Fuel supply issue

[BJedi76]

1986 GMC 4x4 Sierra Classic 1500 SWB Q-Jet feeding a 305” on a 700R4

if I start my truck every single day, it always starts, but if I don’t use it for a couple days, I have to prime the Q-jet.

- rebuilt Q-jet (both pull-offs are new)
- new pump to carb, steel line
- New fuel pump
- New fuel filter in carb
- New in-line canister fuel filter
- New pick-up/sending unit in gas tank
- all new vacuum lines
- thermostatic choke converted to electric & functioning properly.

There are no external fuel leaks.

It is my understanding that fuel should not be able to bleed back through the fuel pump to the tank, so my only idea is that my bowl is leaking dry? Meaning, needle and seat, which are both new.

Does anybody have any suggestions that I may have missed or refute my theory?

I don’t, at all, mind being wrong, if I get results. Lol

 

[Dave M]

@BJedi76

You've covered everything well, perhaps the other issue is the fuel evaporating out of the bowl over a number of days.
I would consider it normal, I think we get too complacent and have been spoilt with the newer EFI cars and electric fuel pumps, that start immediately, hot or cold and regardless of sitting idle for a few days.

 

[Old60Driver]

When you say 'prime' the Q-Jet, are you talking about turning it over for a bit before it starts? Or are you talking about giving the big pedal a few pumps before starting?

If it's the former, did you rebuild the Q-Jet yourself, or have someone else do it? There's a couple of fuel plugs in the QJet that have been known to allow the fuel to leak out of the bowl. This makes for hard starting if you haven't run her in a few days. That COULD be the problem...

If it's the latter, that's just normal.

Good luck friend.

 

[Dave M]

Oh ! and thumping the steering wheel, while cranking, and muttering "start you bastard" sometimes helps.

 

[gotyourgoat]

1986 GMC 4x4 Sierra Classic 1500 SWB Q-Jet feeding a 305” on a 700R4

if I start my truck every single day, it always starts, but if I don’t use it for a couple days, I have to prime the Q-jet.

- rebuilt Q-jet (both pull-offs are new)
- new pump to carb, steel line
- New fuel pump
- New fuel filter in carb
- New in-line canister fuel filter
- New pick-up/sending unit in gas tank
- all new vacuum lines
- thermostatic choke converted to electric & functioning properly.

There are no external fuel leaks.

It is my understanding that fuel should not be able to bleed back through the fuel pump to the tank, so my only idea is that my bowl is leaking dry? Meaning, needle and seat, which are both new.

Does anybody have any suggestions that I may have missed or refute my theory?

I don’t, at all, mind being wrong, if I get results. Lol

We have very near the same stats. What is your definition and method for priming? Or I guess what is your start up procedure?

 

[BJedi76]

I wasn’t aware there were different methods of priming.

I pour a capful of gas or spray a little starter fluid in the carburetor.

there’s not really much going on “inside” a carburetor that could lose the fuel, just from sitting for a couple days.

…needle, seat, Excelerator pump, power valve, & float.

I figure it’s got to be one of those. And if it’s the float, that could also affect the needle.

 

[BJedi76]

…ooooor, I suppose…. Just grasping at all fantastic possibilities.

If the gas in the float bowl, Was open to the atmosphere, through, lets say, a bad butterfly seal or something, a couple of days would certainly evaporate that much gas.

I guess it’s time to pull the carburetor off again.

 

[BJedi76]

After two or three days of sitting, no amount of cranking, or pumping the gas pedal, has any effect on initial start up. Which tells me, there’s no fuel in the bowl for the Excelerator pump to “accelerate” into the barrels.

But if I just pour a capful or two of gas into the carburetor, or spray a little starter fluid, it will start right up and stay running just fine. Which tells me, there is fuel in the line. Which also tells me the fuel pump is holding its seal and not letting fuel run back to the gas tank.

