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15MSH 2 stroke runs rough at light load

E

E4ODnut

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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?
 
I'm not sue how else to describe it. If it is running rough at all I suppose one or both cylinders must be misfiring at some point or other. Could be ignition, fuel, or something else but whatever it is the problem is not there below or above this speed and load range. What I'd really like to know is if this model engine is prone to rough running at this speed and load. If it is, I'll just live with it. If it's not I'll have to find out what the problem is and fix it.
 
I would not think that the symptoms you describe would extend to the entire line of outboards. To follow up on racerone's post, perhaps you could use a timing light on each cylinder, run up to the specified RPMs in question and determine if you have current going to the plug. If the timing light indicates that you have steady strong current at that RPM, then do the same on the other cylinder(s). Presumably you have either changed or at least examined the plugs. Sometimes running the outboard at night can show electrical issues (wires shorting out) that are not visible in the daylight. If all this checks out then you probably have another problem. Sometimes fuel issues can cause the symptoms that you describe.

Mr Spock said " Eliminate potential sources of error systematically 1 at a time. Whatever you are left with, no matter how improbable, is your answer." I would add- "start with the easy stuff."
 
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I would listen to racerone. do a very simple test with a timing light. You don't need to check the timing, just check to see if the light stays flashing constantly when the engine starts to run rough. If it does on both cylinders then you have either a spark plug issue or fuel delivery issue. It cuts the problem in half. Yamaha are about the smoothest running motor at any RPM out there, especially the 2 stroke engines. You may also want to do a compression test to make sure all cylinders are in good operating condition. Simple and quick test like these can get you right to the issue quickly.
 
Thanks Bubba.
That was the answer I was looking for. That tells me that I have a problem particular to my engine, not the breed or model. I was surprised to see this problem, especially on a relatively new low hour engine. I didn't make a study of it, but I was under the impression that Yamahas in general were about as smooth running as they get. The troubleshooting suggestions so far are good and I agree with them.
First chance I get to plunk the boat in the water I'll start with checking for dependable spark and go on from there.
Thanks to all.
 
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