40 hp 4 cyl Merc stalls instantly at full throttle

[Rooster7]

I've searched other threads but have not found a difinitive answer yet. Can someone help?
Last summer I had my carbs and fuel pump gone through on my 40 hp 4 cylinder mercury outboard. (I was told its a 1991 but looking up the serial number I found its a 1989) The motor worked fine last year after that.

2 weeks ago, I rebuilt the water pump and it runs great on the muffs. When I put it in the water it starts and runs good. It runs good at low rpms and I can go full throttle for about 30 seconds and Poof...the motor stalls instantly. I have to crank and crank and it will finally start to fire a little and then I lift my fast idle lever a bit and it will start. I can then go slow but as soon as I go full throttle it stalls again in about 5 seconds. There is no sputtering or slowing down first. Just instantly dies. Is this an electrical problem?
Any idea's??

 

[tredragon]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

because it starts hard and doesn't do well as you add throttle... I would perhaps guess it's running lean?

I'm dealing with a similar issue and have been tuning the idle mix. Have to take it out soon, warm it up in the water, then gun it and see how my tuning did. Bringing a long flat-blade screwdriver with me so I can tune on the water too. Might work for you too.

There is a link that I'll try to add that will give you the diagram for your motor so you can find the parts you may need to adjust.

 

[Rooster7]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

It actually starts easy and runs good cold. It is when I "open it up" for a bit that it stalls right now. Then after that it is hard to start. It is almost like something gets warm and kills the engine instantaneously. Does this make any sense?

I would appreciate that link! Thanks!

Rooster

 

[CharlieB]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

You need another hand, to pump the primer bulb.

I suspect the fuel pump valves are failing and you are simply running out of fuel, the pump is only pumping enough to idle.

Fuel pump, air leaking into a fuel line, pump the primer bulb as see if it stays alive.

 

[Boatleech]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

A friend's boat has a similar problem. Mercury 40 2-stroke, recently overhauled and bought as a replacement for a stolen Yamaha. It was very hard to start last night, and after 45 minutes or so of running fine, it stopped suddenly and stranded us on the lake. I think it's a fuel problem, but don't know (yet) how to approach it. A clue might be that the primer bulb never gets hard when you pump it. I'm going to start by swapping in a portable tank with another hose and primer bulb to verify it's a motor problem. Any tips, and help (especially with respect to the fuel pump on this thing) would be appreciated.

 

[tredragon]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

at any rate it sounds like the concensus is that you are starved for fuel when she dies like she is. If it starts fine and opens up fine at first, and only has problems after running a while... I tend to agree that the fuel pumps are going.

 

[Rooster7]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

I just had a new fuel pump kit put in less than a year ago. The bulb and line are new, stays hard and gas is getting to the motor. One gentleman told me that it could be the plugs since the water pump was going bad it might have screwed them up. But if it was just plugs wouldn't it be missing?

 

[Rooster7]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

A friend's boat has a similar problem. Mercury 40 2-stroke, recently overhauled and bought as a replacement for a stolen Yamaha. It was very hard to start last night, and after 45 minutes or so of running fine, it stopped suddenly and stranded us on the lake. I think it's a fuel problem, but don't know (yet) how to approach it. A clue might be that the primer bulb never gets hard when you pump it. I'm going to start by swapping in a portable tank with another hose and primer bulb to verify it's a motor problem. Any tips, and help (especially with respect to the fuel pump on this thing) would be appreciated.

Sounds like the fuel line is bad. Could be just a pin hole.

 

[tredragon]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

definitely seems like you're starved for fuel when vaccuum is needed. if you use a fuel separator... you could technicallly dab water on the line pre-separator and look for bubbles (ie. the old "hole in the tire" trick). I'm really no expert though

 

[Rooster7]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

If it was starving for fuel though wouldn't it sputter or drag a little bit before killing? This just dies instantly.

 

[tredragon]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

mine would go from nice idle in gear... to as soon as i'd throttle up it'd go bup bup buuu... clunk... dead. Should take much to turn the screws back out CCW a little and try her. just keep track of how many turns your making on each carb and in which direction. a small notepad would be handy if your short term memory is like mine.

