Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

'78 Suburban 'chugging' on long grades

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tayto:
the line that is hardest to get to is probably the problem, murphy's law. haha

RyanAK:
Posts keep getting clipped...

Rigged up a soft line from the pump to carb with a clear plastic inline filter so I can see what

RyanAK:
AAAARRRRGGGGHHHH!!!...

Rigged up a soft line from the pump to the carb with a clear plastic inline filter so I can see what’s happening. Seem to get good fuel on first start on a cool engine. When I shut it down, it immediately drains back. After a 10 min wait on a hot engine, the carb will suck the filter dry before the pump can push fuel to the filter. Maybe 10 seconds from when the engine fires until fuel starts arriving in the filter.

I have a return line on the pump. Are these mechanical pumps supposed to allow fuel to drain back? Could something in the carb not be closing/sealing to allow the fuel to drain?

hiy_po:
I am trying how to understand how the carby can "suck" the filter dry
 I see a suck as a high pressure trying to get to a low pressure. so would need a pressurized line with a low in the carb to move fuel that way. Once pump has stopped and no pressure in line fuel shouldnt be going into carb unless you have heat causing a high pressure in the line. Is it possible your tank is vacuum locking and you are getting a huge low in the tank as the level drops causing movement of fuel back to the tank once pump is stopped?? This would also explain why it would need time to reprime. If you dont have an intank pump then you are rellying on gravity to get the fuel from tank pickup to first pump.

RyanAK:
Thanks for chiming in, Hiy_Po. I

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