Fuel Pump losing prime

[2 Stovebolts]

I'm sure this has been addressed before...Not sure if year or engine makes a difference but I've changed 2-3 new fuel pumps with the same result....After sitting a week or so I either have to crank away or pour a dash of gas down the carburetor to start the couple of trucks I have.....I really don't recall doing this when the trucks were rather new....Is there an AC fuel pump made in the USA ?

[geezer#99]

It’s not the pump!
It’s the gas!
It evaporates faster.
Another thing.
What carb, what choke?
Is the choke set up right?

Some guys use an electric helper pump installed back near the tank and wired up with a push button switch.

[body bolt]

Is it a Quadrajet and could the well plugs be leaking?

[2 Stovebolts]

Perhaps I should have mentioned the engines...Two are 292 & the third is a 250...all have Rochesters

[geezer#99]

Chokes working?
Accelerator pump working?
Also timing can effect starting.
What’s your initial timing?
Stock points or hei?

[Ole Blue 71]

Originally Posted by body bolt

Is it a Quadrajet and could the well plugs be leaking?

+1 Both of mine do this. Definitely check the well plugs

[2 Stovebolts]

Definately fuel starvation....unhooked fuel line at carb while cranking...Took an excessive amount of time for gas to reach carburetor

[chevmn]

Not unusual when they sit a week or more. I have two quadrajets - one big block, one 350, and if they sit, prepare to crank a while to get fuel back into the carb. Neither are leaking, both carbs rebuilt, and well plugs sealed. Fuel is evaporating. Probably is the newer ethanol blend fuel contributing to the problem, but we had a 66 Impala and the car was not very old, and it sat at least a week sometimes more before it was driven, and usually it took a little cranking and few pumps of the accelerator. Newer vehicles with electric in tank pumps have a primer circuit that starts the pump immediately when the key is turned on, and build far more fuel pressure.

[2 Stovebolts]

Gas evaporation could be a factor but how much can evaporate in a rather closed system in a week or two time period?...I'm thinking if the fuel pump has a check valve, there should be enough gas to fire & start the engine with out excessive cranking...With the gas tank behind the seat in the cab, the gas is close enough to be picked up by the fuel pump ???

[chevmn]

Originally Posted by 2 Stovebolts

Gas evaporation could be a factor but how much can evaporate in a rather closed system in a week or two time period?...I'm thinking if the fuel pump has a check valve, there should be enough gas to fire & start the engine with out excessive cranking...With the gas tank behind the seat in the cab, the gas is close enough to be picked up by the fuel pump ???

Not sure if this is a quadrajet, but a quadrajet is not a closed system, and there is a vent (the round tube centered above the choke). I often use that vent to fill the float bowl when attempting to fire an engine that has sat extended periods. I really do not believe you have any significant issue, but you could do as someone else suggested and install a small electric pump between tank and mechanical pump with a switch to prime before attempting to start.

[2 Stovebolts]

OK...Getting a little of course here....Gasoline flow & presence in the line before it enters the carb...This totally eliminates the carb....well plugs, vents ect...An electric fuel pump assist, is not the solution....Yes its an option, but these trucks never came from the factory with one & was not in the design of the vehicle....I'm thinking that the fuel pump should have an internal check valve preventing the gas to go back into the tank once the gas is present in the fuel pump, holding the gas to the entry to the carb....Now unfortunately we are all subject to use the mostly Chinese aftermarket junk that's out there, & in their meager penny pinching to make this junk cheap, some rocket scientist engineer decided to eliminate a check valve...Thus my question in my initial post...Does anyone know a source for a USA made AC fuel pump?....Showing my age here but, I was around when these 67-72 were new, as I had 68 with a 250 as a secondary vehicle that sat around for weeks at a time & never had to prime the carb to get it started.....

