Registered: Sep 2007
Location: Kentucky
Posts: 2713

I got a quick question for any outboard mechanics out there. I got a 96 Yamaha C40 that is bogging down and stalling out at full throttle. I am suspecting fuel pump or fuel line because when it stalls out the primer bubble on the fuel line will be soft. When I pump the bubble back up the boat will start and run for 10 to 15 seconds before dying again.

It started this sunday and got worse today. Sunday it ran fine off the trailer and to the first spot we fished, started it up again and it ran without quitting but would not open all the way up and plane out the rest of the day. Today it ran OK off the trailer again but then got to the point it wouldn't even run more than 10 to 15 seconds. The odd thing is however that it will idle forever without stalling.

Before I left today I drained the gas tank and filled up with new fuel and changed to 2 brand new spark plugs.

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quote:

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Originally posted by Virgil
I got a quick question for any outboard mechanics out there. I got a 96 Yamaha C40 that is bogging down and stalling out at full throttle. I am suspecting fuel pump or fuel line because when it stalls out the primer bubble on the fuel line will be soft. When I pump the bubble back up the boat will start and run for 10 to 15 seconds before dying again.

It started this sunday and got worse today. Sunday it ran fine off the trailer and to the first spot we fished, started it up again and it ran without quitting but would not open all the way up and plane out the rest of the day. Today it ran OK off the trailer again but then got to the point it wouldn't even run more than 10 to 15 seconds. The odd thing is however that it will idle forever without stalling.

Before I left today I drained the gas tank and filled up with new fuel and changed to 2 brand new spark plugs.

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how long has the old gas been in there ..with all the etl they are putting in fuels now adays it is knowen to eat up the fuel lines pretty bad even gel up and clog the jets in the carbs..how soft was the primer ball. if i remb right it will get softer as the day goes along its just made to fill the bowl up and create a syphen{speeling} system..it almost sounds to me like the carb is nasty but im not a mech just trying to help out some

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quote:

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Originally posted by mccracken
how long has the old gas been in there ..with all the etl they are putting in fuels now adays it is knowen to eat up the fuel lines pretty bad even gel up and clog the jets in the carbs..how soft was the primer ball. if i remb right it will get softer as the day goes along its just made to fill the bowl up and create a syphen{speeling} system..it almost sounds to me like the carb is nasty but im not a mech just trying to help out some

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Does it do it when you throttle up slowly? Also does it bog down at mid throttle?

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Registered: Nov 2008
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Sounds to me like you Have a cracked fuel line. The motor will run on idle, when you put the throttle to it you are losing the siphon. My first spot to check would be from tank to bulb and then bulb to motor, this has happened to my dad and myself after they sit over the winter. All it takes is a hair line crack, and it maybe the bulb itself. Not a mechanic just a River Rat!!!

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quote:

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Originally posted by CLJohnson
Does it do it when you throttle up slowly? Also does it bog down at mid throttle?

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may have a bad seal on your gas cap. causing it to lose pressure over a few minutes.

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Mine did that and ass long as I would pump up the ball it would keep running rebuilt fuel pump $30 Fixed it. Mariner 50hp.

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Registered: Oct 2007
Location: Iowa
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The pump ball can go bad. You can get them at WalMart. They pull fuel up and keep it there. They have an arrow on them because it's a one way check valve. Cheapest try.

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Registered: May 2004
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Don't know much about outboards.A little about 2 strokes.Have you checked the exhaust for obstructions?This is a similar sympton of too much back pressure.

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Registered: Apr 2009
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I would say its all in the bulb.....if somebody is in the boat with you, when it starts to bog down, have them reach back and start to squeeze the bulb rapidly. I bet you will see that the boat will continue to run. Replace the lines and bulb from the tank to the motor, and you should be fine....

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Registered: Oct 2003
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Check your water pump. Overheating will cause this.

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change your fuel filter

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My Johnson 115 started doing the same thing a few years ago. It has an inline fuel filter........it was clogged.....changed it......haven't had a problem since. I change it each year now.

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Registered: Sep 2007
Location: Kentucky
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Water pump is fine, has great pressure. The only thing it has that could be considered a fuel filter is a small clear plastic housing that unscrews with a small round screen in it. I have checked it and it looks fine, gas passes through it.

Just got home from work and I am going to run to town and buy a new fuel line and hope that helps it.

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If its not holding pressure its most likely the bulb or the ball in the back flow valve ( anti-siphon valve) thats in the tank fitting..

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Last edited by Jon Millwood on 04-14-2011 at 10:59 PM

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If it was me,I'd change out the fuel line from the tank first.Including the bulb.Sounds like it could be that the fuel line may be deteriorated and maybe sucking air.
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Registered: Sep 2007
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Put a new fuel line and bubble on it and no improvement.

I did notice something today that may help someone determine what the problem is. The fuel filter housing will be full of fuel and stay full at idle. When I give it fuel under load the fuel level in that housing starts to drop and when it gets to about a 1/3 full is when it is dying out. I can pump it back full and build pressure back in the bubble and give it fuel and the same thing happens. For those in the know would that point to the fuel pump being bad?? The fuel filter is in between the fuel line from the tank and the fuel pump.

I think I am going to order me a fuel pump and give it a try, they are only about 35$.

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quote:

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Originally posted by Jon Millwood
If its not holding pressure its most likely the bulb or the ball in the back flow valve ( anti-siphon valve) thats in the tank fitting..

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I thought my primer bulb was bad and replaced it. Found out the fuel pickup tube fell down in the tank. Check that and see if that may be it.

Most of the problems with poor performance is the ethanol. It starts to go bad after 2 weeks of sitting in the tank not being used. I have rebuilt every carburetor on all my small engines. I bought the 1 gallon can of Chem-Tool carb cleaner. Take all the plastic and rubber parts out of the carb like the float and needle and let the carb soak in the carb cleaner a few days and put a new carb kit in it. I usually just put the carb in a pan and clean it with gas to get the chemical off. Don't blow compressed air into any of the orifices or jets, just blow the excess chemical and gas off.

I put StarTron fuel additive in my gas can as soon I buy new gas. The Stabil product is supposed to work good, too.

From what you descibed it sounds like fuel pressure is losing somewhere. It may be the fuel pump, ethanol destroys everything. It's actually a solvent and it attracts water. It just makes a gummy mess out of everything when it sits for too long.

Last edited by heckler on 04-15-2011 at 01:43 AM

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Registered: Mar 2009
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by the vaule in the tank you mean the one that allows air to enter and not create a suckion...pull the fill cap off if that what you are talking bout should allow it to breath with no problem if thats the problem

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quote:

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Originally posted by Virgil
Tried a brand new fuel line and bubble and no improvement.

Is there anyway to test that valve in the tank without just trying a new tank??

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Last edited by Jon Millwood on 04-15-2011 at 06:31 AM

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So did you get it fixed?

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quote:

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Originally posted by Jon Millwood
So did you get it fixed?

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