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Old 21-08-2013, 18:00   #1
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Question Yamaha 15 2-stroke Issue

This has been a great engine for 10 yrs, but it's developed a problem I can't solve. It starts on first pull and idles smoothly. When I open the throttle, the engine sputters and it takes a long time to get my 10ft Livingston dink on a plane. After dink starts to plane, engine will rev up and seems to have almost full power, but not quite. While planed off, the engine does not run as smooth as usual. After it planes off the dink the first time, the 2nd and subsequent attempts to plane off the dink comes out of the hole within 10 seconds, but still not developing full HP.

I have done the following obvious fixes:

Fuel: removed carb and cleaned with spray cleaner at least 3X; put rebuild kit in carb (new float, shut off, and gaskets); replaced diaphragm & gasket in fuel pump; replaced fuel line to tank; cleaned internal and external fuel filters; put fresh fuel in the tank.

Ignition: Replaced plugs; checked spark using a screwdriver in socket and spark jumped 1/2" to block; bought a spark tester light that connects to plug lead on one end and on the plug on the other. I ran the dinghy at night using this light on each cylinder and could not see any misses; I checked AC voltage output of CDI and got around 200 volts and close to shop manual values (one cylinder varied much more than the other, but maybe due to a digital meter); I ran engine under load and removed spark leads one at a time and RPM loss sounded equal; I pulled off the flywheel and looked at the coils and mechanism that operates the pulsar coil... it looked OK, but there was some play in the plastic pulsar-coil lever around the crankshaft. I tried checking the timing according to shop manual, but the info "did not compute" and when I put the #1 piston at 30 degrees BTDC, the timing marks were way off. I didn't mess with it because if it was really that far off I don't think it would run. I'm not sure I understand the procedure so I'll give that another try.

Anyone have any ideas on what to check next? I'm still not sure whether it's fuel or ingnition. Since the problem is intermittent, it has become more challenging.
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Old 21-08-2013, 18:17   #2
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Re: Yamaha 15 2-stroke Issue

I've had that exact problem on that exact engine:
A: the hub on your prop is spun and you need a new prop. or
B: Your main bearings are shot... does it vibrate more than usual and hard to get it up to speed under load?
It's not as hard as you might think to put new main bearings in... I did mine in Trinidad, but had an expert helping me....
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Old 22-08-2013, 07:53   #3
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Re: Yamaha 15 2-stroke Issue

Quote:
Originally Posted by Cheechako View Post
I've had that exact problem on that exact engine:
A: the hub on your prop is spun and you need a new prop. or
B: Your main bearings are shot... does it vibrate more than usual and hard to get it up to speed under load?
It's not as hard as you might think to put new main bearings in... I did mine in Trinidad, but had an expert helping me....
Thanks for the help. The prop is OK. I don't think bearings would be the primary cause since the problem is intermittent. Sometimes "the dink jumps out of the hole" and will do that again on several consecutive attempts. It appears that problem occurs at first attempt to plane off even though motor has warmed up to normal temp. I have tried shutting it down for just 5 minutes and then idled out to open water to open throttle again. Problem came back again and it took 60 seconds to plane off. Again, after the first planning attempt, the 2nd and subsequesnt attempts worked fine.

Anyone have any intermittent condition with ignition parts like coils or CDI? I'm running low on ideas!
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Old 22-08-2013, 08:20   #4
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Re: Yamaha 15 2-stroke Issue

Two strokes are notorious for carbon build up. It will lose power slowly over long time as the exhaust system gets plugged. Remove muffler start it up and see if it feels better. Also there could be small vacuum leak somewhere in head/cylinder/engine body gasket but in this case the motor usually develops hard to start issue.
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Old 22-08-2013, 08:30   #5
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Re: Yamaha 15 2-stroke Issue

Check compression maybe?
Possibly a fuel restriction in the hose or tank? I have seen old fuel hoses and primer bulbs disintegrate inside and cause an obstruction...
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Old 11-09-2013, 18:32   #6
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Re: Yamaha 15 2-stroke Issue

Still working on the engine, but no solution yet. Here are the other things I looked at: pulled the flywheel and checked magneto coil, pulser coil, and checked the timing. Voltage outputs for all were good. I bought a new CDI, but that didn't solve it either. Also removed the exhaust manifold and cleaned out carbon. One new thing I have noticed is spark plug electrodes are always black instead of tan so it is probably running rich.

Thanks for all the good ideas. I'll keep trying them till I find the cure.
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Old 11-09-2013, 19:55   #7
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Re: Yamaha 15 2-stroke Issue

I will add to the guess work. First, I did not see where you changed the fuel filter. A filter has given me problems in past years by restricting fuel flow.

Another idea--- could it be that your prop hit something??? What commonly happens to lawn mowers when they hit rocks or other obstructions, the flywheel key gets bent due to inertia. If one looks at a bent key, they assume the thing is fine, just a little bit bent.

Well it is NOT FINE! The points are set to open by cogs on the shaft that holds the magneto cover--flywheel. That slight bend, repositions the flywheel mounted magnet to some degrees away from where it should be when the point open. Many times, a lawnmower will not even start because of this problem AND IT WILL STILL DISPLAY A SPARK!

Anyway, just some ideas to think about.

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Old 11-09-2013, 20:29   #8
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Re: Yamaha 15 2-stroke Issue

Could possibly be bad seal around crank, allowing too much air in.
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Old 11-09-2013, 21:04   #9
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Re: Yamaha 15 2-stroke Issue

mine does the same thing, if the tank is low on fuel- , fill er up


and she runs fine, I thin it must suck some air as the boats bow rises
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Old 12-09-2013, 05:26   #10
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Re: Yamaha 15 2-stroke Issue

Hopefully you check the fuel tank before buying a CDI unit! If not, make sure there are no blockages in the fuel line or in the tank pickup. Pump up the bulb to firm when the condition appears to insure it is not fuel line or pump related.
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Old 12-09-2013, 08:03   #11
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Re: Yamaha 15 2-stroke Issue

Quote:
Originally Posted by foggysail View Post
Well it is NOT FINE! The points are set to open by cogs on the shaft that holds the magneto cover--flywheel. That slight bend, repositions the flywheel mounted magnet to some degrees away from where it should be when the point open. Many times, a lawnmower will not even start because of this problem AND IT WILL STILL DISPLAY A SPARK!
I don't think points have been used in outboards for many years.

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Old 12-09-2013, 16:32   #12
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Re: Yamaha 15 2-stroke Issue

Unless you dissasemble the carb completely you are not doing anything. There ia a jet that is just above the main jet (Called a "nozzle") and it protrudes into the venturi about 4 mm. It has a series of very small holes (10 or 12) that allow air to mix with the fuel. If they clog it will run rich as you noted.
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Old 12-09-2013, 17:21   #13
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Re: Yamaha 15 2-stroke Issue

Yes I agree, none of what you have done to the carb addresses the most likely issue - that the jets have become partially blocked and are not spraying fuel correctly. I believe Boats.net sells the actual jets as well as the replacement carb.
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Old 13-09-2013, 05:51   #14
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Re: Yamaha 15 2-stroke Issue

Yes. Boats .net uses the OEM parts books and very good service.

Removing all the parts, jets etc allow inspection. I have found that soaking them overnight in lemon juice clears up most issues.
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Old 13-09-2013, 06:02   #15
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Re: Yamaha 15 2-stroke Issue

I was going to say jet the carb BEFORE I read the last few posts....

Methinks that's your next move!
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