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Old 27-11-2013, 19:43   #31
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Re: Yamaha 15 2-stroke Issue

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Originally Posted by S/V Antares View Post
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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Anteres, Your suggestion makes good sense, but I had removed the venturi tube and cleaned it several times and all the little holes were clean. After I cleaned it several times I had a friend take it apart and clean it again. I don't believe there are any blockages anywhere in the carb including air passages. I'm hoping to see a difference between the size of new main jet and the existing one.
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Old 27-11-2013, 20:43   #32
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Re: Yamaha 15 2-stroke Issue

The wire in the jet trick is popular in Rio Dulce. They say the gas corrodes the jet and I don't know if it is true or not but my Tohatsu 8 runs too rich (definitely not the tube in the carb) and I am thinking of putting some wire back in the jet. Anyone think that carbon buildup in the crankcase could be the real culprit? My Tohatsu manual actually recommends a complete disassembly every year to clean the carbon deposits. Pretty hard to do that just for maintenence though. It seems like a lot of people run too high of an oil fuel ratio too which might contribute to carbon deposits. I am reluctant to order new jets though. I have tried jets from different 8hp carbs with no difference and they all look fine (no apparent corrosion). I could order some high altitude jets maybe. But my question is: Could excessive carbon in the crankcase cause a 2 stroke to run rich? and is there a good way to clean it out without disassembly?
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Old 28-11-2013, 07:43   #33
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Re: Yamaha 15 2-stroke Issue

In my post about putting a wire in jet, diameter had too many zeros... it should read .015" (15 thousands).l
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Old 11-12-2013, 05:39   #34
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Yamaha 15 Issue Solved

I received the new main jet and measured the hole diameter using a sewing needle that just barely fit and the size was .042". When I removed the old main jet and measured the hole size it was .053". I looked at the old jet with a jeweler's loop and did not see any sign of corrosion even though the hole had enlarged enough to be obvious with the naked eye. Don't know why it happened, but the old jet had eroded and made the mixture too rich. The small holes in the venturi tube above the main jet and small holes in the low speed jet were also slightly larger than the new parts I ordered, but the primary culprit was the enlarged main jet.

The Admiral and I tested the engine yesterday and the dink jumped out of the hole and developed full power. Wide open throttle was very smooth with no "missing" indications. Case closed!
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Old 11-12-2013, 06:47   #35
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Re: Yamaha 15 2-stroke Issue

Nice to get to the bottom off the issue and even nicer to see it posted. Often we see problems but when it's resolved the OP doesn't bother to post.
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Old 11-12-2013, 10:17   #36
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Re: Yamaha 15 2-stroke Issue

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Nice to get to the bottom off the issue and even nicer to see it posted. Often we see problems but when it's resolved the OP doesn't bother to post.
That is right.

Thanks for the final update.

How old was the engine again??-----------I went back. 10 years. Salt, fresh or both??
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Old 12-12-2013, 14:20   #37
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Re: Yamaha 15 2-stroke Issue

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That is right.

Thanks for the final update.

How old was the engine again??-----------I went back. 10 years. Salt, fresh or both??
Engine is 10 yrs old and only used in salt water. Thanks to all for the inputs.
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Old 12-12-2013, 14:50   #38
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Your gearbox is likely worn and need repair or replacement.
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