that is not normal that means you are too lean that baked on stuff is carbon deposits from caster oil What this means is that your motor had a lot of residual fuel sitting on the piston when the motor was still hot.
Use a toothbrush and dishsoap (the cream one).
If you can clean the black deposit on top of piston troughly (without using metal polish)... then the engine is still in health condition. Just wrong setting, use colder plug and or better fuel.
The (not-perfectly) burnt fuel/oil is creating that deposit.

If.. the carbon still there and the top surface of the piston is like being hammering by thousand needle.. creating a rough surface, then you have a problem.
That is early stage of pre-detonation, your combustion chamber is exploding far before TDC .Could be the plug is to hot / too lean setting . If you keep this setting, result is obvious.. broken con rod or broken crankshaft.