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The engine you have is actually a Yamaha manufactured engine in Mercury cowlings, so either Yamaha or Mercury service people may be able to help you out.

I have some EFI 2 -stroke Mercs, and on those, the idle speed has to be set by timing, not an idle adjustment screw like the old carburator engines used, something I am not able to do. So you may have to have the engine serviced by people with the correct diagnostic equipment. It sounds llike you need a trained professional service person to take a look at it.