Mechanical fuel pump

[Wedged'86]

Can a mechanical fuel pump fuck up so as it blows fuel out with heaps of pressure but it doesn't suck fuel in very well?
can it blow and not suck?

[NeoNasty]

Ive heard this riddle.

What blows but does not suck?.. or was it what sucks but doesnt blow?....

Sorry no idea. If your still running the 4ac, then I dont think fuel presure should be high anyway, maybe 7PSI?..

[75Ta22LT]

As far as I know, they cannot increase in pressure...

If there is a decrease in pressure on the intake, then the output would also lose pressure.

[Wedged'86]

well i'm barely getting enough fuel through it's not even filling up the bottom of the fuel filter
but it shoots out the fuel pump. i took this to mean that it's not the pump so i was wondering if this was a good assumption.

yeah it's a 4ac.

[briney]

i have had good success finding these sorts of problems using a cheap vacuum gauge/fuel pump tester. i normally tee it into the suck side of the pump. more than 2 or so inches of mercury is very bad. it may not hurt to check for damaged fuel lines underneath too.

[shewiththe18RG]

Not possible for it to blow without sucking, or suck without blowing. The rules of atmospheric pressure.

Sorry I can't help though.

[sillycar chick]

It sounds like you are getting an air lock somewhere, check every possible fuel line for air leaks. Dont forget rubber fuel hoses can get damaged by hose clamps if they are clamped on too tightly, so take clamps off and check the ends of rubber hoses that they arent ripped or damaged, and put on new clamps where possible.

[Wedged'86]

Thanks, i've come to that conclusion as well, i'm going to replace all the dodgy looking fuel hose and trace the fuel line to check for leaks.

[rob1]

or fit an electric fuel pump. saves a lot of hassle

[Moppitt]

My 4a-c runs something like 3 or 4 psi of fuel pressure.

and on the topic of blowing out really hard, it could blow out a very small volume of fuel out at high pressure (through a tiny orofice) whilst sucking in a larger volume of fuel at very low pressure (through a much larger orofice... he he. orofice, sorry)

As to a solution to your problem, as you say, check for blocked hoses ect, see if you can blow through them without heaps of resistance.

[Wedged'86]

wait, let me clarify,
I didn't mean can it blow and not suck. Say it had some fuel inside the pump, it shoots that out, but it doesn't suck fuel into it propperly so when that fuel runs out it doesn't shoot it out as hard any more (but i don't see that)
EFI pumps do that. they shoot out at 40psi but don't suck fuel propperly, that's why they're in the tank, or so i understand.
What do you reckon could my pump be doing that?

[lexsmaz]

Is there actually a problem ? is the the car starving of petrol, ( going flat under acceleration, dying off ) or is it just that there's not much fuel in the fuel filter ?..

[Wedged'86]

ohh there's a problem. it just randomly starves for fuel, seems it does it less at higher revs conversly it sucks arse anywhere between 2-3k
the filter not filling up was just an observation i made.

i've got new fuel hose coming today so i'll put that shit in on saturday. see if it makes a difference

[lexsmaz]

I would think you have more of a carby fault, possibly shit in the primary main jet, if you floor it is it ok.? whats it like at light throttle at higher rev's above 3,000 ? ... the filter will not fill up 100 % due to air pockets & low fuel pressure etc .. take off the fuel line from the carby & put the end into a 2 litre clear bottle, start up the car & let it idle & see how much flow you get into the bottle from the pump, over say a 30 second period ..i can't remember the exact figure but i would think around 1/2 a litre to be a good amount ... if there is a return line on the pump make sure you block it so all the flow is coming out of the delivery side of the pump .. Obviously if the flow is around that amount, the problem is else where ...