15MSH runs rough at light load

[Hocus-Pocus]

2010 15MSH 2 stroke. Bought used with low hours. Stock prop. Fresh 87 octane straight gasoline, 100:1 Yamalube. Starts first pull almost every time. Idles well in and out of gear. Idle speed in gear ~800 RPM. Runs smooth up to ~1100 RPM. Between ~1200 and ~1900 it's rough, almost like one cylinder is misfiring. From ~2000 it evens out again and is smooth up to full throttle at ~5200 RPM light boat. I see no mention of carb adjustments in the factory service manual. I didn't expect this roughness from a Yamaha, is it normal?

[sailingmagnus]

Same thing happening for me past few years

Hi,

My 2008 15MSH ran flawless for the first two years. The past 2-3 years I have been having similar rough running. What I discovered was crap in the carburetor bowl seemed to be intermittently plugging the jets. I started cleaning the bowl out several times a year to resolve the problem. I have a racor filter in line so the crap is not coming from the gas. Eventually I traced it down to ethanol in the gas causing moisture/corrosion in the carburetor bowl. You get white like crystals that form, aluminum oxide I suspect in the bowl. Running ethanol free gas has helped reduce the frequency but even then if I don't finish a tank of gas in a few months condensation probably forms so water comes back. I wish someone sold a carburetor bowl that was teflon or plastic coated that would protect the aluminum from oxidizing.