The YX140 engine has tons of compression and requires that you nudge it past tdc prior to kicking it over, to get a good startup. If this rule is not adhered to then it often kicks back, giving a nasty blow to the sole of the foot. I was suprised at the amount of compression this little engine has.
Yes the parts seem quite flimsy and obviously cheaply made, but this engine lasted a good year of me thrashing it on main roads, sometimes doing a 45 mile daily round trip. I think this is OK despite the engine being designed for kids pit bikes, the short bursts of off roading every now and then.
Eventually the flywheel snapped right off at the crank. Its as if the power output is too much for the diameter of the crank with that flywheel on there.
Anyway, I replaced the bottom end and see how long this one lasts.
For anyone interested, the pit bike engines fit into all the cub frames, EXCEPT THE 6V C90. That bike requires some modification to fit the engine. And if doing the swap, I highly recommend addressing the exhaust situation properly, the standard cub exhaust is too small for these engines.
And another thing, I have no idea how they get such cheap, soft, plasticine screws to be done up so bludy tight! The force needed to unscrew these screws exceeds the strength of the metal, resulting in the screw turning to mush. I had to tap most out with a hammer and punch, then drill out the last one.
You want advice on how to fit a pit bike engine to a Honda cub follow this link.