# Mercury 25 HP 2004 Fuel Priming Issue



## flyclimber (May 22, 2016)

Opening this up to the minds on here!

I've got a 25 HP Merc 2 smoke, that wont stay primed when idling. I have to squeeze the bulb when I start and usually after 30 seconds- 1 minute for idling. I have rebuilt the Carb, changed fuel/water separator filter, fuel lines, fuel bulb primer, adjusted idling screw, and timing. I also tried to drain out the built in fuel tank and pulled around 2-3 cups of water from the bottom, then refueled with all fresh ethanol free gas at 50-1 oil ratio.

The motor idles smoothly now but the only thing I can think of now is the reeds being old? The cylinders cold compression is around 95 psi on each cylinder with manual pull start with a cheap compression gauge. There is no electric fuel pump just the one built in to the carb. Should I try to change that out next?


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## Guest (Aug 21, 2018)

I would replace/rebuild the fuel pump, cheap insurance and I reccomend it be done every 4-5 years. It’s just a diaphram pump and they get weak over time. Also, what diameter fuel line are you running? Some of the cheaper ones use 3/8 on all they’re hoses and this causes problems with some smaller outboards. The volume/ weight of the fuel is too great for the little fuel pump to pull. A 25 only needs 5/16 and some 1/4. That is what the factory sends with them. The distance of fuel tank feom motor can have same effect causing the fuel pump to struggle pulling the weight of the fuel. 
Also, I doubt very seriously you have a reed problem! If you do have a reed problem, you will have a lot fuel/air spraying back at you through the carb. A small amount of blow back is normal, you should notice a bad reed easily.


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## flyclimber (May 22, 2016)

That's interesting thought, the hosing from the tank to the water/fuel separator is 3/8" and pretty much a straight shot height wise but its 5/16" to the motor itself. The distance is about 15' total to the tank. 10' of 3/8" and 2-3 or so combined with the bulb. 

So what causes the vacuum to pull the fuel? Is the case under vacuum?


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## Boneheaded (Oct 4, 2017)

Vaccum is created by the closed "air tight" fuel system with the little diapram in youre fuel pump doing the work. Sounds like a fuel pump to me too, should be 2 bolts and two hose clamps. Changed them on both my Honda bf50 and tohatsu 15hp. But also for example, my jonboat has the tank in the front even with 15 feet of 1/4 inch and an inline fuel filter to see fuel flow, i still need to prime the ball sometimes running WOT.

Heres a question though, do you guys think a longer pull through the fuel line stretches out these little diaphrams faster?


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## flyclimber (May 22, 2016)

I can't see why it would. Physics would say that the only real impact would be the height difference between the tank and motor, however there are filters bulbs ect in the way also causing resistance to the flow. I would think the more stuff you have in the way would be the only real cause for the diaphragm wear. I just read elsewhere that on the new gaskets that replace the old ones on the carb don't line up correctly and it blocks some of the passages. I wasn't looking for that when I built it. I will check on it tonight and post back.


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## Guest (Aug 21, 2018)

The further the pump has to pull fuel, the heavier the load on the pump. What happens, since the pump is driven via crank case pulses... the diaphram simply stops moving. Trust me guys, I am no physics professor but this is what happens in real world experiences! If it was running fine and then started acting up then I’d say the length and diameter of fuel line is not the problem in this case provided all was replaced with the same size. However, the weight of the fuel could be causing an aged and weak fuel pump to show it’s typical symptoms early and this could be a blessing in disguise. Also, if previously mentioned gasket is blocking a port in the pump well... ya’ll can figure that one out lol!


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## flyclimber (May 22, 2016)

So I'm not seeing any blocked holes from the gaskets. I guess I try to get a new pump?


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## devrep (Feb 22, 2009)

I had the 2001 version of this motor and had the tank in the bow with no distance problems. Check the tank, fuel lines and primer bulb for pressure leaks.


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## 17376 (May 5, 2017)

Change the diaphragm in the fuel pump(fuel pump kit)


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## devrep (Feb 22, 2009)

take the fuel line off the carb and crank the motor (into a container of course). If you have good fuel flow its not the fuel pump.


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## flyclimber (May 22, 2016)

Travis Smith said:


> Change the diaphragm in the fuel pump(fuel pump kit)


I already have replaced the diaphragm. I just double check to see if there were any holes being covered up and there weren't any. 




devrep said:


> take the fuel line off the carb and crank the motor (into a container of course). If you have good fuel flow its not the fuel pump.


Not sure I get what you're trying to test? How will the motor have fuel? The motor runs great at anything past idle.


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## Guest (Aug 25, 2018)

Ok, so motor runs great at anything past idle correct? Your original post says it idles smoothly now? What exactly is the symptom you are trying to fix besides a soft primer bulb after starting? Is the motor dying after idling a few seconds/minutes? Having to give the primer a squeeze to start a carbed motor isn’t a problem, it’s the nature of some. Even after running a while- heat soak will vaporize the fuel in the carb and basically drain it so a squeeze becomes necessary to fill the bowl again. I’m not trying to be an ass, I am trying to figure out what is actually happining so I can help you solve it.


