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If I prime the carb it will start and run. It will restart 30 minutes later, but must be primed again if it sits overnight. I would like to have a fuel system diagram, and vacuum line routing diagram.
 

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If I prime the carb it will start and run. It will restart 30 minutes later, but must be primed again if it sits overnight. I would like to have a fuel system diagram, and vacuum line routing diagram.
put a 10 micron fuel filter before the fuel pump... the fuel pump check valves are filled with sediment and allowing the fuel to drain back to the tank..

don't believe me.. cold engine.. disconnect the carb... extend the fuel line with a chunk of fuel hose and blow into it.. i bet you can blow bubbles in the fuel tank. still don't believe me.. use some clear vinyl hose on the fuel line.. loop it up to the hood latch.. crank the engine.. it will take a while to get fuel in the hose... when you stop cranking it will go backwards to the tank..

after you install a 10 micron fuel filter before the fuel pump...

perform this test 3 times in a row... it will flush most sediment out of the fuel pump..



you can start with a 1990 ford ranger fuel filter.. its big and bulky.. put it in a loop of fuel hose just before the pump inlet..
 

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The in carb filter has an anti drain back valve in it, unless it was removed.


If it was, and the needle seat assembly is the type with the "low window" (where the fuel flows out) it will cause this issue.


The in filter anti drain back valve looks like this one for sale on E bay:


Rochester Quadrajet Carburetor Long Fuel Filter w Check Valve and Spring Kit X2 | eBay


This Q jet parts place explains it :
http://quadrajetparts.com/short-paper-carburetor-fuel-filter-with-check-valve-p-79.html




Anti drain back can be in a long or short filter
 

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Look through the pictures here: Quadrajet Carburetor Needle & Seats - Carburetor Parts & Kits for Carter, Ford Motorcraft, Holley & Rochester Carburetors


You will see "windowed" and "non windowed" needle and seats.


The windowed kind lets the fuel out at a lower point in the float bowl. On a fuel system with the return line on the fuel pump, it can allow the fuel bowl to be siphoned dry as the fuel system/tank cools down, as that would create some small amount of vacuum on a closed system, which you have. (3410-110 is the windowed style)


The Non windowed kind, forces the fule to flow "over the top" of the seat assembly and is effectively a Stand pipe that wont allow back flow as it is above the level of fuel. This will also eliminate siphon ,or drain back.


I Hope.....No....... I am sure this will help
 

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Wayne has also provided good info on pump performance testing and filtration as well. Good words to listen to.
Probably a real good idea to test the pump just so you are sure you aren't also suffering a starvation issue.


His advice is solid and can help keep other problem from cropping up.
 

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Another very common issue with the Q-Jet is leaking main well plugs in the bottom of the fuel bowl....fuel in the bowl drains away through the plugs into the manifold and the carb goes dry overnight.

YouTube has videos on how to epoxy over these plugs to prevent leaks, but the carb has to be disassembled to do it.
 

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Another very common issue with the Q-Jet is leaking main well plugs in the bottom of the fuel bowl....fuel in the bowl drains away through the plugs into the manifold and the carb goes dry overnight.

YouTube has videos on how to epoxy over these plugs to prevent leaks, but the carb has to be disassembled to do it.
Hit the nail on the head
 

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X3

The well plugs will get you every time. You've got to use some high temp fuel resistant epoxy. Look at products from Devcon or Marine Tex for something that will work.
 

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If I prime the carb it will start and run. It will restart 30 minutes later, but must be primed again if it sits overnight. I would like to have a fuel system diagram, and vacuum line routing diagram.
It's either leaking out or boiling out.

The early Q-Jets leaked out the capped drilling for the main jets, but 1978 is getting kind of far down the road from those early leakers. Plus if it was leaking out it is into the engine which would prove to hard to start because it's essentially flooded and adding a prime charge to start it wouldn't be helpful.

Another is heat is boiling it out. This would dry the bowl the fuel escaping as a vapor. GM spent an excessive amount of time and energy adding heat to the intake and under the primary side of the throttle bores to force fuel vaporization, what they needed was better heads but that had to wait another 18 years. There should be a stainless steel separator between the manifold and the bottom of the carb in this case. If it is missing or burnt through the carb will be fried by the exhaust gasses present and this will certainly cook out any fuel sitting in the float bowl. My recommendation is to plug the feed holes if this system is there. Use a steel, brass, or aluminum rod that just fits tight and drive it in. RTV won't last, don't bother with it.

Bogie
 
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