So recently I wanted to get back on my gy6 project, so last year I installed a big bore kit 58.5 set timing everything finished it fired up and drove it around all night. Went out the next morning wouldn’t crank, cranks over alittle then stops. Figured out if I take out the spark plug cranks fine once I put in the spark plug it seems like it has to much compression. Starter new, battery’s new grounds are all good. I thought maybe timing is off so I redid timing today and adjusted the valve lash and still the same? Wondering if anyone has any ideals what it could be and also took it all apart again today and the piston rings seem fine and so does the cylinder.
Most likely overlooked cleaning and oiling the cylinder bore and piston and rings immediately before assembly. Causes exactly what you're describing and is overlooked on many first-time builds.
Feels like the starter doesn't have enough power to get past compression, but it's really the added resistance from a layer of funk from the factory.