 

[Old60Driver]

After two or three days of sitting, no amount of cranking, or pumping the gas pedal, has any effect on initial start up. Which tells me, there’s no fuel in the bowl for the Excelerator pump to “accelerate” into the barrels.

But if I just pour a capful or two of gas into the carburetor, or spray a little starter fluid, it will start right up and stay running just fine. Which tells me, there is fuel in the line. Which also tells me the fuel pump is holding its seal and not letting fuel run back to the gas tank.

I've not experienced this with my current truck, but I had a 78 Monte Carlo (damn, I loved that car), that did this. Same scenario. Ran great if I ran it every day, but leave it for 2 or 3, and she'd be a B**** to start unless I through a little fuel or laughing gas down the old fire make it happener. For me, it was leaking wells in the Qjet. Not saying that's what you've got, but if you're gonna pull the ol' Qjet anyway, I'd have a look at the wells.

 

[dsteelejr]

@BJedi76

You've covered everything well, perhaps the other issue is the fuel evaporating out of the bowl over a number of days.
I would consider it normal, I think we get too complacent and have been spoilt with the newer EFI cars and electric fuel pumps, that start immediately, hot or cold and regardless of sitting idle for a few days.

I‘ve heard some old school mechanics say that with the E10 fuel we have these days fuel evaporating out of carbs is quite common when they sit for a few days.

 

[rpcraft]

I would pull the fuel inlet needle(s) {sorry I have moved on to fuel injection so I can't even remember how to rebuild a carb at the moment and how many needles. Just one on a q-jet I seem to recall though}. Clean that inlet circuit, inspect for debris and see if the rubber tips off the needle for the fuel bowl are disentegrating, since the oem stuff is really bad with ethanol. Might be an issue with an old kit and they are no good with ethanol and causing the needle not seating and acting like a check valve for the supply line because of premature wear or trash and allowing fuel to seep back down the line once pressure has subsided.

As the vehicle sits for a couple of days on a stock fuel pump it can lose pressure but should still have a volume of fuel to in the line, and your carb should still have fuel in the bowl. Effectively that allows you to step on the pedal to "prime" your carb and then as soon as the engine spins it will re-pressure the line and have instant flow for the most part. If the needle is not seating or sealing then it is the check valve that prevents the fuel from "backing down" the line as it loses pressure. In that case you should still be able to get volume and pressure back pretty well but just depends on how much fuel you have lost but maybe it's a weak pump or something as well, especially if its something like a cardone special or similar auto house brand.

 

[C10MixMaster]

You should be looking at the fuel pump. in warm weather the carb going dry is common. A good fuel pump should resupply the carb and start after the engine turns over a couple time.

 

[rpcraft]

You should be looking at the fuel pump. in warm weather the carb going dry is common. A good fuel pump should resupply the carb and start after the engine turns over a couple time.

Also a good source of the problem perhaps.

 

[DanMcG]

For what it's worth, I have the same issue with a holley.

 

[Backfoot100]

I can only tell you my experience.
Rebuilt QJ with a kit from Cliff Ruggles. I can pretty much guaranty that the Welch plugs don't leak.
Carb works good but I get exact same issue. If it sits for more then 3 or 4 days it won't start right away.

I took the air cleaner off and checked things out. No fuel in the bowl. You can and pump the gas pedal as much as you want but theres nothing in the float bowl to pump. I started using the following process....
Get in and don't touch anything but the key. Crank it for about 5 seconds. Thats all it needs. Turn it off and pump the gas twice. Crank it over again and it immediately fires and runs perfect. Works that way every time.

I think its the design of the carb and newer fuels as mentioned before.
I have a Holley on my boat and I can get maybe a week and a half on that without running and the bowl is dry.
The QJ, IMHO, evaporates off faster. I look at another positive out of this too....cranking it over for five seconds also starts getting the oil moving from the oil pump before it actually fires. That's just a bonus....LOL.