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Old 12-03-2011, 20:47   #1
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Carb Problems - '97 Yamaha 15hp Two Cycle

I picked up a 15hp 2Cycle outboard for the dink. I threw in a carb kit and tried it out today. Idles and runs slow okay but a bit smokey. At full speed it bogs down and acts like it's choked when it isn't.
During the carb overhaul the pilot screw was frozen and broke off in place. I left alone hoping it was in adjustment.

While trying it out under load I pulled the fuel line off at the engine and it started running much better right up until it ran out of fuel.

So, is it the pilot screw, is the float and needle valve out of adjustment, or something else? Timing?

Any thoughts?
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Old 13-03-2011, 07:37   #2
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

Is there a chance the fuel hose is deteriorating and won't supply enough fuel to run at high RPM? Mine goes south every couple of years.

George
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Old 13-03-2011, 07:51   #3
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

George,

Thanks for the reply. The fuel hose is only a couple of months old and works with another engine I have.

Any other thoughts?


Tom
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Old 13-03-2011, 07:57   #4
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

Are you using the correct oil/gas ratio. My 2000 15 and the 2009 that replaced the stolen 2000 both used 100:1 mix. I also have read that some 2 stroke oil is not meant for water cooled 2 stroke engines, apparently air cooled oil needs a higher temp to burn properly.

Did you completely disassemble the carb and spray the innards with carb cleaner? I had trouble with mine last year and used almost a full can.
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Old 13-03-2011, 08:07   #5
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

fuel mix is 100:1, I didn't break out the measuring cups but used the marks on the side of the 2 stroke oil bottle.
I tore the carb apart except for the dual check valve on the side and the pilot screw. The check valve I didn't have a new seal for. As it seems to be running way too rich I don't think the check valves are the issue.

I'll check boats.net Monday to see if they have one in stock. If it has to come from Japan that may take awhile due to recent events. I was hoping to take this engine to the keys with me next weekend.

Tom
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Old 13-03-2011, 08:27   #6
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

Check / Clean the fuel pump, fuel filter. The fact it runs better when not drawing gas up from the tank points to this. You might check this before pulling it apart by running it with tank elevated to or above engine.
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Old 13-03-2011, 08:42   #7
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

The plate on the side of that carb that is held on by four screws is the fuel pump. If the diaphram is bad it will allow unmetered fuel flow into the crankcase and naturally then into the cylinder causing a real rich condition. That would be my bet.
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Old 13-03-2011, 09:32   #8
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

Hmm fuel pump. Makes sense. Still need parts to fix. I'll definately be calling Boats.net tomorrow. since they're only a couple of hours away they can get the parts out quickly, if they're in stock.

BTW yes the outer cover on the side has cracks on the seal. The inner plate didn't come off easy so I left it. I'll take it off later and clean / inspect.
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Old 13-03-2011, 09:50   #9
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

Yamaha pt# 6G1-24411-00-00 & 63V-24411-00-00 = DIAPHRAMS, 6G1-24431-01-00 = GASKET
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Old 17-03-2011, 15:39   #10
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

As the pilot screw was broken and the carb would need to be replaced someday anyways... I just received and installed a new carb. Very impressive service from Boats.Net; again.

The engine fired up on the first pull! On the stand it runs like a champ obviously more responsive to the throttle. I did get a bit of popping from the exhaust, is that just the nature of 2 cycle? It's been decades since I've had a 2 cycle anything other than weedeater.
I checked the timing and it appears to be at 5degrees before TDC at idle. Does that seem right?

Tomorrow morning I'LL put it on the dink and give it a spin, hoping for the best as we head out in the afternoon for Key Largo. (I know, departing on a Friday.. good thing I'm not too superstitious)


Thanks again for everyone's help!!!
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Old 17-03-2011, 16:24   #11
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

Quote:
Originally Posted by amytom View Post
. I did get a bit of popping from the exhaust, is that just the nature of 2 cycle?
Depends on how far you reved it up unloaded, but, yes.

Have a good trip.

Take some bananas with you. The two should cancel each other out.
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Old 02-04-2011, 13:59   #12
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

Nope, 5 Before is wrong at idle. It should be 5 AFTER at idle, and 25 Before at FT.
Remember to slack the setscrew at the carb before setting the timing, the rod can pull the timing ring further (not an enduro)
Your mixture screw should start at 1+3/4 out, BUT be very gentle when bottoming it for the setting. Let it idle for 5 minutes after a fast burn, then kill it and read the plugs. Ideal should be the bottom one a bit rich, and the top just on the rich side of brown. Back to timing, adjust the bar at the top first on FT, then the screw for idle setting. The poor fule in the Caribbean and elsewhere contains other impurities. Bank on changing the idle jet every 2 years. Its a small hole, and even the slightest erosion is a huge difference in %. Your last setting is the idle screw on the carb, make sure its not touching while setting everything else. Best little engine out there!
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Old 02-04-2011, 14:48   #13
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

Try squeezing the bulb when it is running.If it dies the fuel pump diaphragm could have holes in it allowing the gas to bypass the carb.I spent 2 summers with a 8hp Marriner(Yamaha)till I found the problem.The dealer told me to scrap the engine because he couldn't fix it.
$22 for a diaphragm kit and it ran like new.

good luck

Phil
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Old 02-04-2011, 19:53   #14
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Re: Carb Problems - '97 Yamaha 15hp Two Cycle

Thanks for all the advice. I ended up getting a whole new carb with the fuel pump. It fired right up! Ran quite well getting our 10' Zodiac on a plane with 3 people and gear on board. Still sputtered a few times so I'll try the tuneup suggested above.

I agree that this is a great little engine!
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