# Motor idles, but won't accelerate



## srr5008

I have a 1995 25HP Mercury Tracker Outboard that recently started acting up. I recently replaced the starter, ran it hard for an hour or so without any issues. The next time I took it out, it worked fine for the fist few hours. Then, all of a sudden I started having this issue.

The motor turns over and idles fine. The minute I put it into gear, its starts to sputter a little (sometimes it dies). Any time that I give it throttle, it shuts off. As a test, I had someone pull my prime knob on the motor as I was giving it gas, and this worked fine. I don't know much about boat motors, so I am drawing on my knowledge of car motors. This looks like an accelerator pump problem to me - the motor obviously isn't getting gas when I work the throttle, and we are able to bypass it by giving the motor a "shot" of gas with the pull to prime button.

Can anyone confirm my line of thinking? Or offer any suggestions as to other problems that I might be having. I am leaning more towards the accelerator pump problem because it runs fine once I get under full power. If it was a carburetor problem, I would think I would have problems all the time, not just when I try to accelerate from a stop.

Thanks,

Steve


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## Johny25

Well I don't know Mercs very well but it sounds like your link n sync may be out of whack or you have blockage in the carburetor jets? There are usually a couple jets in the carb, low speed and high speed jets. You have a fuel delivery problem most likely, whether to much or to little as the motor advances.

Sounds though like the carb is not opening up at the correct time during the advance. Or it may be opening to soon which may look like it is not getting enough fuel but in reality it is being flooded out and kills the motor. I would check your link n sync or whatever mercury motors have that is similar. And check your carb.


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## PSG-1

If it runs when you hit the primer, or the primer bulb, then it is most likely a fuel delivery issue, such as a clogged jet, or a bad fuel pump.

But the first thing to check, obviously, is if the fuel filter is clogged. Replace it with a new one, and se if it straightens up. If that doesn't work, then it's on to further troubleshooting.

If you're running ethanol fuel, the culprit can also be the gas line or the primer bulb, with the ethanol eating away the hose lining to the point that it collapses on itself, and starves the engine for fuel (I've seen this one happen a few times) Run blue-colored hose for ethanol fuel to prevent this from happening again, this type of hose is specifically designed for E-10.

If it's not the filter or the fuel line, next step is to check the carb jets.

You can check for a clogged jet by looking at the carb when you hit the throttle, and see if you can detect a spray of fuel. If you don't see it, then it could be a jet, or a clogged internal filter on the carb (I know that a lot of Mikuni carbs on jet skis have internal filters, not sure about merc outboards, but I would bet they probably do) 

But before you tear into the carbs, go ahead and verify that it is a clogged jet, by trying it again, while priming the bulb. If you tear into the carbs, you will need new gaskets, etc, so, it's best to avoid having to do it unless necessary.

If you see the jet spraying into the carb when you hit the primer, then you know the fuel pump isn't delivering enough volume or pressure, and that's your culprit.

If not, then, it's an issue with the carbs.

Good luck with it.


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## srr5008

Thanks guys. I was planning on testing my carb in the exact method mentioned by PSG-1 to see if the pump worked, but I hadn't gotten a chance to do it yet. 

I guess it should have been rather obvious on my part to check simple things like the fuel filter and fuel bulb/line first before assuming the worst. I will also look into the link-n-sync. I've got enough experience tearing down carbs to not be intimidated by the task, but that doesn't mean I want to do it if I don't have to.


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## PSG-1

Yeah, I'm bad for assuming the worst, and it being something simple.

Had that issue a couple of weeks ago with my 50 merc, I changed the plugs, and it ran worse. Couldn't figure out why it had sorry acceleration, etc. I was about to take it to the dealer....glad I didn't, because I would have felt pretty stupid. Came to find out that the connector plug to one of the coil packs had somehow been partially disengaged #-o Guess I bumped it when I changed plugs. Ran fine once I figured that out and re-connected it!

I also went through a massive ordeal with the old tigershark engine in my jetboat, too. Couldn't figure out why it ran erratic most of the time, but every once in a while, it would run fine. Went through all kind of horse-sh!tting monkey business with changing fuel pumps, cleaning the carbs, etc...only to figure out that the wire to the rear spark plug was vibrating and pulling loose from the connection inside the boot. #-o Again, once I figured out what should have been obvious, it ran fine!

As far as the merc 25.....I had a 2005 model, and every once in a while, it would get sluggish, due to water/crap accumulating in the filter bowl. All I had to do was unscrew it, dump the junk into a drink bottle and cap it (for disposal at shore when I got back) screw the bowl back on, prime it a few times, and it ran like a champ.

So, I've learned to start with simple things first, and if that ain't it....THEN assume the worst! :mrgreen:


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## Johny25

Well if the fuel is making it to the carb from the fuel tank then I doubt it is the fuel pump. Like I said I don't know Mercs very well but if the primer on the motor is pulling fuel from the carb then I don't suspect it is the fuel pump itself. Now if you are saying that you have to pump the primer ball on thge fuel tank to keep it going then I would say fuel pump, but it does not sound like you described it that way. Also if it was the fuel pump then you would have an issue running at WOT also and it does not sound like that is what you described.