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## devrep (Feb 22, 2009)

if its running great at high rpm's how could it be a fuel pump problem?


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## Guest (Aug 26, 2018)

devrep said:


> if its running great at high rpm's how could it be a fuel pump problem?


From the sound of it, it idles good too! IDK what the actual problem is at this point lol! Says he has to sqeeze the primer but idles and runssmoothly...


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## 17376 (May 5, 2017)

There is a contradiction to what you’re saying...the bulb won’t be rock hard after you have ran it just as long as it isn’t empty.. so what is your issue?


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## flyclimber (May 22, 2016)

The problem I have is before I run it (Cold engine or Warm engine) I prime the engine so that the bulb is rock hard any more than that and it squirts out the carb. I start up the engine and if it idles for more than a minute or so It will die from fuel starvation. (Symptom is that bulb still be softer than a flaccid ...). I give it a few squeezes and she'll fire right back up and idle for the same time. The only difference if I run the boat there aren't any hiccups any no fuel problems. Timing appears to be correct. 

The problem is the idle.


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## 17376 (May 5, 2017)

Eliminate the tank and lines as an issue.. get a portable tank and line plug it in and check it.


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## Guest (Aug 26, 2018)

Ok, place in a bucket of water full enough to cover cav plate. Try adjusting you idle screw in 1/4 turn at a time and see if it improves or worsens, if improvment is noticed this is the direction it needs to go. If you need to go the other way then do so. 1/4 turn adjustments at most at one time and let run a minute before making further adjustments.


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## Guest (Aug 26, 2018)

Also, check your idle rpms! Adjust according to spec, some of these little motors are designed to idle faster than one might think and will not idle much lower than spec! Use a tiny tach or shop tach to verify if your boat does not have one.


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## flyclimber (May 22, 2016)

I get to around 800 rpm without a problem. Anything less and it's start sputtering and kicking.


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## Guest (Aug 27, 2018)

You are probably trying to idle too low, like I said... some of these are idled high from the factory and won’t idle if set too low. What year is it, I’ll try to dig up idle rpm for you.


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## Guest (Aug 27, 2018)

Ok, a quick search led me to... 750+/- 50 rpm in forwad in water. Set it to idle at 800 in gear under load and let it be. This will make it idle around 1000 rpm in neutral but that is what it wants and ya can’t change it without re jetting and a lot of tuning.


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## flyclimber (May 22, 2016)

I finally got around to running the boat again. 
I adjusted the carb Clockwise a 1/4 turn and it gave me the big old middle finger. It did not want to start or run what so ever. I turned it back around 1/4 turn counter-clockwise and it fired right up. Didn't get to run very far but it idled just fine after the carb rebuild. I am thinking I must not have tightened the diaphragm side of the carb enough when I rebuilt it the first time. I will keep this active as I get to run the outboard more.


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## Guest (Aug 30, 2018)

freeclimber said:


> I finally got around to running the boat again.
> I adjusted the carb Clockwise a 1/4 turn and it gave me the big old middle finger. It did not want to start or run what so ever. I turned it back around 1/4 turn counter-clockwise and it fired right up. Didn't get to run very far but it idled just fine after the carb rebuild. I am thinking I must not have tightened the diaphragm side of the carb enough when I rebuilt it the first time. I will keep this active as I get to run the outboard more.


What makes you think the diaphram side was under torqued? Just curious.


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## flyclimber (May 22, 2016)

Boatbrains said:


> What makes you think the diaphram side was under torqued? Just curious.


I think there was a slight amount of fuel leaking when I would give a good firm squeeze on the new bulb. I probably didn't pay enough attention to the way I tightened them down.


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## DuckNut (Apr 3, 2009)

If the bulb is going dry with low suction and is fine with higher suction then I say it is an air leak.

You described testing every item in the fuel system except for 2. The fitting that snaps onto the motor and the fitting that snaps onto the tank.

The male fittings should be fine, the female may have an oring that is allowing air in when not enough suction to pull the two pieces together.


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## flyclimber (May 22, 2016)

DuckNut said:


> ...
> The male fittings should be fine, the female may have an oring that is allowing air in when not enough suction to pull the two pieces together.


I replaced the fittings between the fuel tank and the outer side of the motor including the plug in adapter. Is there a chance air can leak through the air fuel separator? All fresh hose clamps (male barbed fittings) and they're tight. I haven't checked the hoses inside the motor. Might check that next it's better but still doesn't like idling for long amounts of time.


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## DuckNut (Apr 3, 2009)

If you disconnect it and pump it hard and it stays then you should be good from tank to fitting. If it leaks down when plugged in the leak is between the fitting and carb, including the carb itself.

Does it leak down if you pump it up and just let it sit?


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