When I would run my engine on the gasoline program, I'd get carbon buildup on the back of the exhaust and intake valve. I pretty much stopped burning gasoline altogether. I still use it to flush the fuel system, but I avoid running the engine on it. When I disassemble the engine after running only alcohol, everything is clean. The exhaust valve will have a very thin coating on it. Probably from the little bit of oil that gets past the rings. No more chunky carbon buildup.