If it starts good, idles good and runs at WOT good then there seems to be an issue with fuel delivery when the timing is advanced on the motor. Whether to much or to little? Running good at WOT means the carb is delivering the correct amount of fuel when the motor is fully advanced.

Do you have a lean sneeze? Are your plugs and plug threads wet with unburnt fuel?


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## Gramps50

I was having some isues with my 20 hp Mercury and it turned out that it was sucking air and causing problems, my case it wouldn't idle, would be hard to start & finally it dies while backing up and would not restart. More or less by accident I discovered that the quick disconnect at the motor was worn and allowing the fuel pump to suck air. 

The quick disconnect didn't leak fuel so it took a while to figure out what the probem was.


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## srr5008

Well I checked the filter and there are no issues there - clean as a whistle. I am also thinking that my carburetor is not the issue... It was running smoothly at WOT once I gave it a shot of gas with the "Pull to Prime" button. If it was my high speed jet, then it wouldn't run smoothly while under power. 

Gramps comment gave me another line of thought - I had the O-ring on one of my portable tank dry rot last summer. Now whenever I trailer my boat, I notice gas leaking from around the cap on the tank (the top of the tank is always wet). Could I be introducing air into my fuel system here? I am a little dubious to this due to the fact that I had no O-Ring most of last summer without any issues.

I did notice that while my ball got harder when I pumped it, it did seem to still be a little "spongy." I don't know if that indicates that the fuel line/ball may be the culprit or not.

I also have a question about some previous comments. Several people on this forum (and others) mentioned a fuel pump as a potential culprit. I originally mentioned an accelerator pump. Are these one and the same in outboard motors? In a car/truck, they are two separate pumps, and the fuel pump keep supplying fuel to the motor (which I have no problems with once my motor is up and running) while the accelerator pump gives the motor its first "shot" of gas to get things going (which is where I am having problems).

Sorry for writing a novel, and thanks for all the helpful posts.

Steve


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## PSG-1

Fuel pumps and accelerator pumps are 2 different things. The fuel pump delivers fuel constantly. An accelerator pump only delivers fuel under certain conditions, such as hard acceleration.

On an engine with EFI, the fuel pump is typically an electric pump, that pressurizes the system to a certain PSI and delivers it on demand. 

A conventional fuel pump, like on a 2-stroke engine, uses a 'pulser hose' that's hooked to the crank case to operate the diaphragms to pump fuel. On the downstroke of the piston, air is pushed one way through the diaphragms, creating positive pressure, pushing fuel to the engine. On the upstroke, it generates a vacuum, which draws fuel from the tank. Internal check valves within the fuel pump also assure that the air pressure used to operate the diaphragms only runs one way. If one of these check valves goes bad, this can cause an issue with the proper operation of the pump.

My Tigershark 1000 had an accelerator pump. Basically, it was a diaphragm that was triggered by a cam on the throttle shaft, when you hit the throttle, this diaphragm opened up, and gave the carbs an additional shot of fuel, to keep it from bogging on takeoff. It had a single hose that came from the accelerator pump, hooked to a fitting at each carb, with a small orifice for fuel to spray from.

Additionally, on some outboards, there's also a mechanism called a "dash-pot" which is kind of the opposite of an accelerator pump. It actually holds the throttle open for a little longer, as you back off, to keep the motor from gagging and dying out.

Gramps is right....you should also check your fuel line connections, make sure that the hose clamps are tight. If not, it could be sucking air at one of these points, and that would cause it to run erratically.

As far as the O-ring on your gas cap.....while you don't want fuel leaking into your boat, this is not the cause of your problem. I say that because portable fuel tanks have a vent on their cap, and that vent is supposed to be left open, for pressure to vent off, or equalize, within the tank, which keeps it from swelling like a balloon, or collapsing. If air being introduced through the cap area was an issue, they wouldn't put a vent fitting on the cap. So, I seriously doubt the missing or damaged O-ring is the culprit.

The spongy primer bulb sounds suspect to me. Once it's pumped up, it should remain rigid, it shouldn't get spongy. If you've been running ethanol fuel, the issue could be the internal components of the primer bulb, or even the hose itself being eat away by the ethanol fuel.


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## srr5008

PSG, thank you for the VERY helpful reply. It really helped me wrap my head around how the fuel delivery works on an outboard engine. I will look into a new primer bulb and replace the hose while I am at it this weekend. The bulb wasn't completely soft, as I was able to get it "mostly hard", but it definitely had some play in it that I don't remember and wasn't rock hard. Seeing as how the setup is 17 years old, it is probably due for replacement anyways.


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## Pappy

Once running, the primer bulb should not stay hard. It should get spongy and soft!
The primer bulb is on the suction side of the engine driven fuel pump so there is no way it can retain the pressure induced by you pumping it up initially. Fuel is continually being sucked through it while running.


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## Captain Ahab

Gramps50 said:


> I was having some isues with my 20 hp Mercury and it turned out that it was sucking air and causing problems, my case it wouldn't idle, would be hard to start & finally it dies while backing up and would not restart. More or less by accident I discovered that the quick disconnect at the motor was worn and allowing the fuel pump to suck air.
> 
> The quick disconnect didn't leak fuel so it took a while to figure out what the probem was.





X2


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