To me it looks alot like premature combustion.
If your piston is 49mm in diameter, your displacement is a little over 75cc.
Given that the original engine has a compression ratio of 11 and a displacement of 50cc, it is found that the combustion chamber is 5cc, but it might be a little bigger because of your piston design. Anyway, if we think that your piston has not changed the combustion chamber size, your compression ratio is now at 15, but since these engines are often only around 80% efficient at filling the combustion chamber at the best rpm, your final compression value is most likely 12 (compression ratio x pressure in cylinder at end of intake). It is usually recommended to use 100+ octane when having a final compression value of above 11, according to
Elsberg Tuning.
Premature combustion is when the highest pressure comes too early and is caused by the mixture either being ignited too early or that the mixture temperature is too high due to exessive compresion. Because of the way gases behave, the temperature rises even further when this happens and the problem becomes worse.
It is usually handled by having the right mixture and either decreasing the compression ratio or using higher octane fuel, both along with adjustments of the ignition timing, which is 28 degrees before TDC (above 4000 rpm) with the original ignition.
Another kind of controversal way of dealing with it is to cool the piston with an oil squirter, so that the temperature of the mixture wont raise too much.
But hey! It could also be as simple as some dirt getting into your engine through your air filter and wedging between the piston and cylinder, breaking the piston it seems and then the metal pieces has been thrown around in the combustion chamber. :p
Topic: Found the issue of no compression after high revving! Pics! + Question (Read 5556 times)