1989 9.9hp Mercury loads up at low RPM

I have a 1989 - 9.9hp 2 cycle outboard. I do not know the history. The engine starts ok, runs great at WOT but has real problems at low RPM. At low RPM the unit runs rough and stalls frequently. If I increase the idle RPM it runs better.

If I change the plugs it runs better for 2-3 hours and then the plugs load up. They are black and "wet".

I have had the carb rebuilt, but no change in operation. The low speed idle adjustment has little effect on the "rough idle". The front panel idle RPM adjustment has little if any effect on the idle speed. The mechanics of the unit seem to be okay.

I have been fighting this problem for over a year. I took the engine to a repair shop but they just changed the plugs and charged me $100.00 and said the unit was running properly.

I am open for suggestions.

Re: 1989 9.9hp Mercury loads up at low RPM

Might you be running a longshaft motor on a shortshaft boat? That could do it. It could also have carbon build-up in the exhaust making it breathe hard. If you happen to troll a lot with it, I'd suspect that. If you don't, then I wouldn't. If yours has the ability to install a thermostat, try that. The 9.9s and 15 of that era use the same block I believe. Most 15s came with a thermostat. Most 9.9s didn't.