Merc fuel issue

[belercous]

My buddy's boat engine has problems. It is a 1994-96 (?) Mercury tracker, 115 hp. It starts (with push-to-prime key switch) and runs for a bit, then slowly idles down til it dies. Pushing the primer keeps it going and revs the engine. Same if squeezing the primer bulb on the fuel hose.
The gas is fresh. Switching to another tank, same result.
The fuel filter is clear (blew thru it, no resistance).
The fuel pump seemed fine (check valves worked, no holes in diaphragm) but we rebuilt it as the diaphragms were stretched. No change.
Just for kicks, I changed the plugs out for new ones. No change.
We can keep the engine running using either the primer bulb on the fuel hose, or the push-to-prime key switch.
The boat sat out last year out of the water. We really don't feel comfortable taking it out for a full throttle run as it is now on the Mississippi and has no back-up engine.
I am a mechanic, and am familar with 2-strokes, but have never worked on a boat motor before. I am fairly certain it is a fuel issue, and the motor seems to be carburated. No fuel leaks are apparent, the primer bulb (on the main inlet fuel hose) stays hard and we have switched tanks & hoses (2 removable tanks) with no change. It runs good when started (& primed) then slowly runs out of fuel. Lifting the prop out of the water and engaging the transmission only allows the engine to spin at about 2,000 rpm. Squeezing the primer bulb or pushing the primer in makes it rev to 5,000 and sounding right.
Any advice would be most appreciated.

 

[usskydvr]

Re: Merc fuel issue

Sorry if I get on your case a bit, but you state that you are a mechanic (no outboard experience), but you say the engine 'seems' to be carburated. It is, as long as your yr/model is correct Mercury does make a 115 EFI . Clearly you have a bad fuel pump.

 

[belercous]

Re: Merc fuel issue

I worked on aircraft which have inlet port injection, not throttle body injection. These look like carburators to me from what I can see, but I haven't had one off yet. I can see the outlet of the main jet at the rear of the carbs, but no idle jets near the butterfly valve.

The fuel pump is good, I replaced the diaphragms even though there were no holes in them as they could have been stretched. The pump does pump, ops check normal. Cranking the engine makes it spit gas. As it is a pressure operated diaphragm pump (low pressure output), I assume I'm dealing with carburators.

We've ran about 4 gallons of new gas with injector cleaner mixed in at the dock. Still the same. Under a load, the engine leans out & dies. It doesn't want to idle & the idle speed screw does nothing for the idle speed.

I can see no pinched or collaped hoses.
With the intake plenum off, it gets too much air to run. I am pretty sure its a lean mixture, but I can't figure out why.

 

[Coloradolakeboy]

Re: Merc fuel issue

Try running clear hose in between fuel pump and carbs. Check for air bubbles
It's sounds fuel side related.

http://forums.iboats.com/showthread.php?t=283363

 

[belercous]

Re: Merc fuel issue

Good idea, and I'll check it. However, as I just replaced the diaphragms in the fuel pump, I can't see how air could get in the system. Also, even if air did get into the system, the floats wouldn't shut off the fuel intake in the carbs. From what the output of the fuel pump looked like, the engine could not be using as much gas as it put out.

 

[Coloradolakeboy]

Re: Merc fuel issue

My merc manual says if the engine idles rough and stalls or hesitates on acceleration to check:

1. Water and dirt in fuel
2. Fuel tank vent restricted
3. Pinched, cut or restricted fuel line or filter.
4. Low fuel pump pressure
5. Anti siphon valve.
6. Needle or seat in carb is stuck preventing fuel from entering carb
7. Restricted jet
8. Improper carb float level.

 

[belercous]

Re: Merc fuel issue

No water is in fuel, we put in alcohol which is agroscopic, so that is not an issue, espescially after running 4 gallons of gas thru it. Fuel filter is clear of obstructions.
Fuel vent is open, on both tanks (ship & portables).
All fuel lines are open, none pinched or collapsed.
Fuel pump has been rebuilt, output is more per stroke than engine could possibly use.
The engine runs, but lean. I would think that after 4 gallons of new fuel, with carb/injector cleaner mixed in heavily, that this would not be a problem. Espescially with 4 carbs and all cylinders seem to be running, none missing/not firing. All plugs look the same so its not like I have a cold cylinder or two.
If all 4 carbs have a restricted main jet, I don't see how. I've dropped in straight "Sea-Foam" cleaner to each of the main jets and let sit. The engine is getting gas in all 4 carbs, but just not enough it seems. Very puzzling.'t't
It doesn't seem probable that all four carbs would suddenly get an improper float level. If the floats sank, it would run rich. The engine seems to run lean as hitting the push-to-prime ignition switch button makes the engine rev up, but holding it in makes it flood.
This is a baffler.

 

[j_martin]

Re: Merc fuel issue

Put a piece of clear (vinyl will work) hose in the line just before the engine. You'll probably see bubbles there. Sucking air will often kill it at idle. Can also lean it out at WOT and burn down a cylinder, so needs to be fixed. Often the culprit is the pickup tube in the tank.

hope it helps
John

 

[belercous]

Re: Merc fuel issue

j martin; that sounds like it could be it. We switched from the onboard tank when I checked to see if the fuel pump was working & noticed the gas was turning. The portable tanks always worked fine before, but we had to get a different hose when we noticed the engine would only run good up 3/4 throttle. The old hose was for a smaller engine, but was a Mercury part. The new hose is not a Mercury part and doesn't snap into the portable tanks like the old one.
If its drawing in air, this would be the place. I really hope so as I'm not looking forward to removing the carbs. It still looked like (when cranking the engine) that the pump was putting out more fuel per stroke than the engine could use, but I'm gonna try this before I start taking more things apart.

 

[belercous]

Re: Merc fuel issue

Just thought I'd let you all know the outcome.
No air in fuel line using clear tubing.
We used a bunch of mechanic-in-a-can (sea foam, b-12), which helped a bit. Got the motor running of its own volition, but not totally right. On by-pass (transmission) mode it would run at 2400 rpm, but hunt up to 3600. Got closer & closer betwenn steady & rev. Still not good enough under load. It would die when shifting from neutral to fwd. or rev.,but hitting the push-to-prime hept it alive. This was a real joy manuevering around the marina.
I then decided to look at the accelerator pump fuel filter as the engine would die on sudden advances of the throttle. I kew it was there only because of a schematic drawing. Took the intake plenum off and moved the oil resiv. Finally found it. It was clean.
While I had the plenum off, I took off a carb. A bit of crud was found in the main jet are & float bowl. So I took off the other 3 carbs, and more crud/shellac was found. Put back together, and it ran like a sewing machine. We tested on the river (Mississippi), which, at Alton, was about 20+ ft. over flood stage. We ran it upstream to Grafton (about 20 miles away), and the engine didn't miss a lick.
This was a classic case of not runnig the engine dry (and then choking it to run some more) before letting the boat sit a year. This could have been avoided by proper "pickeling" of the engine.
If I had to do this job for money I would have charged the owner over $200.00, IF I KNEW WHAT THE PROBLEM WAS, not including trouble-shooting. And working on a boat on the water made it that much more challenging as dropping a tool or part/screw in 17 ft. of water would not have helped.
I would like to thank all those who have shared advice with me as I am new to marine engines.