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Old 03-10-2014, 18:13   #1
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Yamaha 15hp 2 Stroke vs 55lb Delta

Ok I need help. We have a 2001 yamaha 15hp 2 stroke that is giving me fits. It started with a stuck float from old nasty gas. Thoroughly cleaned the carb and she ran great for an hour. Motor seemed warm, came home wouldn't idle and died but would run with choke half out. Replaced the waterpump and checked t stat. Cleaned carb again, put together, motor idled like new for 20 min, re started no problem. Great power up on a plane, backed down and she died again. Same situation except eng never got over 150 degrees. But will only start with choke half out, then at higher rpms with choke in.
Replaced fuel water separator, cleaned carb,and filter on motor, emptied fuel and put new rec fuel and oil, new fuel line and motor ran great, idled for 20 min, Up on a plane great, then at idle sputtered and died will run fast higher rpms, without any choke but. I can clean idle jet, and she runs fine until on a plane and then wont idle but runs fine off idle, im thinking I might have. Replacement for my delta anchor, but it would probably float.
I am out of ideas. New fuel system, spotless carb, new fuel, problem keeps repeating itself.


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Old 03-10-2014, 18:57   #2
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Re: Yamaha 15hp 2 stroke vs 55lb delta

I assume the reference to the anchor is to attract attention of folks that want an argument about a contentious topic in the hope that someone would have useful advice.
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Old 03-10-2014, 19:50   #3
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Yamaha 15hp 2 stroke vs 55lb delta

Have you cleaned up the fuel pump diaphragms?


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Old 03-10-2014, 20:09   #4
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Re: Yamaha 15hp 2 stroke vs 55lb delta

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Originally Posted by amytom View Post
Have you cleaned up the fuel pump diaphragms?


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Yes they looked perfect. Also checked the membrane on the top.

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Old 03-10-2014, 20:19   #5
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Re: Yamaha 15hp 2 stroke vs 55lb delta

If it will only start with the choke half out then she's running lean. Which means you have a fuel restriction somewhere. Or your idle settings are off.

When you say cleaned the carb how in depth are you talking? Complete rebuild kit or just took apart the obvious stuff and sprayed it? Have you ever been taken a class on how carbs work or is your knowledge grandad's teachings?

I can almost guarantee your problem is from it sitting too long with un-stabilized gas and there is still a partial clog somewhere. 9 times out of 10 small engine problems are strictly fuel delivery related. They're almost never air or electrical.

Lastly like I tell anyone in your situation. If youre going to make it an anchor let me know and i'll pay for the shipping to my house.

Good luck!
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Old 03-10-2014, 20:21   #6
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Re: Yamaha 15hp 2 stroke vs 55lb delta

Also check your idle screw and fuel mixture settings. Could be as simple as they are off.
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Old 03-10-2014, 20:39   #7
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Re: Yamaha 15hp 2 stroke vs 55lb delta

A new carb is around $180 and it will be a new lease of life.

You should have the carb ultrasonically cleaned. There are balance pipes going "over the roof" inside the carb. They are very tiny, and can be blocked in such a way as to be an intermittent fault.

Your idle jet is undoubtedly too big by now, erosion by bad fuel and cleaning with anything hard. The idle jet should be replaced every 2 years or so. The Idle screw should be 1.5 to 2 turns out, but the needle on the end must not be "necked" by tightening too much before counting out.

So When all is said and done, and after buying new jets, new needle, a carb kit and all that jazz, you could have spent another $100 and got a new carb.

The description you have given above clearly indicates a carb problem. It is not a blown middle bearing seal, or a broken reed valve, or an electrical problem.

Do you know someone with the same motor nearby? Swap carbs and test it.

In future, never let fuel evaporate in the float bowl, drain it or run it dry, and never leave the fuel hose connected.

If you really cannot afford a new carb, get the body cleaned ultrasonically and use copious amounts of compressed air to blow out the passages. Do NOT use wires.

Test the passages one by one, especially the smallest ones.
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Old 04-10-2014, 05:16   #8
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Re: Yamaha 15hp 2 stroke vs 55lb delta

Onthe rocks, yes I have yearssss of mechanical education and experience. I have also had more ase certs expeire than most techs ever get. Thats not the point,
After cleaning she runs perfect, the idle adj screw is set 2 turns out, the issue seems to only occur after high speed operation. Almost like the heating up of the engine is causing a change somewhere in the engine creating the leaning out situation.
Dumb question what are the 2 tubes running fore and aft on the starboard side of the power head?

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Old 04-10-2014, 05:46   #9
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Re: Yamaha 15hp 2 Stroke vs 55lb Delta

Could it be a soon-to-fail coil, or an intermittent short on the primary side of one of them? Since you've pretty much eliminated the fuel side issues (short of a worn out carb), the electrical side might need some attention. The intermittent nature of the problem sounds electrically related to me anyway. A weak coil can still spark at no load and not fire properly under load.
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Old 04-10-2014, 06:36   #10
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Re: Yamaha 15hp 2 Stroke vs 55lb Delta

If you can 100% absolutely rule out the carb then it could be an unmetered air leak in the intake. After it gets warm and starts sputtering you can try spraying brake cleaner around connections and gaskets. If the rpms go up then you found your air leak.
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Old 04-10-2014, 06:41   #11
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Re: Yamaha 15hp 2 Stroke vs 55lb Delta

I'm with everyone else on the carb. Too many times I have chased this problem not believing that the carb is the issue because I KNEW I cleaned it thoroughly.

A new carb or extensive rebuild (jets, needles, etc with ultrasonic cleaning) solved the problem every time.

Do yourself a big favor and get a new carb.

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Old 04-10-2014, 06:56   #12
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Re: Yamaha 15hp 2 Stroke vs 55lb Delta

A few seasons ago I had problems with a nearly new Yamaha 15hp 2 stroke due to crappy fuel in the fuel hose. I did not clear the fuel line hose for summer storage and the bad fuel got into the carb when I started the motor in the fall. I took the carb off and cleaned it with carb cleaner and air. Put it back together, same problem. Took carb off again, cleaned it again. I think I had to do this at least three times before I got it right. I knew it had to be the carb so I kept at it. Once the carb was properly cleaned the motor ran like new. Now I empty the fuel line hose when I lay the boat up for the summer.
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Old 04-10-2014, 20:59   #13
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Re: Yamaha 15hp 2 Stroke vs 55lb Delta

Went thru it again, apparently I had 2 problems, now 1 problem. The first issue was there was a piece of something, stuck against the wall in the idle circuit that was moving to block the jet, then it would lay flat again I am thinking when cleaned with carb cleaner, I believe it became dislodged at some point from the interior of the carb. The bigger issue uis after sittin gjust one day with clean fuel, there is considerable corrosion building up in the bottom of the bowl. Sorry for the furstreation earlier, . I spoke with a yamaha dealer and they say over time the bowls and interiors get soft, and corrode easily and the carbs just never stay clean, AFter seeing what formed overnight, I believe them. New carb ordered $219.00

Thanks for the advise!
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Old 05-10-2014, 01:37   #14
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Re: Yamaha 15hp 2 Stroke vs 55lb Delta

Have had the same problem many times due to bad fuel. I would try an additive to assist cleaning and stabilising the fuel. I have been using 2-stroke fuel that is 2 years old with no issues at all since using an additive.
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Old 05-10-2014, 15:17   #15
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Re: Yamaha 15hp 2 Stroke vs 55lb Delta

Stabil. It will save you headaches.
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