Is this carbon buildup? And how can I fix this?

Jadaharabi said
Post 1, third sentence. It pops when he steps down on it.
To me that means a back fire thru the carb. That will build carbon deposits from the cylinder to the air cleaner faster than you can imagine.

Bingo! And all 4 that I mentioned can and will cause a backfire through the carb doing exactly what you said, build carbon deposits.