Do mechanical fuel pumps get weak?

[72farmer]

I have a 72 that is not getting gas to the carburetor.I took the sending unit out and they are some rust I can see in bottom of tank.I took a old plastic lawnmower tank and plumbed it to the 3/8" steel line going to the fuel pump at the sending unit connections.Of course the lawnmower tank was a lot higher than the pickup on the sending unit.
Turned engine over and got a steady stream of fuel out of the lawn mower tank.So I thought gas tank rust was my problem.Then I hooked a long fuel line to connection on top of tank going to fuel pump.I dropped it down in fuel through the sending unit hole just enough to cover end of the rubber fuel line not getting into rust.
No pumping fuel when I did this.Have you ever seen a mechanical pump get weak?Not lifting fuel out of the tank.

[firedemon]

yes there is a rubber diaphragm in the pump that goes bad

[72farmer]

So they can still pump but be weak and not lift out of tank?I hate to replace it and it still have the same problem.

[davepl]

There's a diaphragm as mentioned, and you could get a hole in it and it'd pump a little but not well. Still, unless I'm confused with the water pump, I thought the fuel pump had a weep hole such that if it ruptured or leaked it'd leak out the body of the pump...

[El Dorado Jim]

I have heard of the diaphragms rupturing and leaking gas into the crankcase and not leaking through the hole on the outside of pump,

[frankslagoon]

maybe ya got crap in the line and or pump. put a air hose (low pressure) to the line at the carb end and blow a little air back thru the pump into the tank. then see if it'll start.

[Andy4639]

Take lines loose at carb and fuel pump. Blow that out. Then take the line back to tank loose blow it out.
Check the oil make sure you don't have gas in it also. Fuel pumps do go bad just like any other part.
There should be a sock filter on the tank hose also. If not you have a clogged filter maybe.

[midniteblues]

sometimes you gotta prime the pump/line when its been drained.
try dumping a cap full of fuel into the carb and turn it over.
get it to fire up for a few seconds that should be enough to pull the fuel up the line and too the pump.

a fuel pump is only pumping as fast as the engine turns when cranking.when the line isnt full of fuel the slow spinning pump has a hard time "pulling the fuel".
abit more rpms(from the cap full of gas) should pull the fuel up the line.

check filter in the carb
and blow those lines out
make sure your not collapseing the rubber cab to frame line when cranking.and the frame to engine rubber line.

i would do everything that has been suggested. and clean the tank.

[Gromit]

Maybe I am too cautious, but once that sender is removed at the upper tank, any tiny static electricity spark (even a spark not visible to the eye); could potentially ignite that air/fuel mixture in the upper tank. I believe this would occur faster than any feasible human reaction time.

I've removed those senders myself; but I think of it as one of the sketchier things I have done on these trucks.. The worst thing in my opinion would be the initial blast would take out your eyesight, and it might make it more difficult to respond or know what to do when you cannot see; and if you are drenched in flaming gasoline from the burst tank.

But anyway to respond to your actual question; the mechanical pumps are only $20 or $25; so it would be easy to just try out the spare.

It is also pretty easy to put a hose pinch off clamp on the lower hose by the frame rail and disconnect the rubber connector there and at the fuel pump and blow out the steel line (and make sure it isn't crimped (for example by someone trying to force the trans support back too far) between the tank and the pump).

I do remember the first time I blew out that line I didn't notice that the fuel pump supply pointed back toward me under the truck and when I hit it with compressed air it blew all the contents of the line back all over me.. I only did that once.. (I'm not claiming to be a genius here, just a regular guy with the bulk of my brain cells devoted to thinking about a woman)..

The previous owner of my truck did fry the pump diaphragm and fuel did get past it and enter the crankcase to mix with the oil.. That is a pretty fatal design flaw IMO.. a good argument for a separate electric fuel pump unless there is a mechanical pump that is sealed off from the crankcase..