I had to take the diesel gen apart yesterday, to try and figure out how bad the carbon build up was, and how to fix it. It was bad. This is the 'head' the top of the gen where the piston hits:
This is the top of the piston, with a copper gasket around it.
This is the piston head after I spent time cleaning it off. Here's hoping I didn't screw something up with my cleaning efforts: