1985 mercury 25hp issues

[MarsAngler40]

Bought a boat used and motor was running fine. Only got the run the motor three times. Twice short runs on lake and another 4 hour trip on a river. Sent to boat up to my Dad for the winter where he uses it for steelhead fishing in the river. Ran okay but then it started dying on him. What he figured out is when its running the bulb is loosing pressure and then it dies out. It idles fine for a couple hours then starts bogging out. If he pumps the bulb when running its fine.

Now I found out he was using regular file and not premium, and pretty sure ethanol fuel. The hose bulb and tank are all new. Any ideas? These problems do not always occur. Could it just be the cold? I never had issues in the summer but I always used premium.

Anyways feel free to ask for more info, I get the boat back in March to start working on this. Thanks for any help.

 

[crazymanme2]

If he pumps the primer bulb & it runs ok odds are fuel pump needs to be rebuilt.

When was the last time the impeller was changed?

 

[MarsAngler40]

If he pumps the primer bulb & it runs ok odds are fuel pump needs to be rebuilt.

When was the last time the impeller was changed?

Not sure on the impeller, would the fuel pump work on and off? Sometimes we go out and have no issues, and others it has issues on and off. It doesn't always need the bulb pumped, its on and off.

 

[cliff58]

In a fuel pump there are little one-way valves that work together with the vacuum diaphragm, constantly opening and closing. One little piece of dirt hanging around by one of those valves could do exactly what you are describing when it keeps the valve from sealing. Also, an ethanol-hardened diaphragm might perform well enough with warm fuel but not so well when it's cold out. So yes, you can have intermittent fuel pump problems.

Another possibility is a fuel line connector or the pickup tube inside the tank could have a bad seal allowing air to get sucked into the line. If you use two gas tanks pay attention to which one is hooked up when you're having trouble.

I would consider rebuilding the pump, and put an inline fuel filter in the line leading into the pump. You want your filter to be after the last place where dirt can be introduced, which is at the fuel line connection to the outboard. Also if you use clear fuel line between the engine's connector and the filter, you can easily see if you have any air leaks between there and the fuel tank's pickup!

That's all i can think of, best of luck getting it sorted out

 

[slick]

Is the cap for the tank tight? You may have a vacuum on the tank and when you pump the bulb it gives enough fuel to keep it running.

 

[KMixson]

Make sure all of you fuel hose connections are tight. If it runs fine sometimes but not others it may be a loose connection at a fuel hose nipple getting air sucked in. Then it may run with the hose situated in a certain way and not run when the hose is another way. I don't believe the fuel bulb should stay hard while the motor is running. Before you start the motor you pump the primer bulb to get fuel to fill the fuel bowl on the carburetor. Once the bowl fills the needle valve closes and the bulb gets hard. As you run the motor the fuel pump keeps the bowl full so it is pulling fuel from the tank through the bulb which actually has a small vacuum on it as the motor runs. I would be looking for an air leak somewhere.

 

[DaleH]

Run the OB w/ the tank fill open ... if the motor runs OK, you have a blocked vent on the tank. Fastest and easiest way I know to diagnose fuel delivery problems.

 

[slick]

Thanks DaleH
I didn't word my thoughts very well.
I've had this issue once myself.

 

[onthewater102]

Premium vs. non-premium gas is not the issue - the only difference is how much compression they can take before spontaneous ignition becomes a threat. I'm assuming this is basically the same engine as my 20hp '84 Mariner - in which case the "fuel pump" as so named is really not much more than a cast plate with a rubber membrane screwed to the side of the carburetor body - IE no big deal to "rebuild". You've already got a fuel filter between the quick-connect and the intake to the pump, so you're all good there assuming the filter is not half full of water.

Parts and diagrams are available here:

https://www.marineengine.com/parts/mercury_marine_parts/sn/06535686/%20%207020204/274_250
https://www.marineengine.com/parts/mercury_marine_parts/sn/06535686/%20%207020204/274_90

Not knowing about the impeller is definitely cause to replace it too - you're asking for big problems running a used motor with no idea about the impeller maintenance history. Fortunately, that's another easy DIY fix on these motors, parts & diagram here as well:

https://www.marineengine.com/parts/mercury_marine_parts/sn/06535686/%20%207020204/274_180

All in for $150 you should be able to get the full carb rebuild kit & the impeller/gaskets and spend 2 hrs and you're working with a top shelf setup. These engines are beasts and will run forever, but if you overheat due to a bad impeller then you're running the risk of ruining your powerhead and trashing a really wonderful motor. There are plenty of you tube demonstrations for how to do this too - any of the mercury/mariner 18hp, 20hp and 25hp videos will work.

 

[MarsAngler40]

Thanks for the responses, I get the boat back in 2 weeks and ill update once i start working on it.

 

[onthewater102]

I've rebuilt mine top to bottom so if you have an questions feel free to hit me up.

 

[Boat2fast]

I had an older Merc with problems like that. I wasn't opening the tank vent. It was already mentioned; I'm repeating it.