Even a hairline crack in the casting that is invisible until the engine is warm has been known to happen. Any random loss of air pressure within the crankcase will cause inconsistent running and make a guy think he doesn't know how to set a needle. Another cause of inconsistent running is bolting the engine to mounts that aren't absolutely flat and parallel, causing the case to distort. I like to set the high speed with the nose of the model pointing straight up and just momentarily pinch the fuel line to make sure that there is some reserve fuel in the mixture. Learning a couple of simple ways to diagnose a problem quickly is how the pros at the lawnmower shops feed their families and what they know applies to our hobby 100%.