1985 Mercruiser 120/MCM120 2.5L Fuel supply issue

[ScooterMcTavish]

Hi All,

Been working on the restoration of a 1986 Glasstream runabout, and had a fuel issue that I hoped someone could help with. Problem is that there are no boat part dealers near where I live, and I'd rather diagnose before ordering parts. The engine is a 1986 Mercruiser MCM 120 (GM) I-4 153 CID/2.5L engine with the "old style" fuel pump on it.

When I bought the boat last year, I knew this boat may have sat for some time before I bought it. So, I drained the fuel tank, put in fresh fuel with stabilizer in it, and it started, though it took quite a bit of cranking. Did other maintenance items such as new plugs, and I have a few more new parts to put on, plus have what I need for an oil change. FYI, I am a noob to 4 stroke boat engines.

Never did have time to get it on the water last year, though when I closed it up for the winter, it wouldn't start. And when I went to open it this summer, again it would not start.

On the outside chance something in the carb was sticking, I sprayed carb cleaner in. With the carb cleaner in the system, the boat will fire. So obviously I had spark and compression, and was dealing with a fuel issue. So first I opened the gas cap, and there was no sucking or pressure noise. The gas tank vent seems clear, and there are no kinks in the lines.

So when I disconnected the fuel line from the tank to the fuel pump, there was a couple of drips of fuel in the fuel line, but there was backpressure in the pump, as a decent stream of fuel came back out of it.

A couple of questions I have:

#1 - Is the choke automatic on these boats? I have no choke lever anywhere, and there is no "push to choke" on the key like I've seen with 2-strokes.
#2 - Should there be a fuel primer bulb? 2 stroke boats have these, but cars do not, so I figured "no"

There is no water separating filter between the tank and the pump.

So my thoughts here are one of two things:

#1 - The fuel filter in the pump is plugged
#2 - The diaphragm on the fuel pump is shot

To me, the back pressure suggests this could be as simple as a filter, as the pump is pulling - there's just nowhere to send the fuel to.

However, I welcome the opinions of those more experienced than me on these things.

And once I get this solved, I need to figure out how to change the lube in the outdrive.

 

[alldodge]

Howdy
1) Yes choke is automatic and is a plate on top the carb. With the engine cold it should be closed except about an 1/8 inch opening.
2) I/O do not have primmer bulbs.

You fuel filter should be part of the fuel pump and it does sound like it's clogged. If the fuel pump was not change to an automotive type there should be a thumb wheel to remove which will allow access to the filter. If it does have an automotive type pump "GET IT OFF AND BUY A MARINE TYPE". If you don't we could be reading about you some day soon.

Spraying carb cleaner down the throat of the carb doesn't help a lot. What you probably need is to rebuild the carb

The link below should be the type of pump on your engine
http://www.iboats.com/mall/partfind...&gd_poid=112251&gd_row=1&session_id=049747381

 

[ScooterMcTavish]

Thanks for the answers, and you can tell I come from the world of two-stroke outboards.

Fuel pump appears to still be the original pump, and matches what I saw in the service manual, so I think we're all good there. Not expecting to explode any time soon.

Not sure the carb needs a rebuild yet, but once we get the filter changes up, I should hopefully have a better idea.

 

[ScooterMcTavish]

So I pulled the fuel filter last night to see if the boat would start without it. New ones are in the mail and should be here tomorrow.

Regardless, lots of firing, but the boat just will not start. It almost started the last time I tried it with a long cranking period, but then the battery got a little too weak.

So, is this a case of the boat needing to build pressure and refill the lines after a full season off, or could there be an airlock in the lines, or could we have a bad pump diaphragm, or could the carb need a rebuild?

If anyone had some ideas on a next step, it would be great.

 

[MikDee]

Take off the flame arrestor (boat air cleaner) and open the choke flap manually then look down into the carb after pumping the throttle a coupla times to see if it's getting gas? If so, I'd say your fuel pump is working.

 

[ScooterMcTavish]

Success!

Boat started tonight, but only after I put the throttle in "Throttle Only" mode, and put it about halfway to full throttle. Engine ran at a good idle speed.

So this makes me think that my neutral throttle position is not good, and will need some adjustment. Would likely be easier if the tach someone added was working but the fuse holder was shot and the accessory switch is broken off.

Any tips for adjusting the idle? And is this simply what it could be or should I be concerned about something else? Have to say the throttle pieces seem to have a bit of play in them and one rod looks non-factory bent.

 

[ScooterMcTavish]

Actually answered my own question - I took the inside panel that the throttle was attached to off last year to clean everything up. I'm guessing that when it was put back, it may have been put back a little farther back than it was originally there. This may have introduced some slack to the cable.