I'm just wondering because today I believe it gave up the ghost. The way I see it it, it can only be one of three things. Completely worn lever, bad diaphragm or a bad check valve. I was going for a test drive after changing to a 500 cfm edelbrock from the 1406 and I made it about an 1/8 mile up the road and it quit. I was lucky enough to be able to roll backwards and right into my driveway. I put a half tank of gas in it thinking I had ran out of fuel but still can't get fuel past the pump. I blew out the lines and the FPG still registers zero and can't get any fuel past the pump. Tomorrow I'll be putting on a new pump and all soft lines and a canister filter that is good to 10 microns. hopefully that will fix it. Anyways, how can these things fail? 351C btw.

 

I have personal experience only with one, and another is theoretical.

1) The diaphragm can fail; when this happened to me, the crankcase filled with gasoline (it siphoned from the tank).

2) Incorrect installation... the pump's arm needs to be correctly oriented with the, um, whatever that part is that attaches to the cam and pushes down on the arm, I forget its real name.

 

I thought the same about the fuel going into the crankcase. I already smelled the oil. I didn't smell any gas. I guess that rules out the diaphragm.

 

Kind of strange that it quite immediately after changing the carb. Any chance you pinched a hose between the pump and the carb?

As for answering the initial question, there has to be a check valve in the pump or it couldn't pump. So, it is possible for it to stick shut. And, if you don't have a hose pinched that sounds like what the problem is.

Btw, what 10 micron filter are you installing? I need to consider that aspect as heretofore I've been using the cheap see-through plastic filters.

 

No pinched or clogged lines anywhere. I've blown them all out with the compressor just to be sure they weren't clogged. The filter at the top of the page is the one I picked up here locally.

Professional Products Online

 

If you suspect it may have run out of fuel, but cannot get it to pickup fuel after adding some to the tank, try pouring a little into the carb and start the engine. This may need repeated a couple times.
Mechanical pumps can have a hard time getting the fuel drawn from the tank, and pumped up to the carb, at the lower RPM of the starter. Especially if the rubber line(s) have even the slightest hint of a crack allowing air to be sucked in.

As for the ways in which these can fail, 2 main symptoms are leaking or not pumping fuel. Leaking can be internally or externally, from either a bad diaphram or cracked housing.
Not pumping fuel can be from the check valve (bad or crudded up), broken lever, or busted housing/pivot pin where the lever pivots.
There's surely more ways they can fail, but those are the main causes in 90% (or more) of the failures.

 

Rogue has a great point. These older lines will develop cracks that might not pour fuel, but still suck air.

A new fuel pump (and filter) doesn't cost a lot, but takes some wrenching to replace.

There's always the option of going electric and those are pretty cheap.

Jim

 

Fuel Pump Essentric is the word you are looking for Chris.

As was said, there are only a few ways that the Pump fails.

1: Diaphram Rupture: (Pumps gas into oil)
2: Check valve malfunction: Does not pump fuel at all
3: Mechanical failure with arm or fuel pump essentric.

As for the pump not being able to pump fuel from the tank, without failing, there are several. Filter Sock in the tank attached to the sending unit sump, plugged. Bad Line or hose. Vapor lock, wrong type gas caps etc...

 

I tested the pump today and can't find any fault in it. It holds vacuum so that rules out the diaphragm. I can't blow threw it so that rules out the check valve. All the lines are new in the engine compartment. Looks like I'll be dropping the tank here real soon. Fun fun.

Good point about being hard to draw fuel from starter power Rouge. It didn't cross my mind but makes absolute sense.

Another thing I noticed is when I was blowing out the lines I had a lot of pressure build up in the tank but didn't notice immediately. I took the gas cap off as soon as I noticed and scared the crap out of myself when the pressure released. Apparently I'm having venting issues so that's another thing I need to take care of. For now I'm just going to leave the cap off until I drop the tank. I don't think the venting is the source of my problem though. It wouldn't happen suddenly, I wouldn't think.

Another odd thing was when I took the fuel line off the carb when I switched them it did something I've never seen before on this truck. It relieved fuel pressure like from a schrader valve on an EFI truck. It was only 7PSI but I've never seen that before on this truck and I've taken that line off multiple times.

 

I don't get this "sucking air" claim....

In my mind, the only way a pump can "suck air" is if the intake tube is exposed to air and not fluid.

The output is under pressure; a leak or hole there will manifest itself as a leak of fuel coming out, not air going in.

Similar to if you drill a hole in one of your copper plumbing pipes, or slice your wrist (artery). Pressure is going to push out, not allow something to be sucked in.

Now, there *is* the possibility of a hi-pressure line having a hole, leaking out enough fuel out to relieve the pressure (imagine engine and pump not running) and then, if there is a secondary air intake, I guess it's possible for a fuel line to empty out and become filled with air over time.

An analogy is to put a drinking straw into a glass of water, cover the end with your thumb and lift it straight up out of the water... the liquid won't leave until it can be replaced with something else (usually air) else a vacuum would be formed.

So, even though the bottom isn't sealed, it's not gonna leak.

Which makes that explanation seem like a stretch to me - the holes would be noticed as leaks when pressurized.

So, can somebody please explain this "sucking air" phenomenon?

 

If the inlet line has any kind of leak in it air will come in. Had that on the 82 Explorer and it caused problems with running out of gas on the road but not at idle. Finally replaced all the hoses and even discovered a poor seal on a hose/tube connection.

 

I let my truck sit for a month due to carburetor problems. I installed the new carb and filled the bowls up with fuel and she fired right up then died when I was adjusting the choke and idle mixture. It was getting dark so I returned in the morning. I pumped the gas a couple of times and the streams of fuel were very weak, after a few times it quit pumping. So I filled the bowls up again and it ran, I backed up and went down my drive 1/10 of a mile and it died again. I pulled it back home with the tractor.

The thing I noticed both times is that my clear filter between the pump and the carb was only about half way full and there was a constant stream of bubbles.

I went to Autozone with another vehicle to buy a new Airtex fuel pump for $25. (If I knew how bad Airtex pumps were, I wouldn't have bought it).

With the new pump installed my inline filter was full with only three seconds of cranking, other than a pocket of air trapped in one corner. She fired up without putting any fuel in the bowls. I went for a drive and it was fine, but I still had a steady stream of airbubbles going into the fuel filter coming from the pump. I got under the truck and the rubber fuel line around the tank selector is so dry rotted that I'm amazed it even held fuel. I replaced the line and I haven't had any air in the lines since.

Makes me wonder how the old carb would do with all the problems I have fixed.

 

Both of you guys are describing holes on the *inlet* side, such a situation is totally understandable and is not what I was trying to ask.

 

You can't "suck air" on the pressure side.

 

I'm starting to suspect float issues with this carb. It's basically new. I ran it twice and took it off and boxed it up. It's been sitting for about a month or two now. I think the float valves may be stuck. I tried to blow fuel down the fuel line from a pint sized reservoir and I can't seem to get fuel to the bowls. It would seem that if I can't get anything past the float valves the pump has no where to push the fuel and I think that is my problem. I'm going to swap carbs again to see if that fixes the fuel problem. If it does it's time to tear into this "new" carb that just came from summit. It had issues with fuel leaking from the shaft for the primary butterflys so the floats need adjusted anyways would be my guess.