Mercruiser 120hp 2.5 wont start....fuel pump??

[Reeddawgy]

Hey guys, I took the boat out on the lake and after cruising around for a bit, fishing and stuff it started acting like it wanted to cutoff. Finally it did and I couldn't get it started again. I remember it being hard to start at the put -in ramp as well.I removed a plug and checked it for spark (good) and replaced the fuel filter. I believe that it is the fuel pump. Can I just unscrew the line from the fuel pump to the carb. and check it for flow. Maybe there is another technique for troubleshooting this. Could there be other reasons for it not starting. 1984 Mercruiser Inboard 120 hp, 2.5., MCM 120, Serial #6769768. What is the PSI for this fuel pump, and can I put in a Electrical one in place of the mechanical one. I noticed that there is the main aluminum fuel line going from the pump to the carb and also a smaller yellowish clear plastic/rubber line doing the same. I assume it is a vent of some sort. Just trying to figure out how to set this up with a Electrical pump in its place. Thanks in advance. Any help would be greatly appreciated. New to the Boating game.

 

[Coors]

Re: Mercruiser 120hp 2.5 wont start....fuel pump??

Get a cup under the line and spin it over; put that in a glass jar and let it settle. Water will collect at bottom of the jar if it is present. You don't use a marine electric pump, and you ain't set up for one. Fix what you have.

 

[searay3]

Re: Mercruiser 120hp 2.5 wont start....fuel pump??

The clear line is marine standard that flows fuel back into the carb when the diaphragm fails. It should be clear with no fuel in it. If there is fuel in the clear line, you have a failed fuel pump. There is also an anti-siphon valve at the fuel tank that could be your issue. Try running it on an outboard tank and see if you still have the issue. You could put on an electrical pump, but why? The original pumps work well for many years. If yours has failed and you want minimum hassle, replace it with what it has...imo.

 

[Haut Medoc]

Re: Mercruiser 120hp 2.5 wont start....fuel pump??

Well, one way to check is to pour a little gas in the carb & see if it wants to run....
Did you see any fuel in that yellow line?
If you do need a new pump, I would stick with a mechanical one....
It will bolt right on.....You will need to add some wiring & switches if you go electric....
There is no real benefit to an electric pump on your rig......
Fuel pump pressure is 4 PSI......​

 

[Reeddawgy]

Re: Mercruiser 120hp 2.5 wont start....fuel pump??

Ok, Thanks guys for your quick responses. Now, I opened the fuel line and caught some in a cup. Did not see any separation of water from the fuel. I cranked it with the line still opened and saw fuel pumping out. Looks like the fuel pump is working. I poured fuel into the carb and still no start, just cranks. The Yellow clear line had alittle oil/fuel mixture in it, not much at all. I drained it, It was dark colored and it seemed to be oil/gas. I smelled it but really couldn't tell. Maybe its probably something else im guessing.

 

[achris]

Re: Mercruiser 120hp 2.5 wont start....fuel pump??

Start with a compression test, then check points and condensor (get a 'tuneup kit' from Mercury, that's got points, condensor, rotor). Check timing, then start looking at the fuel system.....

Chris............

 

[Reeddawgy]

Re: Mercruiser 120hp 2.5 wont start....fuel pump??

Thanks for the advice ACHRIS and everyone else also. Will try that and will let you know how it turns out. Thought about that alittle already.

From one Chris to another!

Chris

 

[Reeddawgy]

Re: Mercruiser 120hp 2.5 wont start....fuel pump??

Started thinking about it today at work and remembered that the butterfly on the carb stayed wide open and never moved when i was working on it. Came home and tried starting it after I manually partially close the butterfly and it started. The butterfly does nothing when I throttle up and down. Its like a spring or a attachment rod is missing from it. Or does the idle screw control it when you throttle. Carbs kill me.

 

[Don S]

Re: Mercruiser 120hp 2.5 wont start....fuel pump??

The choke is not attached to the throttle linkage at all. Your's should have a tube going from the choke to the manifold for hot air to go thru. Later models used electric chokes.
They are normally closed when the engine is cold, when started the choke coil is heated and opens the choke completly.
They should be wide open with a warm engine.

 

[Reeddawgy]

Re: Mercruiser 120hp 2.5 wont start....fuel pump??

Ok, just looked at it and it does have an electric choke bolted to the front side. I opened it and it has a coil in it. I closed it back up and all of a sudden it seemed to be closing the butterfly. I moved the butterfly with my fingers and now it seems to have some tension on it. I then started it and it opened up a few seconds later. Maybe the choke coil slipped of of the lever that controls the butterfly and when i put it back on it started working properly. Strange, is this common? Does the coil act like a thermocouple/heatsink (expand/ extract)? When it goes bad what are the symptoms? Dont want to get stuck out on the lake if all i need to do is replace the electric choke. Maybe I should just replace it!?