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try to figure this one out....


B0MBTRACK's Avatar
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12-08-06, 04:24 PM   #1  
try to figure this one out....

i have a yamaha 55hp older one, carbed. It goes full throttle for 5 minutes then bogs down to about 1/2 the power. If I shut it down and start it back up it will continue to run at about 1/2 the power. However, to avoid this i can run at about 3/4 throttle for however long i want and it won't bog down or anything. Compresion is good. fuel is clean, no water, primer bulb is fine. spark plugs are clean. fuel filters are fine.
I was thinking fuel pump, or carbs, and if neither of those work it might be some ignition thing where it cuts out after a bit for some reason. any ideas?

 
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thezster's Avatar
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12-08-06, 06:04 PM   #2  
Sounds to me as if......

You're starving for gas...... fuel filter/fuel pump/fuel line restrictions.... something somewhere

 
Ed Imeduc's Avatar
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12-11-06, 10:55 AM   #3  
Im with thezster. Fuel?? Is the fuel filter new???

 
B0MBTRACK's Avatar
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12-11-06, 05:29 PM   #4  
the one on the engine is perfectly clean. its a plastic one that you can take off and look inside and empty and all that. the one before the engine i'm not sure how old it is but doesn't look old. although when i pump the primer bulb i can see the fuel enter the clear plastic one on the engine with no problem. i'll see what happens when i change that one off the engine anyway.
thanks

 
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12-11-06, 07:10 PM   #5  
Personally, I'm guessing fuel pump or fuel pump diaphram.... cheap, and easy to replace...as I recall - though it's been years since I owned an outboard...

Crap... as I was leaving I thought... Eureka.. he's got carryon fuel tanks.... Another possibility is that your fuel line connections at the tank/motor are leaking air at high speed... and need new O-rings. but.. I'm still betting on the fuel pump..

 
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12-12-06, 11:32 AM   #6  
How old are the hose on the tank? They get bad and soft inside them . Is the tank vent clear. I have had gas filter that look clean can blow through them. But the motor would run good only if I put in a new one.

 
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12-18-06, 07:47 PM   #7  
just letting everyone know i'm not going to be able to check this out till spring time but i'll let everyone know what happens

 
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12-20-06, 01:14 PM   #8  
I think it is electrical
If it is that constant then I think the windings in the coil are breaking down when run wide open and reconducting when cooled.
The ignition coil is just a step up transformer.
If a short happened when warm it would bypass many of the internal windings and the windings it bypassed would be directly propotional to the voltage out put.
Coils wear out after a while because of the constant "swelling" of the windings when heated up. This wears out the shellac insulation and the next time it is run at that temperature then the windings expand to a point where the live short happens and lessens the voltage output.
I would do a primary and secondary resistance check when cold and when hot and compare them to specs. I dont know what the specs should be but it could be researched.
Or if you could borrow a known good coil to try .
Guess we wont know till next year

 
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12-20-06, 01:44 PM   #9  
I'll betcha a buck it's the fuel system..... holler at me next spring when he identifies it.... so you can pay up... :-)

 
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03-13-07, 05:58 PM   #10  
just letting you guys know i still haven't checked it out but i haven't forgotten, its coming up soon

 
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08-09-07, 04:46 PM   #11  
spark plug wires...had the same problem....

 
duigoose's Avatar
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08-10-07, 07:55 AM   #12  
I had almost the same problem on my '76 Merc 850 last summer. Turned out one switchbox would drop in and out so it was running on only 2 of 4 cylinders. My symptoms sure were very similiar to yours though.

 
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