 

[Rooster7]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

mine would go from nice idle in gear... to as soon as i'd throttle up it'd go bup bup buuu... clunk... dead. Should take much to turn the screws back out CCW a little and try her. just keep track of how many turns your making on each carb and in which direction. a small notepad would be handy if your short term memory is like mine.

I'll try it. Thanks!

 

[Boatleech]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

I tore into the fuel system on the motor mentioned above, and found:
- good flow of clean fuel from the primer bulb on the input side of the fuel pump;
- weak flow of dirty fuel on the output side of the fuel pump with the starter spinning the engine with spark plugs removed. The dirt in the fuel was like tiny black sand. Carbon, maybe? Where did it come from? No idea how long it's been accumulating.

I disassembled the fuel pump. It was loaded with the black particles, but the diaphragms were intact (but looking pretty ratty). I rebuilt it anyway, and now I get a weak flow of clean fuel out of the pump when testing. I also replaced the inline fuel filter, which was full of the black particles.

The engine now starts OK; will try some running tests later today.

Can anyone comment on why the primer bulb never gets hard? That gas has got to be going somewhere. The only places I can see are through the oil-line check valve to the oil pump, through the fuel pump into the crankcase, or into the caburetors.

I'm pretty sure this is a 1994 engine, based on dates on the starter and switchbox. Transom mount s/n is bad, have yet to find any other s/n info on the engine itself.

 

[FLBoy70]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

Does it have an electricaly opperated choke solenoid connected to vacume tubes that connect to the intake manifold behind the carbs?

If so check them for leaks too.

 

[CraigC]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

Boatleech: Those little black particles are most likely your fuel lines disintegrating, or the primer bulb, either way I'd replace them. Those little pieces are going to get into your carbs and cause problems at some point if they're not filtered out. It also could be junk from the fuel tank, if you can see into it I'd shine a flashlight in and have a look.
If it's the tank add a big inline filter to the line before the primer bulb - just get one at the auto parts store for the correct size fuel lines. They have some made of clear plastic, I'd use one of those so you can keep an eye on it. (FWIW- I put one on my boat for extra filtering, works fine)

 

[Boatleech]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

The particles don't show up until after the fuel pump, but the fuel lines from the tank were replaced recently, so they may be residue from disintegrated fuel line, primer bulb, etc. Everything looks clean now, so we shall see...

 

[Rooster7]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

Ok.........back to my problem...

 

[Boatleech]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

Right. Sorry to hijack your thread. Was the inline fuel filter replaced on your motor when the pump and carbs were done? One thing to try -- as soon as it fails, take the "out" hose off the fuel pump, replace it with a length of clear tubing, and see if cranking produces good fuel flow. You could also replace the hose from the fuel filter to the upper pump with a piece of clear tubing (temporarily only) and see if there's good fuel (i.e., clear, no air, no crud) in it when things fail. You could also run the motor with a fuel pressure gauge T'd in to that line to monitor fuel pressure.

Much as I like the idea of a fuel problem, though, I'm more inclined to be suspicious of the electrical/ignition system. It's possible something is breaking down when it gets hot. I had an airplane once with old magnetos, and it wouldn't start at all when hot. Until I replaced the mags, I just sprayed them with a water hose and it'd fire right up once they were cool. I'm assuming that your motor starts and runs OK (for a while ) if you let it all cool down and try again.

Since everything shuts down instantly, it's something common to all cylinders, so it's likely not a plug, plug wire, or coil problem. Common elements include the stator, trigger, mercury switch, stop switch, and switchbox. I don't see how heat would affect either the mercury switch or the stop switch, but you could disable them just to eliminate them. You could try pouring a bucket of water over the top of the motor to cool down the stator and trigger (better, maybe -- shoot them with a C02 fire extinguisher), and see if it will start when they're cold. You could take off the switchbox cover and give things in there the sniff test -- electronic components have a distinctive order when they're overheating and failing.

Good luck...

 

[bass186]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

I woulndn't pour cold water over any hot motor lookin for some warping/cracking issues.

 

[Rooster7]

Re: 40 hp 4 cyl Merc stalls instantly at full throttle

Yeah I don't know about the cold water or fire extinguisher. I do agree though that it might be something electrical. I'll keep ya posted when I find it. Just haven't had time to mess with it much lately.