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Old 13-09-2013, 10:06   #16
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Re: Yamaha 15 2-stroke Issue

I work on small engines and outboards all the time. I have a set of Jet Reams that allow me to clean out even the smallest 4 stroke jets (Clean...Not enlarge)

https://www.wolfgangint.com/store/pr...eams-t1129305/

Most, if not all of the newer 4 strokes have brass plugs over the idle air /mixture jet so one cannot adjust the settings. One can drill these plugs out for "cleaning purposes". If you do not know how or where it is you should not attempt this. Honda's use a loctite set cover on the adjustment screw that only comes off if you heat it. If you do not know how you will break it off.

The good folks in the EPA driving around in their belching blackout Chevy Suburban's do not want you to adjust the factor setting.
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Old 13-09-2013, 18:44   #17
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Re: Yamaha 15 2-stroke Issue

Cattails, This comes from a dealer, ME.

You said your props ok. How did you check it? As said before there is a rubber hub between the splined hub portion and the prop shell. The rubber can slip until it get’s warm and then grab. Borrow another boater’s prop that will fit.

As far as the plugs being black instead of tan – That’s a good thing, it’s a two stroke. Tan would indicate lean aka lack of fuel.

“engine does not run as smooth as usual” There is an RPM limiter built into the CDI that interrupts the ignition on that engine. Hub slip = to high RPM - limiter does it’s job by interrupting the ignition system causing it to miss = rough.
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Old 13-09-2013, 19:46   #18
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Re: Yamaha 15 2-stroke Issue

Your timing problem in original post is confusing.
Motor stopped, in fwd, and wide open throttle, you should line up the white plastic pointer with the second line on the flywheel while the 30 before mark lines up with the mark on the black plastic cover.
You say you pulled the flywheel, was the key intact?
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Old 14-09-2013, 17:46   #19
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Re: Yamaha 15 2-stroke Issue

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Originally Posted by colemj View Post
I don't think points have been used in outboards for many years.

Mark
Could be Mark. I have never pulled one of mine apart. I know the so called electronic ignitions used on small engines use SCRs for switches. Not close enough to a circuit diagram to predict if SCRs exhibit the same problems as point with respect to positioning the flywheel magnets to the SCR firing controls.
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Old 14-09-2013, 18:03   #20
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Re: Yamaha 15 2-stroke Issue

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Could possibly be bad seal around crank, allowing too much air in.
I was thinking the same thing or maybe reed valves.
A propane test would verify a vacuum leak.
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Old 14-09-2013, 18:09   #21
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Re: Yamaha 15 2-stroke Issue

Reed valves worn and not closing properly until temp allows is also a point to be explored
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Old 14-09-2013, 18:29   #22
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Re: Yamaha 15 2-stroke Issue

The OP stated the spark plugs were dark.
Bad lower crank seal = lower plug color lighter tan than upper plug.
Upper crank seal = oily mess below the fly wheel
Bad reeds = light colored plugs - lean - again a light colored plug
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Old 14-09-2013, 20:48   #23
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Re: Yamaha 15 2-stroke Issue

I try and think simple with these problems and eliminate the obvious first... You said you cleaned the fuel line, but did you try and use another one?

I hade an outboard that had a small hole near the hose clamp at the bulb. It would suck air intermittently and cause similar problems you are experiencing. I replaced the hose and everything was fine.

Also when you cleaned the gas can, did you find excessive residue, if so go back to the carburetor and completely rebuilt it including the jets, as suggested.
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Old 27-11-2013, 14:35   #24
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Re: Yamaha 15 2-stroke Issue

After months of trying everything I could think of, we finally made some progress on the engine. I knew it was running rich based on the fact that choking the engine at all would stall it out and I sprayed starter fluid in the carb while running and engine stalled out quickly. The spark plugs were normally black or wet and all the signs pointed toward a mixture that was too rich. I had cleaned all the jets, "boiled the carb" in strong cleaner, sprayed all the passageways with carb cleaner etc. The carb was "clean as a whistle."

