Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

How to Measure Inlet Pressure Capability?

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Shiny:
Question:  How to measure fuel inlet "holding power" on or off the car?

Goal is to correct flooding that may have been caused by rust but could also have been a more basic fuel system issue like too much pump pressure for the inlet on my carb.

I ran across a thread on PY where Cliff had described measuring the "static" capability of an inlet, presumably by increasing pressure until it "let's go".  This sounded like a great way to determine how much "margin" my carb has and whether I should reduce the inlet dia, change the float, find a different pump, install a regulator, replace the carb, etc.

Here's the thread for reference.  Cliff's post is what I'm asking about:
http://forums.maxperformanceinc.com/forums/showthread.php?t=531861

In the past, I measured the fuel pump pressure and thought it might be on the "high side" and possibly contributing by overpowering the inlet.  The last time I attempted to diagnose the flooding, the fuel pressure gauge while cranking suggested "peak" pressures higher than the 6.5 psi which I understand is a desired "max" for most early carbs.  Here is a video of that measurement:

https://1drv.ms/v/s!AiIRc33Ia4jVivY3WlzPJMu3pVZA3Q

So how do I tell if my carb's inlet is holding the pressure my pump is putting out?  How do I measure the pressure the inlet will hold?  My preference is to do this on the bench but can do it in a car.

And no, I don't have an electric pump and regulator I can crank up until bad things happen.  ;D

I want to avoid randomly buying parts and "fixing" things that aren't broken.  This is probably what led me here...

Thanks if you've read this far and can guide on this measurement.

Mike


Stock 1970 Pontiac 350
   Modified by:

   Summit 2800 cam
   71 4bbl Intake
   69 Buick QJet core 29240MD
      Rebuilt with Cliff kit and new APT screw in 2009
      Rebuilt again with Cliff parts in 2018, but ended in flooding episode and rust discovery
      Fuel inlet seat diameter is UNKNOWN, possibly larger than stock, hence this question
   Aftermarket "tall can" fuel pump, unknown specs but stock 3-line with a return

Cliff Ruggles:
The 1969 Buick carb will easily handle 6.5 psi fuel pressure without any issues.

Did you check the gasket sealing surface under the fuel inlet seat?  Any leak there will cause flooding at any fuel pressure and combination of parts used......

Shiny:
Thanks Cliff... was hoping not to open it up again, but understand your point.

I don't remember if seat dia was stock or not, which is part of the uncertainty.  What is a stock seat dia for my carb?  It has been ~4 years since I put your parts in the carb so don't remember...

The other uncertainty is the pump.  While cranking, the pressure pulses, with peaks way over 6.5.  And I've heard others on PY report recent aftermarket "stock" replacement pumps generate more than 6.5 psi.

So before I go chasing anything, I wanted a direct way to measure the inlet holding pressure before chasing my tail (again).  I plan to try it on the bench, then decide whether to open up the carb, replace the pump, or both.  On PY, suggestion was to watch for fuel overflowing boosters on a "static" test.  Is this OK or is there a better way?

quadrajam:
Tee into the fuel line close to the carburetor and note fuel pressure while the
engine is running. This will be more accurate than watching cranking pulses.

Shiny:
Thanks and understood but car is not running.

Long story about uncertainty...

Carb is unproven, newly rebuilt, modified per guidance from Cliff and Kenth.  Pump is new.  Last time I tried to get it running, it flooded and I'm addressing as much as I can before trying again.

The pump and carb were both suspects.  I opened carb, found rust inside.  Replaced needle, seat, & float.  Installed a filter and tried again.  Still flooded.

I have finally replaced the gas tank and sender, which was the source of rust.  I flushed the lines and want to verify inlet seal before deciding next steps.  That's why I'm asking how Cliff or others have measured max inlet pressure capability for a carb.

Once I know what the carb can tolerate, I'll be able to better interpret fuel pressure readings...hopefully when running.

I've read too much on PY badmouthing aftermarket mech pumps for overpressure so I don't trust mine.

I also know the flooding may be caused by something other than overpowering the float.  I may not have blades closed, idle may be too high, etc.  If I gain confidence in the inlet seal, I will guess less..

Hope this is halfway rational.
 

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