I was wondering could anyone explain the way the carbon is building-up on the piston and button in the Murnan 35+21 Rex I'm running? And if it's normal?

I'm having a problem with the engine not staying in tune. It can be tuned to run brilliantly, but then will on occasion begin to go rich or lean within the space of a few laps or 5 minutes. Sometimes after letting the engine cool down, and restarting it, (and it being brought back up to normal temperature etc..) it will have gone very rich without anything being different.

On the exhaust port side, there is a lot of carbon which makes sense, but on the opposite side the piston and button get gradually cleaner to the point where there is no carbon build-up at all. It seems strange that there's such a difference.

I've attached photos, but if they're not clear enough let me know and I'll upload new ones. They were taken following 1 days worth of running since the piston and button had been cleaned.

Thanks!