4 stroke outboard low and mid range intermittent misfire

[JimW]

Took the boat out yesterday and had a minor issue. The boat ran great at high speed. On a plane at over 3500 RPMs was as good as it ever ran. 4900RPMs gave us 39 mph on the GPS on a C-19 with the bimini top up, without even trimming the motor out for max speed. It's a Yamaha 115 4 stroke, 2003. At idle and 1000-2000 RPMs had an annoying intermittent misfire. Over the years when I've has old fuel issues It would idle fine but be totally fuel starved when throttle was increased. So This is not old gas, clogged filters, it runs awesome at high throttle settings. I bought a new set of plugs to try. They don't have many hours on them but many years. Any thoughts? 2003 motor has 500 hours on it. Do plugs get bad first at low RPMs as they start to wear out?? Thanks in advance guys.

[wadestep]

Plugs would also be my first guess. Then maybe injectors. Does the motor have many recent hours on it? May just need to have plugs changed and run...
wade

[JimW]

Bingo. Our first guess "appears" to be it. Changed the plugs, #1 was "black" and others were "brown sugar". I will test later but the visual evidence of #1 slightly fouled enough to cause intermitent low speed misfires was compelling. Thanks Wadestep.

[Aripeka Angler]

Jim, single fouled spark plug in a four stroke could be a symptom of something more serious than simply a bad plug.
It might be a bad coil or a fuel mixture issue with that cylinder. I would keep a close watch on it for awhile. A compression test might be in order as well...

[Cracker Larry]

I agree with Richard. A fouled plug is probably caused by something else. Injectors, fuel pump, bad compression..I think that engine has a high and low pressure fuel pumps.

[Steven]

I noticed some mid-range roughness after the engine hadn't run for a while during the Console rebuild and T-Top install. I hadn't been using fuel conditioner as the boat was getting run regularly. I used Yamaha Ring Free additive and the problem went away after a few runs. I add both the Yamaha and Stabil Marine whenever I get gas.

[peter-curacao]

E Te... nahhh!! don't bother my comment