This motor has about 150 hours and runs good at idle and WOT. But when running at slower rpms (1500 to 1900) the engine feels like it's got a fuel problem, maybe too lean. I tried Sea Foam but no dice, and tried adjusting the mixture (which is now at 1 1/8 turns out) to no avail. Running clean gas with QuickSilver and new NGK plugs - BUHW. The boat was originally rigged in Ft. Collins, CO. (elev about 5000 ft) and that's about the altitude I'm running at. Don't know about the carbs if set for altitude or not. Is this a characteristic of the engine or where's the best place to start trouble shooting?