I have a small aluminum bass boat with a 40 HP Mercury (1999 model motor). The motor runs great over about 2500 RPM, but when running at low RPM while trailering, idling, etc., it runs very rough and frequently stalls. I'm a newbie with outboards (had the boat for a year), but it almost seems like a carb or idling adjustment to me (total guess on my part). Are there common causes for this? Plugs were replaced and carb "rebuilt" (carb kit installed?) last spring right before I bought the boat.

Sounds like old fuel.

Maybe. I probably haven't burned through a whole tank of gas this spring. But maybe I will take the boat out on the lake and make sure I burn a whole fresh tank. I fish smaller lakes (Gibbons Creek, Bastrop), so it's definitely true that the engine doesn't run much on most of my trips.

Any other common issues I should investigate?

I think Gary was right. Burned through two tanks of gas on Caddo and now the motor has never run better.

Not only is it old fuel, but the old fuel has varnished you carb over the winter. Will it run better at low rpm with the choke on? You're going to need another carb rebuild. Be sure to treat your fuel before you put it away for the winter.

Cheers,

George

It did run better at low rpm with the choke on, but after running a tank of fresh gas the engine runs smooth at all rpm's. Hopefully there wasn't significant varnish buildup. I will definitely be more careful in the future to avoid this problem. I do use Stabil on a regular basis.

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