My friend helped me lean out the mixture by clamping a vice-grip on the carb fuel intake and gradually tightened the thumb scew while we were at high RPM. As we progressively clamped the fuel line tighter we found the "sweet spot" where the engine smoothed out and RPM
increased and we ran at this higher speed and engine sounded normal. Since the main jet has no adjustment to lean the mixture, we put a small piece of wire inside the jet and bent over both ends to capture it in the jet. Wire was about .0015" and we wanted to slightly limit the amount of fuel that could flow through the jet. The wire probably reduced the area inside the jet by about 10%. When we ran the engine, the performance improved dramatically. The dink jumped up on a plane and ran smoothly at high speed. We ran the engine for a good half-hour at various speeds including wide open throttle. To summarize... the engine was back to normal!

Based on this experiment, I ordered new jets for the carb and will try to measure the hole size in the jet between old and new jet. Has anyone ever heard of a jet going out of tolerance by the hole growing larger?? To the naked eye, there didn't appear to be anything wrong with jet. I feel pretty good about the pending fix, but it would be really nice to know if anyone else had the same experience. Since this problem was a bear to isolate, I would also be interested to hear that someone fixed a similar problem by replacing the carburetor (that was my next step)?
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Old 27-11-2013, 14:44   #25
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I've got a little boat that I use for cave diving that has a Mariner 25 on it. Pretty sure it's actually a Yamaha. Anyway it had the same symptoms you have. I pulled carb apart and through careful looking found that there was a small ball and spring missing from the enriching circuit, so even with the knob pushed in, with this missing ball and spring the enriching circuit was actually full on. Replaced the little BB looking ball and spring and now it runs beautifully.
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Old 27-11-2013, 17:04   #26
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Re: Yamaha 15 2-stroke Issue

SV Cattales, It is very unlikely the main jet got enlarged unless someone reamed it out. It is much more likely that the air mixing tube , which has 12 very small holes, was gummed up. This makes the mixture rich by not providing enough air to the air/fuel mix that forms in the tube just above the main jet.

With the main jet removed one must gently the tube sticking into the venturi down. It will pop out if it is not too gummed up.
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Old 27-11-2013, 17:07   #27
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Re: Yamaha 15 2-stroke Issue

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Originally Posted by a64pilot View Post
I've got a little boat that I use for cave diving that has a Mariner 25 on it. Pretty sure it's actually a Yamaha. Anyway it had the same symptoms you have. I pulled carb apart and through careful looking found that there was a small ball and spring missing from the enriching circuit, so even with the knob pushed in, with this missing ball and spring the enriching circuit was actually full on. Replaced the little BB looking ball and spring and now it runs beautifully.
Normally you'd be spot on. But Yamaha 2 stroke 15's do not have an enrichner circuit, they use a simple choke. I really doubt that if this OB ran well before that and all of a sudden the high speed jet decided to increase in size.
I'd put my money on the fuel pump leaking fuel thru the pulse hose to the crankcase.
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Old 27-11-2013, 17:14   #28
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Re: Yamaha 15 2-stroke Issue

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SV Cattales, It is very unlikely the main jet got enlarged unless someone reamed it out. It is much more likely that the air mixing tube , which has 12 very small holes, was gummed up. This makes the mixture rich by not providing enough air to the air/fuel mix that forms in the tube just above the main jet.

With the main jet removed one must gently the tube sticking into the venturi down. It will pop out if it is not too gummed up.
Wouldn't that have a greater affect at idle and mid range?
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Old 27-11-2013, 19:04   #29
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OK, I stand corrected, sorry. But I do feel myself that re-jetting it is not the fix, and if anything is likely to lean it out and cause it to sieze. You need to find out what has changed. Only time I have ever had a pump diaphragm rupture, it leaked fuel on the outside of the engine.
Stupid question, but you sure the fuel float in the carb doesn't have fuel in it? Most floats are made from plastic and when fuel gets in them, it obviously causes the engine to go excessively rich, cheap easy fix too.
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Old 27-11-2013, 19:06   #30
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Take the float out and shake it to see if there is liquid in it
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