'76 Merc 850 Leak at Fuel Pump

[BR]

I just rebuilt my fuel pump replacing the gaskets and diaphram. Also have recently replaced the fuel lines and done a carb clean and rebuild. My problem is when I put everything back together and pumped the fuel line bulb to prime the carbs, I noticed fuel leaking from around the plastic fitting over the fuel pump. I don't like this design because there's a fuel line hose fitting on a circular plastic piece which is held over the fuel pump and fuel filter screen with a single screw at its center. There's a rubber "o" ring under the piece which is supposed to seal it but apparently isn't. I don't want to over-torque the screw holding this plastic piece because I'm afraid of cracking it and its a $50 part!! I went about as tight as I thought I could go and it's snug.

I double checked the check valves in the fuel pump and they are installed properly. I did think it took a surprising amount of breath to force the inlet valve open. Do these check valves need to be cleaned / replaced periodically? My only thought is that the valve might be sticking allowing too much pressure to build in the fuel line and causing the leak at the "o" ring seal. I don't really want to add a gasket-maker sealant and am even doubtful if anything would last in direct contact with gasoline. Any suggestions / tips?

 

[SeaKaye12]

Re: '76 Merc 850 Leak at Fuel Pump

I have the same engine and have had the same problem.

I find that even when an o-ring looks fine, it can be compressed from previous use . Have you replaced the O-ring with a fresh one?

The last time I purchased an O-ring; it was identified as a #2-217. O-rings come in a lot of sizes, and it would be very easy to select one that would be a size or two off from correct. You might try one just a step or two larger from what you have now.

Let me know what happens...

Chuck

 

[BR]

Re: '76 Merc 850 Leak at Fuel Pump

Chuck,

Thanks for the reply. I did replace the "o" ring with a new one that came in a Quicksilver fuel pump rebuild kit so I assumed it was the right size. I would say that the diameter was right on the money but the thickness of the ring looked a bit thin to me. Maybe I'll go up an "o" ring size.

Thanks,

BR

 

[Yepblaze]

Re: '76 Merc 850 Leak at Fuel Pump

If the plastic part is cracked or broken, it must be replaced.

 

[SeaKaye12]

Re: '76 Merc 850 Leak at Fuel Pump

I've seen metal versions of the plastic cover; beefier looking....maybe an aftermarket replacement. It had a larger bolt with washer at the center instead of the tiny little screw that comes original equipment.

I'd look for one of those if I ever need to replace the plastic one on my engine. The plastic one is rather feeble; especially weak looking where the fuel line attaches to it.

Chuck

 

[Chris1956]

Re: '76 Merc 850 Leak at Fuel Pump

BR - Loosen the screw a bit, and wiggle the plastic cap side to side, and then tighten it a bit. They don't usually leak. The o-ring is on top of the screen, correct? You have the nylon washer under the phillips screw head?

 

[BR]

Re: '76 Merc 850 Leak at Fuel Pump

Chris,

Yes, I have the "o" ring on top of the filter screen, and yes, I have the nylon washer under the phillips screw. When you say wiggle the piece and re-tighten I assume that the plastic cap piece is supposed to fit inside the rim of the metal casting of the fuel pump, right? The diameters are so close I can't really tell if the plastic cap is seating on the "o" ring or the rim of the aluminum casting of the fuel pump body. I'm pretty sure it's supposed to seat on the "o" ring for a more positive seal, right? Also, what do you think of my theory of the check valves causing too much back-pressure on the "o" ring seal? Has this ever happened to anyone? BTW this motor hasn't been run in a long time. Would this have an effect on the check valves at all. Should I soak them in fuel to soften them up?

Thanks....BR

 

[Chris1956]

Re: '76 Merc 850 Leak at Fuel Pump

Replace the check valves if you have trouble with them. The plastic cap fits inside the metal pump body, and pushes down on the oring causing the seal. I find that they do not need to be too tight to seal well. When I said wiggle it, I thought you should move it side to side to seat the plastic cap on the oring.

BTW- under normal operation there should be a vacuum in the fuel pump at the plastic cap, not pressure. If you have pressure, the check valve isn't working.

 

[BR]

Re: '76 Merc 850 Leak at Fuel Pump

Chris,

Agreed that under normal operation (engine running) there is vacuum under the plastic cap. However, I experience the leak when I am pumping the fuel bulb which forces fuel into the fuel pump under pressure. Even slow pumping of the bulb causes it to leak. The check valve is supposed to open to allow the fuel to pass through to the next chamber but instead it's leaking around the cap. I'm gonna take it apart again and start over.

Thanks....BR

 

[Chris1956]

Re: '76 Merc 850 Leak at Fuel Pump

Fuel will only pass thru the check valves until the carbs are full. Then the fuel pressure will build as you pump the bulb. If you pump hard enough, something will give, and leak.