[67 twins]

Originally Posted by 2 Stovebolts

OK...Getting a little of course here....Gasoline flow & presence in the line before it enters the carb...This totally eliminates the carb....well plugs, vents ect...An electric fuel pump assist, is not the solution....Yes its an option, but these trucks never came from the factory with one & was not in the design of the vehicle....I'm thinking that the fuel pump should have an internal check valve preventing the gas to go back into the tank once the gas is present in the fuel pump, holding the gas to the entry to the carb....Now unfortunately we are all subject to use the mostly Chinese aftermarket junk that's out there, & in their meager penny pinching to make this junk cheap, some rocket scientist engineer decided to eliminate a check valve...Thus my question in my initial post...Does anyone know a source for a USA made AC fuel pump?....Showing my age here but, I was around when these 67-72 were new, as I had 68 with a 250 as a secondary vehicle that sat around for weeks at a time & never had to prime the carb to get it started.....

You will not find what you are looking for. AC Delco parts are all made in China now, so you will not find one unless it's NOS

[BigBird05]

Pull the air cleaner, open the chock blade and look into the carb while some one pumps the accelerator pedal. Do you see gas being squirted by the accelerator pump? If not then there is no fuel in the float bowl. If the float bowl is empty and needs to refill then you have a "loosing the prime issue".

[Caddylackn]

I think this is a combination of issues: Fuel pump and carb

Inside the factree style fuel pumps there are two one way valves.

Often if just one valve fails or is dirty the fuel pump will still pump but allows gravity drainback when its not running. If your carburetor is perfectly running you will not even notice, since there should be enough fuel in the bowl to start the engine and resume pumping without starvation. If your carburetor bowl is also leaking, or the fuel is evaporating you will have the hard starting issue since you have to pump fuel from the fluid level of the tank every time on a cold start.

Remove the fuel pump and open the diaphragm to access the two one way valves. Sometimes the seal and the mating surface is just gummed with old fuel and can be cleaned and revived. You may be able to just replace the fuel pump one way valves, if you can find them, they are pressed in.

With a properly working fuel pump, if your carb is still leaking, you will still need more cranks to start since you have to prime the bowl to get the accelerator pump working.

A leaking carb bowl is bad, since the raw fuel washes down the lubrication on whatever cylinder has an intake valve open when the motor is off.

[HO455]

It sounds like the fuel pump is sucking air when cranking. At idle it is able to overcome the air leaks and pump fuel to the carb.

Check your hoses on the suction side of the pump. Old hoses will allow the pump to suck air, not fuel .

Old cracked hoses on the suction side usually won't leak fuel as there is no pressure in the line.

Some years/models have a hidden piece of hose under the cab towards the rear of the cab next to the frame.

If your unsure of the age of the hoses replace all of them.

Good luck and please post your findings.

[suth1968]

Smell your oil..... If it smells like gas your carb could be leaking and emptying the float bowl.

[rockyrivermark]

Your gas is evaporating from the bowl as stated here already.
It’s just that simple and very common. Especially with new fuels.
I tied an in-line check valve and it made no difference.

[tdangle]

I used to have the same issue with a quadrajet carb. The plugs were good and not leaking, but if it sat for more than a week I would have to crank and crank to start. Switched to a Holley and no problems, now am running a Edelbrock and no problems.

[2 Stovebolts]

OK...Given the float bowl is empty....It fills real quickly if there's gas in the line ready to enter the carb....Exactly my point, no gas...if the fuel pump was filled with gas, so should the line from the pump to the carb.....

[RichardJ]

>>have to pour a dash of gas down the carburetor to start the couple of trucks I have.<<

>>Two are 292 & the third is a 250...all have Rochesters<<

>>.After sitting a week or so <<

>>I was around when these 67-72 were new, as I had 68 with a 250 as a secondary vehicle that sat around for weeks at a time & never had to prime the carb <<

>>how much can evaporate in a rather closed system in a week or two time period?.<<

It would be possible to have a leaking suction hose or even a bad pump, but NOT on three trucks at the same time.
The Rochester B float bowl is not a closed system. The bowl is vented to the atmosphere, but the bowl itself is completely closed on the bottom and cannot leak.

The common denominator is the fast evaporating fuel used today. Much different fuel than you put in your trucks when they were new.
Pour about four oz. of fuel into a open tin container(tuna can) with a level mark and let it sit the same number of days as your trucks.

