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Old 07-11-2014, 04:50   #31
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Re: 5 HP Yamaha won't stay running

When trying to diagnose what the problem is, having a full bowl eliminates your fuel delivery system. But the fact that it runs fine for 5 minutes makes me think that fuel delivery is your problem. but you need to check the bowl right after it stalls. If your spark plug is dry that means you're not getting fuel.
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Old 07-11-2014, 04:57   #32
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Re: 5 HP Yamaha won't stay running

Awhile is maybe 5 minutes. This last time I was actually ready to celebrate that the problem was fixed, it then stopped. Pumped the bulb until it was hard, started the engine, then a few minutes later it stopped. The bulb was soft.


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Old 07-11-2014, 05:07   #33
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Re: 5 HP Yamaha won't stay running

You can pretty much confirm or eliminate a fuel pump by keeping the bulb full by keeping it pumped up, not a whole lot of pressure, but enough to keep the bulb firm.
You can also get home in the event of a pump failure doing this.
Obviously if it runs fine the whole time your pumping the bulb but dies soon after you stop, it's likely the fuel pump.
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Old 07-11-2014, 05:10   #34
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Re: 5 HP Yamaha won't stay running

On the pump, it's most likely a diaphragm pump that uses the pulsations of crankcase pressure to pulsate the diaphragm which then pumps fuel. failure mode is usually the diaphragm tears, but there is an intake and an exhaust valve, check to see if one is stuck when you have the pump apart, easy fix, diaphragm is usually stupid expensive for what it is, and it's fragile so be a little careful.
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Old 07-11-2014, 08:26   #35
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Re: 5 HP Yamaha won't stay running

These carbies a not easy to work on. I swear they make them in one of those dust free labs with people in white overalls and face masks. If one of the check valves is even slightly damaged it won't work. I have stripped them down, found a fault, reassembled only to find it won't work because of a hairline crack in a gasket that I caused by not aligning things perfectly. I think that you' re on the right track but will struggle to actually see a cause such as a speck of carbon in a valve.
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Old 07-11-2014, 09:36   #36
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Re: 5 HP Yamaha won't stay running

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Originally Posted by landonshaw View Post
Awhile is maybe 5 minutes. This last time I was actually ready to celebrate that the problem was fixed, it then stopped. Pumped the bulb until it was hard, started the engine, then a few minutes later it stopped. The bulb was soft.


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Try putting the tank slightly higher than the engine and see if it keeps running via the siphon. If the fuel pump is weak it will not be able to maintain much head but with the fuel tank high it should keep running.

What is curious is the bulb has a check valve on each end. So how is it going soft?

If the fuel pump is weak and the check valves on each side of the bulb leak I guess the bulb could go soft by pulling air in from the carb bowel as the fuel backflows to the tank.
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Old 07-11-2014, 10:38   #37
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Re: 5 HP Yamaha won't stay running

I like the idea of (1) continual pumping and (2) setting the tank higher than the engine. I bought the gaskets and would of course prefer to eliminate everything else first. Or verify that this is my problem. The gaskets are very fragile so want to do the change only if necessary.


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Old 07-11-2014, 14:49   #38
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Re: 5 HP Yamaha won't stay running

I have a 4HP which is almost identical but no external tank.

I had a similar sounding problem and after I tried and failed to fix it, I sent it to the Yamaha dealer, It took him about a week but eventually he found the main jet was the problem, apparently, the quality of the fuel around these parts is so bad that it is not an uncommon problem for them to wear. The jet is not expensive

He had a spare carb which worked OK and he swapped put parts until he found the problem.

Good luck
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Old 08-11-2014, 12:58   #39
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Re: 5 HP Yamaha won't stay running

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I have had a problem on my Yamaha 5 hp outboard. I will start it and it runs for awhile then cuts out. I pump the bulb from the outside gas tank, start it then nothing. I can try and try, will not start again

I have replaced spark plug, fuel filter, one of the fuel lines in the engine, pulled and cleaned the Jets and needle. Sprayed the carb with cleaner.

Two things that I am thinking. One that gas is not being pumped to the carborator. If so, then wouldn't me pumping the external bulb between the gas tank and engine alway me to start the engine again at least until the float chamber has been emptied of gas?

Second, I am not getting a spark to the plug. Had can I test this? I would think it would be easy.

If it is not getting a spark, possible my red key that goes under the cutoff is worn down, or maybe a malfunction in the cut off??

These are theories, any suggestions from people with mechanical experience?

Thanks, it has been frustrating.


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Make sure the vent is open on the fuel tank. It might be pulling against a vacuum.
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Old 08-11-2014, 13:12   #40
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Re: 5 HP Yamaha won't stay running

Also check the fuel hose and fittings carefully. Better yet, try a new hose. Should have a spare, anyhow. Might be sacking air into the bowl more than fuel.

Spark is easy to check. You can get a checker at auto zone, or simply ground the threaded part of the plug to the engine block. Easier to see spark at night. Be sure no fuel in engine, and no fumes present, tank well away from the area. Seen a doozy of a fire start that way.

Try what Triumphants mechanic did, swapping out the entire carb. If it runs, then try swapping out each individual part. Good to have a whole spare carb anyhow.

If no spark change the condenser. They are cheap. Point type ignition? Make sure points are clean and dry.

I'm betting you actually have spark and you are pulling fuel from a tank with the vent closed, or through a fuel hose assembly that is letting lots of air leak in.

If the hose is leaking it should still run if the tank is higher than the carburetor. Try holding the tank up. Watch for fuel leakage if you try that. Could start a fire or explosion. Sucks when that happens.
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Old 09-11-2014, 15:53   #41
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Re: 5 HP Yamaha won't stay running

Had similar problem.
Cleaned carby but not completely or well enough, SO;
Disassemble the carby COMPLETELY, every single part, and clean ALL the minute holes(there are a lot), jets,etc. polish all the brass bits. You must get it totally clean!
Next time before storing, start, turn off fuel tap, and let engine run fuel out and stop.
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Old 24-11-2014, 10:02   #42
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Re: 5 HP Yamaha won't stay running

So here is what I found. Changed the fuel pump on the carb, 130EC, 50US, sprayed and cleaned the jets, float bowl, visually checked that it was working properly, sprayed the openings in the fuel pump and put it all back together.

Engine started right up and ran full throttle for 20 minutes. Ran around in the marina and everything seems fine. I am in a slip having my Bimini replaced so should be out this coming weekend. Will give it a good test then but don't anticipate any problems.

Easy to change the 2 diaphragms and gaskets. Although they are very fine.


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