Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
'78 Suburban 'chugging' on long grades
RyanAK:
Thanks for chiming in, Hiy_Po. I’ll try to get video. The tank isn’t under vacuum. The vent is clear. It will do this with the gas cap removed... I checked. Pump is mechanical on the block. Not sure if it is supposed to have some sort of check in it to keep fuel from draining back... either through the supply line or the return line. But it drains as soon as the engine is cut. As for the fuel remaining in the filter after drain-back... after a few seconds after startup, it gets pulled from the filter to the carb. I’d guess the low vacuum in the intake manifold (20”) might be enough to pull the fuel from the filter to the carb. A few seconds after that the pump is pushing and refills the filter. If the engine were under load right after starting, the carb would get sucked dry before the pump could get fresh fuel to it and the engine would stall.
Cliff Ruggles:
I tried calling yesterday, then saw this morning where you called again.
Was out of the office most of the afternoon running parts and working on my shop and driveway.
I'll try again in a couple of hours, orders don't ship out till around noon......
tayto:
--- Quote from: RyanAK on July 30, 2020, 04:57:09 PM ---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
--- End quote ---
trying pressing "Preview" button first then "Post"
hiy_po:
lol I think I may have read it wrong. I read it as the carb was emptying the fuel filter after shut down. With no manifold vacum or pressure in fuel line I couldnt see how that was happenning. I get it that the carb is running dry before the filter is primimg on start up.
RyanAK:
I'm thinking that hot fuel / vapor lock is contributing here. I'll check the fuel return line is functioning from the fuel pump and explore ways to keep the pump and lines cool, along with the carb rebuild. Anyone have thoughts?
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