[SCOTI]

I see some serious validity to the evaporating fuel suggestion as a key part to be considered in this issue. It may not be THE problem, but could def be a factor.

We drained the remains from a tank on Thursday early evening last week (about a 1/2 gallon of recent 87grade @ about 6pm). It was captured in an open pail & set beside my trailer.

Friday evening when I left work (~9pm), I stopped by the shop & noticed the pail/bucket still sitting beside the trailer. I went to bring it inside & was surprised @ how little was left. It was windy that day.... but to vaporize over a 1/2 gal was just a moment for me.

[HO455]

Originally Posted by RichardJ

>>have to pour a dash of gas down the carburetor to start the couple of trucks I have.<<

>>Two are 292 & the third is a 250...all have Rochesters<<

>>.After sitting a week or so <<

>>I was around when these 67-72 were new, as I had 68 with a 250 as a secondary vehicle that sat around for weeks at a time & never had to prime the carb <<

>>how much can evaporate in a rather closed system in a week or two time period?.<<

It would be possible to have a leaking suction hose or even a bad pump, but NOT on three trucks at the same time.
The Rochester B float bowl is not a closed system. The bowl is vented to the atmosphere, but the bowl itself is completely closed on the bottom and cannot leak.

The common denominator is the fast evaporating fuel used today. Much different fuel than you put in your trucks when they were new.
Pour about four oz. of fuel into a open tin container(tuna can) with a level mark and let it sit the same number of days as your trucks.

My bad, I didn't realize it was 3 different trucks. I thought it was one truck with 3 different fuel pumps being installed.

Highly unlikely all three trucks would have bad hoses. Unless there is a conspiracy involving sabotage. Got any practical joker friends?

[body bolt]

After a week, the bowl should still hold enough fuel to start the engine and stay running as the bowl is replenished. Without a leak it must be evaporation.

[2 Stovebolts]

Fuel pump, On my 70 C30 this is the 2nd. replacement....The first replacement was either Auto Zone or Advanced.. China junk, the second was NAPA & if I remember made in Israel....no matter, they are both sealed units, no screws to take apart to inspect diaphragm. I did replace the 6"? rubber hose from the steel line coming from the gas tank..The outlet is a steel line to the carb....As the gas tank is behind the seat, I would detect a weeping hose by the smell of gas in the cab....No smell....OK given the carb bowl is empty from a leak, the amount of time cranking would not be as long if the gas was at the ready to enter the carburetor.... Filling a carb bowl is rather quickly.....
Speaking of Gas....I only use Non ethanol premium, very expensive for a thirsty engine...Premium is the only grade in non ethanol, & grateful that I can get it....I use it in my tractor, quad, chainsaw, mower....ect... Evaporation? Its not like having an open container as a coffee can filled with gas out in the open...Can't make this argument on fuel injected vehicles with pressured fuel systems, have a couple of those too that sit for months at a time...No priming there, they fire right up !!!

[SCOTI]

Originally Posted by 2 Stovebolts

Fuel pump, On my 70 C30 this is the 2nd. replacement....The first replacement was either Auto Zone or Advanced.. China junk, the second was NAPA & if I remember made in Israel....no matter, they are both sealed units, no screws to take apart to inspect diaphragm. I did replace the 6"? rubber hose from the steel line coming from the gas tank..The outlet is a steel line to the carb....As the gas tank is behind the seat, I would detect a weeping hose by the smell of gas in the cab....No smell....OK given the carb bowl is empty from a leak, the amount of time cranking would not be as long if the gas was at the ready to enter the carburetor.... Filling a carb bowl is rather quickly.....
Speaking of Gas....I only use Non ethanol premium, very expensive for a thirsty engine...Premium is the only grade in non ethanol, & grateful that I can get it....I use it in my tractor, quad, chainsaw, mower....ect... Evaporation? Its not like having an open container as a coffee can filled with gas out in the open...Can't make this argument on fuel injected vehicles with pressured fuel systems, have a couple of those too that sit for months at a time...No priming there, they fire right up !!!

You're right.