How to Measure Inlet Pressure Capability?

[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. 

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.
 

[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]

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.

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]

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?

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.

[Shiny]

I got this done.

I poured gas in the carb, filled a tube, and pressured with air using a low-pressure regulator.

I cranked it up while holding the float pin.

The needle and seat held fine until I hit 12 psi, at which point it was obviously "overwhelmed".

FYI, I measured the seat ID at 0.128"... likely way more than I will ever need.

The good news is I can eliminate overpowering the needle and seat as a cause for flooding as long as my pump pressure is close to what it should be.

Rather than guess on cleanliness and performance of my pump, I bought a new Delphi brand.

Onward.

[ourkid2000]

Shiny what fittings are you using there to test that inlet? I'd like to pick up something similar.

[Shiny]

I bought a low-pressure regulator off Amazon.  I always try to find stuff local but I struck out on this, at least for a reasonable price.

https://smile.amazon.com/dp/B01BPQDG62?ref=ppx_yo2ov_dt_b_product_details&th=1

The only tricky fitting was a 3/8" male flare to 3/8" barb adapter for the carb inlet.  I found it on the rack at O'Reilly's.

This might be it.... double check, though...
https://www.oreillyauto.com/detail/c/dorman/dorman-fuel-hose-fitting/dor0/785402d?q=dorman+fuel+hose+fitting&pos=4

Here's another style that might work.  Again, double check...
https://www.oreillyauto.com/detail/c/american-grease-stick/american-grease-stick-3-8-inch-90-degree-clamp-elbow/ags0/chf28b?q=3%2F8+male+flare+to+3%2F8+barb&pos=4


The other fittings for the regulator were common 1/4" NPT fittings I had laying around: one 3/8" barb and one male quick-connect for my air line.  I set the pressure on my compressor to 20 psi and fed it to the low-pressure regulator.

I filled the carb body and tube with gas, then plugged it onto the regulator.  To keep gas out of the regulator, I had to have the tube and the regulator above the carb which was a little awkward.  I had a big plastic tub under it all to catch the spills.  It took all of about 2 minutes to measure the spill-over pressure... so a lot of cost and effort to get this done but I was tired of wondering.

Speaking of fittings, does anyone know of an easy way to measure the fuel pressure at the carb inlet?  Would be nice to find a tee made for this that had a male flare to the carb, a female flare to the hard line, and a tee to a barb for the pressure gauge.  The hard line between carb and pump makes it a pain to hook up a gauge.

Mike

[ourkid2000]

Thanks so much for this information. I think you provided everything I need!

[bry593]

I got this done.

I poured gas in the carb, filled a tube, and pressured with air using a low-pressure regulator.

I cranked it up while holding the float pin.

The needle and seat held fine until I hit 12 psi, at which point it was obviously "overwhelmed".

FYI, I measured the seat ID at 0.128"... likely way more than I will ever need.

The good news is I can eliminate overpowering the needle and seat as a cause for flooding as long as my pump pressure is close to what it should be.

Rather than guess on cleanliness and performance of my pump, I bought a new Delphi brand.

Onward.

Nice work.  That's similar to my method, but I just turn my devilbiss regulator down to 5 psi and blow through the fuel inlet with a standard rubber tip blow gun. 

A lot of the aftermarket mechanical fuel pumps will spike much higher than 7 psi.  The fuel regulator is a great way to address this issue.