Had a conversation with a domestic engine manufacturer some time back where he was describing an engine and customer problem to me. Seems he sent this engine out new and shortly thereafter received calls about how the engine would not perform correctly. So the engine came back and he checked it out. A little needle turning and all was well so he sent it back.

A week later the calls about poor performance start again, so it gets sent back. Once again a little needle turning and all is as it should be, so back to the user. This goes on two more times. Turns out the problem resided with the "gas engine expert" at the field the engine was being used at. The "expert" was, surprise, a gas engine newbie. Every time he put a screw driver to the carb things turned to crap. Owner of engine company finally sent note back with the engine prohibiting the "expert" from getting within a given distance of the engine with a screw driver in his hand.