue, as I indicated before, since I using oil premixed with gas (40:1). The soft seizures come when I am riding it at high rpms (over 30mph) and the engine tend to come to a halt; and once it comes to a halt it is much more difficult to start again and it tend to seize and halt easily again and again (not necessarily at high rpms from that point on). I could only imagine that thermal expansion coefficient for the piston/cylinder caused the problem, especially considering that the problem was very localized (only the upper portion of the piston and upper portion of the cylinder walls were full of scratches). It is very rare indeed to have new cylinder walls with geometry issues (I would like to to have a manufacturer to point my fingers to…), but it seems to be brandless. So I contacted the seller and asked for the measurements of the cylinder and the measurement of the piston, so that I can check them out with measurement tools. Again, it can not be a lean condition; I know people running with stock carb and jets for months with no apparent problem; and my bike is more on rich side (and was running on the rich side for months with its previous 49cc bore). I really don't know what else to do. Could it be somewhat related to bearing or the crank?

Thanks for any help,
pdwald