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multi species angler
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Discussion Starter · #1 ·
My recently purchased 25hp mariner seems to run great, but when held wide open it cuts in and out. It is a 1983 built by Yamaha. Anybody have any suggestions as to what the problem might be? Thanks
 

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Check for any air leking into the fuel line - epeciallythe quick connects.

If the bulb goes soft it's a good indication.

ALso - fuel pump.

Air leak can be very very small and it will suck air instead of fuel.
 

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Might make sure both cly are fireing at idle then pump the bulb at full throttle. If no change and still breaks up choke it! If the carbs plugged itll pick up power. I would have to think the gas is good if it idles.
 

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Lumberjack
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It has been a long time for me as a wrench so I may be off the mark a little here, but from my worn memory, here are my thoughts.

Spark plugs tell you alot...are they black and wet after running the engine? Are they are a nice tan color and dry?

Two strokes have two jets in the carb and if either is clogged, gummed or dirty, they will affect the fuel/air mixture and affect the performance. It sounds as if the idle jet is okay, but the fuel jet may be gummed. Sticking floats will also flood the intake with fuel and the ingition system cannot burn the excess fuel, thereby fouling the spark plugs and affecting performance at WOT (but would also affect it at idle). A good cleaning of the carbs will eliminate that as a source. If the carbs appear to be clean and eliminated as a source of the problem, the piston rings may be shot. With little fuel at idle, worn rings are not such a big deal, but as the engine rpm is increased and therefore the fuel intake is increased, a loss of compression becomes more apparent and when that occurs, the extra fuel/oil passes by the piston rings which, again, the ingition cannot burn fast enough causing a plug fouling issue again. Use a compression tester to check the compression.

I am certain an up-to-date mechanic would explain this and diagnose your problem more accurately than me, so take it for what it is worth.
 

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multi species angler
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Discussion Starter · #8 ·
Thanks guys for all the info so far. The motor starts very easy and stays running, but it does seem to idle a little rough. The gas should be good. It was winterized as usual and some fresh gas has since been added. It ran fine with my 9.9. I just switched the fuel line connection to fit the 25hp. The ball felt firm when I checked and I did give it a couple pumps and it didn't seem to help. It seems to run great until it gets to top speed and then it starts doing that.
 

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Being Japan made it probably dont have points let alone a mixture screw for high/low adjustment?
 

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Air leaks into fuel line can cause "cycling" as the engine runs. Been there.
Replace any questionable fuel lines. Double check clamps. Plastic, quick connect strap clamps weaken with vibration and temp changes.

Carb cleaner thru system might help if its a carb problem. Easy/cheap.

Other possibilities above.

Good luck.
 

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multi species angler
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Discussion Starter · #11 ·
Ok this is where I am at right now. The gas, tank, hose, and connections are good. The compression is the same on both cylinders. I changed plugs and ran it for a little bit and checked them. There was some wetness. It does seem to smoke a little more than I would think would be normal, but not sure what normal is for this motor. The fuel filter is fine.

While running I can take a plug wire off and it stills run just a little rougher. I put that wire back on and take the other off and it stalls. Is this a clue to any problem.

If it is in the carb, how difficult would it be for me to take apart, fix, and get back together with out having any parts left over:D

On a side note, I was wondering if anyone knows if there is an adjustment to make the throttle stick in any position? The only way to keep trolling speed now is to continuously hold it in position. If I let it go it goes back to idle speed. My 9.9 would hold open in any position.
 

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useless poster
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Motor Not Revving Up Like it Used To : If all other normal things have been checked out & the motor will not run at the faster speed, sometimes if it has sat for some time with the fuel not run out of the carburetor when it was put away, the main jet can get partially plugged. The bowl of the carburetor can also accumulate some debris. If this happens, the motor may start & run at a medium speed, but is not getting enough fuel to allow it to run at a faster speed.
 

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Lumberjack
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Ok this is where I am at right now.

While running I can take a plug wire off and it stills run just a little rougher. I put that wire back on and take the other off and it stalls. Is this a clue to any problem.

On a side note, I was wondering if anyone knows if there is an adjustment to make the throttle stick in any position? The only way to keep trolling speed now is to continuously hold it in position. If I let it go it goes back to idle speed. My 9.9 would hold open in any position.
Sounds like your getting close...as freyed suggested, the spark may be weak. First make sure the gap in the plugs is correct. Before changing the coil, switch the location of the spark plugs and try it again...just in case you got a bad plug (even new ones fail sometimes). If the result is the same, then check the plug wire cap...sometimes they can loosen and not make good contact, reducing the current to the plug. Then pull both plugs, insert them into the respective plug wire and lay them across the cylinder head so they are grounded. While watching the spark plug, give the starter a yank and see what kind of spark you have...should be a solid blue spark and both should be the same...if the coil is bad on one of them, the spark will be a small yellow spark (or no spark) rather than a bold blue spark. I hope this helps you further trace to the source of the problem...isn't it fun;)

I cant remember what your motor type is, but my Evinrude had a set screw to tighten the tension on the throttle. It was located on the underside of the tiller handle.
 

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MOD SQUAD
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does sound like a possible bad coil/wire.
 

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multi species angler
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Discussion Starter · #16 ·
The spark is good on both wires. I guess the next thing is going to be a carb overhaul. I do not think I will be able to do this. Any one know what kind of price range I will be in to have this done? Any other things I should have done while it is apart?
 

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Seems to me that you have pretty much qualified the problem as spark associated. I would bet that your problem is "OLD COILS". That motor is twenty four years old! Coils do age, even when sitting unused and the material used to seal them cracks. I had the same problem 9 years ago with a 1957 Johnson 10. Replace the coils, points and condenser plus plug wires and no more problems. You can get the parts at NAPA. Give yourself the best shot at having a reliable motor. Leave the carb as the last resort. Also replace the water pump impeller.
 

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The Inferior Fisherman
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I think it is a spark problem too. When you pulled the plug on the cylinder that stalled the motor, that would mean that is the good cylinder. The one with little change is the weak one. I would change points, condenser, coil, distributor cap, plugs, and wires. And if it didn't work, so what, at least you know you got a tune up when you finally do figure it out.
 

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Defensor Fortis
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My motor was doing something similiar last year. Look and see if you have any unspent fuel in your exhaust. It will look like muddy water. If you don't have that, then I would say it is not your carb. It cost me about 80.00 to fix. I attached a photo of what it looked like.
 

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