Need Yamaha 15 HP 2-stroke help/advice - SailNet Community
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post #1 of 5 Old 02-27-2008 Thread Starter
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Need Yamaha 15 HP 2-stroke help/advice

May have damaged my Yamaha engine two summers ago and would like to try to repair it first, as I'm really poor right now and local dealer repair is wicked expensive. I have reasonable mechanical skills. Still, I'm wise enough to let them do it, if I'm in over my head or lack the special tools.

2004 Yamaha two-stroke short shaft. The engine was almost new and goes like a bat out of hell on my 13-foot Duranautic alum tender, GPS'd at 24 MPH. I was trying to get a message to my cousin, a windsurfer across the bay, on wicked choppy waves (Maybe 18" waves about 36" apart). I was effectively smashing across every wavetop because the message was urgent. The boat didn't like it; I would not normally have run so fast. After about two miles of this, at nearly full throttle, one cylinder quit. Fortunately, my cousin sailed my way, I got him the message, and I was able to get back to harbor on one cylinder. I haven't touched the engine since.

Any ideas what I might be looking for?

Thanks for any guidance you may have!

Last edited by coreywoodworking; 02-27-2008 at 07:55 PM. Reason: typos
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post #2 of 5 Old 02-27-2008
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I'm no mechanic, but given the description of that run, I would first check the spark plug-to-cable connection - could easily have jolted loose. Or, possibly the plug had fouled with excessive carbon.

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post #3 of 5 Old 02-27-2008
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My major concern is having the engine sit, untreated for almost two years... which may have made things much worse. The gasoline may have clogged or gummed up the carburetor or turned to varnish.

Check the interior of the engine for corrosion... sitting unfogged for two years isn't all that good for an engine in the best of circumstances...if it isn't badly corroded/rusted, then take the thing apart and clean the parts fairly well in kerosene or gasoline.

If it cleans up well, then I'd recommend replacing both spark plugs. If one cylinder wasn't firing, that spark plug has been sitting with unburnt oil and gasoline on it for two years... not good.

I'd also replace the cooling water pump impeller. The impeller that is in there is probably dried out and has taken a serious set, since the blades have been in a single position for almost two years.

Good luck.


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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

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post #4 of 5 Old 02-28-2008
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This is two-cycle motorcycle territory.
Classically, the jets on the carb can wax up, but they are easily cleared.
I don't know the motor, but you should be able to get the jets out of the carb fairly easily and clear them, if they are gummed.

Chech for spark first. Take out each plug in turn, and lay it against the cylinder head. Turn the motor to check. You should see it easily. If one does not spark, swop the plugs to see if the problem follows the plug. If it does, then new plug is needed.

If you have spark on both pots, then you are in a strong position. If you have a wee compression meter.... the rubber-ended ones are about $20.... push it into the plug hole and check compression. Spray a wee bit of WD40 in there to give you a wee seal and some lube. My olde 500 cc Suzuki has about 150 psi on the meter, kickstarting.

If you don't have a compression meter, your thumb end will do as a coarse check. They should be about the same. If there is a hole in the piston, it will scream at you... no compression worth talking about.

If it won't start with the spark, pour a wee dribble of fuel into the pot, turn it over once with the plug out (it must not lock up with the plug in), then put the plug in. See if it starts now. If it starts for a wee moment, then dies, then I really would suspect blocked carb jets.

It's not a big job to clear them. Take off the carb bowl, and have a wee look. You may have to take the carb off to get at them... I know not. Take off the carb float, and screw the jets out. They can be cleared with a wee fine wire. The purists say not to use wire, but don't say what else you are supposed to use if compressed air and solvent does not work.

If the motor has been rested a long time, it is helpful to pour plenty 50/50% oil/fiel mix into the crankcase down the carb throat. Let is soak to lube the crank seals. The two-cycle motor has crank seals, and they are vital. Often they get dry if the motor has been rested a longtime, and they burn when the motor revs to 3000 rpm the moment it starts.. let it soak for, say, 8 hours, then, WITH THE PLUGS OUT,turn the motor over to purge the 50/50% mix. Put the plugs in again.

With the jets cleared, carb back on, and some new gasoline in there, I will be surprised if it does not start.

I ressurected an old GT500A from 1977 that had lain for 8 years unused. The soaking of the crank seals was critical. The carbs were gummed out of sight. Now, some 6 years on, it is still running and has 150 psi compression. Maybe I got lucky. It vibtates like sin, but then they all do Sir.

All a two cycle motor asks is not to be thrashed when cold, and not thrashed out of sight at all when hot. Taken easily, they can last a long time. I know of one dude that had 169,000 miles on an old MZ 301 motorcycle, a communist design, and probably more by now.

Even if you start the motor for a few seconds every few weeks, they will behave.

They detest being stored unused, and often they don't forgive it without TLC, like the TLC above.

Good luck.

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post #5 of 5 Old 02-29-2008
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Its more than likely ignition. Check the sparkplug boots and wires at the spark plug and the distributor/mag/coil to see that they are connected. Fix your electrical problem and I'll bet it'll start right up, even if it has been sitting for awhile. It's pretty hard to kill a 2-stroke outboard.
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