115 Yamaha running rough

[caper]

Yes - fuel - carbs have idle circuits and high speed jets.

[sidelock]

Thanks for clarifying, when I see the word circuits my brain automatically thinks electricity.

[sidelock]

Put the boat in this afternoon and did Dunk's decarb procedure with a 16 oz. can of seafoam mixed with 3/4 gallon of fuel and 3 oz. of Yamalube. Issue still persists, checked prop and hub, all seems OK. I did notice that the rough idle only seems to occur when it's in gear and under load, not saying that it's not missing at all in neutral but if it is, its not obvious enough for me to notice. Seems like the vibration is gradually getting worse or more pronounced.

[76Red18]

Warm the motor up and run it for 15 seconds at the rpm that it misses at (5 seconds for a high rpm mis)then shut it off without moving the throttle. Pull all the spark plugs as quickly as possible noting what cylinder they belong to. Look at the electrode on each plug, noting coloration and wetness. 1 should be slightly different. If 1 plug is drier than the others, its a fuel issue on that cylinder. If 1 is wetter than the others, it's an ignition issue on that cylinder. A lighter color is a fuel issue, a darker color is ignition.

Plugs should be "read" as the first step in troubleshooting a misfire. The method above works most of the time. The motor should be under load, so it needs to be ran in the water or in a tank. Hopes this helps.

[Bushwacker]

Originally Posted by sidelock

. . . I did notice that the rough idle only seems to occur when it's in gear and under load, not saying that it's not missing at all in neutral but if it is, its not obvious enough for me to notice. Seems like the vibration is gradually getting worse or more pronounced.

That's because it takes more fuel/power to spin the prop in gear than it does to just overcome friction in the system in neutral, so if one cylinder is lean, it will be more obvious when it's under load. Red's idea of reading the plugs MIGHT reveal the suspect cylinder if you do it under load. But to fix it you're gonna have to disassemble the offending carb, soak it overnight in some strong carb cleaner, and then blow out all the passages with some compressed air. Snake oil ain't gonna fix it!

[sidelock]

Oops, double post!

[sidelock]

I mentioned in my original post that the technition at Bass Pro pulled the carbs disassembled and cleaned them last fall. Having said that while visiting Harkers IslandI last fall I brought up the issue with a Yamaha dealer while buying Yamalube and he stated that the holes in the small jets are so tiny, sometimes cleaning them just won't cut it and they may need replacing.

[Bushwacker]

You may be able to unplug them with small wires. I once had a car with a 2 barrel Holley, and the idle air bleeds on top of the carb were so small that about every 6 weeks I had to spray the top of the carb with Gumout to get it to idle smoothly! Finally changed manifold and installed a 4 barrel Carter Thermoquad and eliminated the problem while also improving gas mileage and power! The racers love Holleys but I've found them to be pretty temperamental for daily street use.

[bumpdraft]

I've been watching thread with interest to see what could be found wrong. I guess I missed the part that you said it was worked on last fall, we are coming up on this fall now(might need to go over everything that was done last year again). I got so disgusted with my 2001 Yamaha 225 with several different mechanics unable to find problem, I traded it in.

[Fr. Frank]

Here's my 2 cents:
For $27 each (4 total required)

Change the idle speed jets.
Change the idle speed jets.