Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
How to Measure Inlet Pressure Capability?
von:
If your pump is an Airtex brand, I'd be leery. I had trouble with those having too much pressure. An AC Delco pump fixed mine. I always test the seat gasket sealing by putting soapy water around it (disassembled carb), hold the needle in place with your finger, and blow about 20 psi air into the inlet hole (no filter in place). If bubbles blow around seat gasket or needle, you have a leak. I think that method is in Cliff's book. Make sure there isn't more than one seat gasket in place and that the gasket sealing surface is clean and true, no nicks or erosion.
quadrajam:
OOPS I missed the part about it not running.
Testing the carb as you describe is messy, complex and involves some
specialized testing items.
The easiest way otherwise is to use Cliffs parts properly assembled,
do the usual blow and suck test at the fuel inlet and when that is good
you can move on to the fuel pump.
Is it possible some of the tank rust clogged up the return line orrafice
inside the pump? IS the return line clear back to the tank?
Shiny:
--- Quote from: von on January 05, 2023, 02:10:36 AM ---If your pump is an Airtex brand, I'd be leery. I had trouble with those having too much pressure. An AC Delco pump fixed mine. I always test the seat gasket sealing by putting soapy water around it (disassembled carb), hold the needle in place with your finger, and blow about 20 psi air into the inlet hole (no filter in place). If bubbles blow around seat gasket or needle, you have a leak. I think that method is in Cliff's book. Make sure there isn't more than one seat gasket in place and that the gasket sealing surface is clean and true, no nicks or erosion.
--- End quote ---
Thanks. I don't know the brand of pump but your warning is one of the seeds that has me wanting to better diagnose. I did pull a vacuum on the inlet last time with the carb upside down and it held. This may give a false sense of security as it only checks the needle and seat but not the ability of the float to resist pressure on the needle.
Good input on the gaskets and sealing surface. Seems the same feedback Cliff provided. I'll open the carb and revisit.
Shiny:
--- Quote from: quadrajam on January 05, 2023, 05:25:37 AM ---OOPS I missed the part about it not running.
Testing the carb as you describe is messy, complex and involves some
specialized testing items.
The easiest way otherwise is to use Cliffs parts properly assembled,
do the usual blow and suck test at the fuel inlet and when that is good
you can move on to the fuel pump.
Is it possible some of the tank rust clogged up the return line orrafice
inside the pump? IS the return line clear back to the tank?
--- End quote ---
Thanks for the reply.
I will pull out the book and read again about the blow and suck test. Per my last post, I think I understand the vacuum test but don't recall a pressure test.
Yes, I suppose the pump could be fouled but I did flush the return line and pulled a pipe cleaner through it when I was replacing the tank. Maybe I can open the pump and see? I've never seen the inside of one so will learn something even if I don't find it fouled.
Maybe you can clarify - while the pump is a "3-line" pump and there is a return line to the tank, the return line is smaller diameter. I also understood there's an orifice in the pump return that will significantly reduce flow. So...
Is the return just a way to allow vapors to escape and reduce risk for vapor lock?
The return is clearly not designed to be a "recirculation" path with a lot of flow. So will a clogged return line actually cause the pump pressure to increase? Hmmmmmmm...... good question I think I need to check out.
quadrajam:
Actually the spring in the pump will determine its pressure.
a hot fuel pump and lines can boil todays crappy gas and
cause excess pressure but sounds like youre not at that
point yet.
I second VONs advice.
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