My 3 cylinder 1990 mercury 90HP runs fine at idle and WOT. It will run fine for a time at any mid range throttle position, but occasionally with a throttle position which is giving me 4000 RPM, the engine surges between 3000-4000 RPM. It will continue to do this as long as I am willing to allow it to continue. Changing the throttle position will stop the surging ie WOT or idle, but the problem will eventually return if I try to run at 4000 RPM. So far, it is an intermittent nuisance, but I am growing weary of it. This motor will run at WOT all day long without a hiccup, but I don't like to listen to the noise or burn all that gas. Since I purchased the boat 3 years ago, I have accomplished the following:
1. Overhauled all three carbs.
2. Changed the spark plugs.
3. Replaced the fuel filter.
4. Replaced all fuel lines including the priming bulb from the 20 gallon, well vented tank all the way to the carbs.
5. Ran several cans of sea foam through the engine.
6. Replaced the stator.
1. Overhauled all three carbs.
2. Changed the spark plugs.
3. Replaced the fuel filter.
4. Replaced all fuel lines including the priming bulb from the 20 gallon, well vented tank all the way to the carbs.
5. Ran several cans of sea foam through the engine.
6. Replaced the stator.