If the fins are dirty, it stops cylinder block and head cooling. That leads to warped aluminum blocks, bores out of round, valve seats popping out, valve guides coming loose and so on. It overheats the engine. These are air and oil cooled engines. No coolant to cool them. Should be cleaned yearly or more if you bag or side sling. When you scalp in the spring, tons of dirt and grass fly airborne. The flywheel sucks all that cloud into the fin area. Older engines leak oil which attracts dirt. Grass locks onto it like mad. You are not just cleaning surface dust which you should do as well.Thanks guys, I never cleaned the engine and now that you mention it, looks dirty and dusty.
? I always thought the guys cleaning the engine was for the look ... but now that you mention it, is does make sense to keep it clean.
I'll also try checking the gasket.