Old Merc Fuel Delivery Issues

I’ve got a 74 Ranger with a 74 Merc 650. I’ve had some little fuel delivery issues here and there that have, over time, been cleared up in a number of ways… new primer bulb or lines, new tank, Seafoam (of course).

Every time I experience a problem, I do something small to fix it, and it’s good for a while…. this last time, however, couldn’t even get her started for a while. FINALLY, I got her running. After she got warmed up, I’d have to keep priming in order to keep her going. She’d start to die… give the bulb a squeeze, and it revs back up.

Okay, now this is the same kind of issue I’d had several times before… and like I said, replacement parts (which were needed anyway) or Seafoam with fresh gas always did the trick…. but not this time.

So, I pulled apart and cleaned the carbs yesterday, and the only thing that really stood out to me was the needle valves were a little sticky. Freed those up, cleaned everything else, and and started puting everything back together.

I’ll give it another try tonight… but what can you guys suggest if I continue to have problems?? What else could it be if the only way to keep it running is by continuously priming it? Fully primed, it runs great and sounds great… until the gas in the carbs burns up.

As far as I can tell, fuel should be flowing great from the tank to the carbs right now… So we’ll see tonight.

You must be a master mechanic to keep a rig that old running. Boats are not or never were made to last for 30 years. They have a newfangled thing called EFI now that is something every boat should have.

Bad fuel pump or fuel line restriction/ gas tank venting problem are my first guesses. What are you using for a gas tank?

Tank is plastic, bought new last fall – its vent is… venting.

Definitely no restrictions in the fuel line.

So, if I still have problems tonight, you think the pump is the likely culprit?

The fuel pump is new territory to me…. how does it even function??? There’s nothing electrically associated with it from what I can tell. Fuel goes in, fuel comes out, and into the filter. Basically, there’s a gasket and a diaphragm… I’ve never felt inclined to remove either of those things. They seem to be doing their jobs, however.

What else is there to the pump?

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You must be a master mechanic to keep a rig that old running. Boats are not or never were made to last for 30 years. They have a newfangled thing called EFI now that is something every boat should have.

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2 weeks ago I would have told you to bite your tongue since my 200 carb yamaha was running like a champ. Unfortunately, I blew her up since then

complex,

Your fuel pump is probably operated by crankcase vacuum, which occurs in pulses due to the pistons moving up and down. Outboards have been using vacuum fuel pumps since the dawn of time, so take a close look at the diaphram, which make be cracking or have a pin-hole in it.

Found this at: http://www.ehow.com/about_5171996_parts-outboard.html

The Fuel System

An outboard motor fuel pump sends fuel from the tank through a diaphragm located inside the pump, separating the crankcase side from the pump side, according to Thiel. The unit works on pressure rising and falling, which is generated by the piston moving up and down in the crankcase. It is the simplicity of this outboard system which ensures its reliability and it is rare that an outboard fuel pump fails. On the negative side, because of their small size, outboard fuel pumps are unable to pull fuel all the way from the tank. They need to be primed manually through the use of a squeeze bulb located in the fuel line. Once primed, the fuel travels through the flapper valve into the carburetor where it is mixed with air and sent into the engine. In the event the engine is cold, outboard motors have a second flapper valve upstream from the one in the carburetor, called the choke plate. It is activated by a solenoid, which opens the throttle when the key is turned. Thiel reports that newer outboards have solenoid-operated systems that shoot gas into the carburetor, eliminating the need for a choke plate.

HRG

Good info!

So, while I had the pump off last night, I took a good look at the diaphragm after priming it up. If there had been a pinhole or crack in the diaphragm, I assume I’d probably have noticed it at that point… but I didn’t observe anything out of the ordinary. I applied a little pressure to the diaphragm looking for leaks, and it looks good, so I’m probably in good shape there, no?

Aside from all else, I guess the only other thing could be air entering the fuel lines from one or more of the hoses somewhere.

There is a short length of hose leading from the pump to the filter that is cracking some… I’ll replace that tonight, and check the others again while I’m at it.

Tanks for the help everyone… and my offer to sell you the Ranger still stands, Phish… Just sayin’

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Good info!

So, while I had the pump off last night, I took a good look at the diaphragm after priming it up. If there had been a pinhole or crack in the diaphragm, I assume I’d probably have noticed it at that point… but I didn’t observe anything out of the ordinary. I applied a little pressure to the diaphragm looking for leaks, and it looks good, so I’m probably in good shape there, no?

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Not allways the diaphram itself gets stretched to the point it will not function as it should. If the diaphram was not nice and flat when layed on the bench it`s about time to replace it. Also the little flapper on the diaphram that opens and closes to allow fuel to the carb could be leaking allowing fuel to run back through the pump.

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so take a close look at the diaphram,
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I’d agree with this statement. And if the part isn’t available, it can be made. Good luck. Let us know how it turns out.

Alright… not-so-good news.

I decided to take another look at the diaphragm, as well as the float needles. Diaphragm is definitely stretched out, and one of the needles is shot.

Ordering a couple bits and pieces right now.

In the meantime, the fuel line that delivers gas from the pump to the filter… while it looked kinda bad, I had no idea it was about ready to fall apart entirely! Got that replaced.

Also, just to see how things were progressing in terms of the fuel lines, I attempted to prime it up tonight only to have fuel dumping out everywhere!

Man, I must have REALLY overlooked something in my haste to get it back together. Unfortunately, it was dark at this point and the mosquitos were getting bad, so I called it a night. All I can say for sure is it appeared to be pumping out near where the lines connect to the float bowls. Gonna have to tear it apart again tomorrow!

well, the good news is that you’re starting to find the problems. The better news is that you found them before you had a fire out on the water!
HRG

And the better good news is it’s still easy to find these kinds of parts

Replace all fuel lines and in your case with an older motor you really, really should be running non-ethanol. Ethanol and old gaskets and fuel lines (though you should be replacing all the fuel lines) do not go together well. I would replace everything, make sure nothing broke up into your fuel system and try it again before messing around with the pump.

I did a quick Google search not sure of the correct part number but a fuel pump kit for that motor is roughly $2.95 to $5.89 so why not replace it. The new material should be more compatible with todays fuels.

That’s the plan

Fuel lines have all been replaced, all the gaskets were replaced last year.

I’m waiting on the order right now.

As far as I can tell, the pump kit and new needles/seats are going to do it for me. Needles probably haven’t been replaced in a decade. I just need to figure out how to adjust the float level.

There isn’t a whole lot to it, just a little spring coming out of the top of the floats.

??

Should be back on the water this weekend