Bought a used bike with a 48CC engine and I have been having the same problem as the original poster. The old plug was as black as coal and I have also noticed unburned fuel at the muffler and the muffler appears to be modified (thinking about ordering a new one). Running a mixture of 32 to 1.
I have ordered a new plug, I'm sure there are no air leaks, I've adjusted the float, and noticed that the cover on the ignition coil is split with the wire exposed (waiting for a new coil) and a new motor mount for the back of the motor (currently missing/also on order). Unable to test the changes until the new parts arrive.
Can anyone advise what I should set the plug gap at? Anything else I should look at regarding the problem?