A/B Fuel Pump Pressure

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Paul Ahnell
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A/B Fuel Pump Pressure

#1 Post by Paul Ahnell »

Why can't I get a Workshop Manual spec reading of 1.9 - 2.6 psi from a newly rebuilt A/B fuel pump between 1k & 3K rpm? It's been a loooong time since I took the mandatory Fluid Mechanics class, and I'm stumped.

The pump actuator rod throw falls between 29 & 33 mm as measured on a genuine VW Tool 328a.

I've tried 2 pumps, both rebuilt by one of our highly regarded and knowledgeable 356 Registry Maestros (he was kind enough to send me a second to try when I couldn't get the spec pressure). The best momentary readings I've had is ~1.5 psi at 3K rpm. The readings are mostly ~1 psi or less. I've used 2 different 15 psi gauges, one liquid-filled, the other not.

Given the expertise of the Rebuilder, it's clear to me that the pumps should not be the problem.

Could it be the way I have the gauge hooked into the system? I'm using standard FLAPS 5/16" ID fuel hose with gauge and tee to connect the stock fuel lines from the pump to the carbs. See attached pix. Do I have too much hose?

Now, the intriguing part. Today my '60 Normal and I clocked an honest 95mph while pulling a long 15*+ grade. Could probably have done more but ran out of room. No stutter or farting along the way, just strong. I guess my quest for an answer above is moot, and simply to satisfy my curiosity. But, I sure would like an answer.
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2nd gauge setup.JPG
2nd gauge setup 1.JPG
normal connection.JPG
Paul Ahnell
'60 Normal Coupe

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John Brooks
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Re: A/B Fuel Pump Pressure

#2 Post by John Brooks »

Try another gauge, here is a banjo bolt with 1/8 NPT
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John Brooks

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Dick Weiss
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Re: A/B Fuel Pump Pressure

#3 Post by Dick Weiss »

Paul,

Did you rebuild the fuel pump correctly by pre-loading the diaphram flat to the lower body surface when adding the top 1/2
and snugging-up the 6-screws.
Also, if the diaphram isn't flexible, you won't get the pressure (or flow) you need. Are both check valves in good shape?

Dick

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Harlan Halsey
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Re: A/B Fuel Pump Pressure

#4 Post by Harlan Halsey »

A friend of mine had no end of trouble with a fresh rebuild recently until finally he found that the spring in the brand new rebuild kit was weak. An old spring cured the problem.

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Re: A/B Fuel Pump Pressure

#5 Post by Dick Weiss »

Paul,

In your photo, you should raise the oil filter can midway in the strap clamp to remove the kink from the inlet oil line.

Dick

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Paul Ahnell
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Re: A/B Fuel Pump Pressure

#6 Post by Paul Ahnell »

All: Thank you for your comments.

Dick Weiss: Remember, I tested two pumps with the same result. The rebuilder bona fides are without question and he assures me that the springs he uses (Harlan) are top notch without problem in over 100 rebuilds. The seats on my original pump were shot along with other deficiencies so he recommended I take one of his shelf. I assume they have new seats, one of his specialties. Diaphragms give a nice squawk when cycled indicating to me that tool 328b was used to set them level. The rebuilder's competence should not be in question. I used two different gauges with essentially the same result. I the sled my usual Springtime La Dow tune up from bottom to top and I was able to haul up that grade with no problems, just running strong. I thought it would starve for fuel with such a low indicated pressure. Thanks for the tip on raising the oil filter.

John Brooks: I like that. Elegant. Does the gauge screw into the bolt at the carb thereby eliminating the hoses? Where did you get that bolt and what gauge do you use with it? Where can I get that setup? Please give me the details.
Paul Ahnell
'60 Normal Coupe

Steve Kram
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Re: A/B Fuel Pump Pressure

#7 Post by Steve Kram »

Interesting to read this.
I recently was installing a newly rebuilt fuel pump on my B car.
Used a new "phenolic" spacer. Found out the hard way that some of the new spacers are wider than the original ones.
Using them gives you low fuel pressure that you cant cure. Found an original one and the pressure was right on the mark.
FWIW.

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Jim Clement
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Re: A/B Fuel Pump Pressure

#8 Post by Jim Clement »

begs the question.. how thick should they be ?
 

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Harold Singh
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Re: A/B Fuel Pump Pressure

#9 Post by Harold Singh »

Jim Clement wrote:begs the question.. how thick should they be ?
I’d like to know this and I do know that I’m within .02” based on a measurement from another running C. It appears that it should be .25”. Someone tell me if that is correct.

So that’s all that I can tell you about the C spacer.

Thanks,
Harold
Harold
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Steve Kram
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Re: A/B Fuel Pump Pressure

#10 Post by Steve Kram »

Here's the 12mm banjo bolt that takes the fuel pressure gauge:
Russell 640700 Fuel Pressure Take Off Banjo Belt available from Amazon $7.18
http://www.amazon.com/Russell-640700-Fu ... Banjo+Belt

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Re: A/B Fuel Pump Pressure

#11 Post by Dick Weiss »

Paul,

The pump's pushrod actuating range is 29-t0-34mm; It doesn't seem it's not much, but depending on the amount of shim-gaskets are needed, it could solve the problem. I use a 0-to-5 PSI gage w/an adaptor at the R/H carb's inlet.
My gage came from my former employer's hydraulic department. (35+ years ago)

Dick

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Eric Wills
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Re: A/B Fuel Pump Pressure

#12 Post by Eric Wills »

As Dick stated, even a perfectly rebuilt pump needs to be setup on the engine. We use the tool the factory workshop manual recommends, VW 328a. VW 328b is used to preload the diaphragm during assembly. Correct setup is easy with these tools and if you have a shorter pushrod than should be, it will show during the setup.

Given you have driven the car over 90 MPH and did not have any stumbling, I would suspect a faulty pressure gauge. Having more than one is handy. I had a gauge that was off by 1 lb. It was sh*t-canned immediately. Not a big deal on higher pressure applications like an MFI 911, but can be an issue for carbs that require 2.5-3 lbs.....

Summit racing or Pegasus will have decent pressure gauges that do the job.

Edit....I see the OP used the vw328a tool to setup the pump. I guess anything I have added will only be of use to members that search the forum....
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vw328atool.jpg
vw328a&b.jpg
fuelpressuregauges.jpg
Eric Wills

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Paul Ahnell
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Re: A/B Fuel Pump Pressure

#13 Post by Paul Ahnell »

I used both factory tools to initially set up and check the spacer and two gaskets I received with my original rebuilt pump. The "b" tool was a homemade dimensionally correct block of wood which gave me a nice "squawk" from the pump when actuated. The "a" tool was a borrowed original made in the Fatherland. I found that the spacer supplied by the rebuilder was so thin (~5.6 mm ) it required making and fitting NAPA gasket material to bring the pushrod back to the generally recommended 33 mm mark on the "a" tool with fasteners torqued to 15 lb-ft. After many tries, I gave up and got a spacer from Stoddard which was on the order of 7.6 mm thick and, with a couple of 0.45 mm paper gaskets, brought new pushrod into battery at <~33 mm on the "a" tool, with fasteners torqued to 15 lb-ft. I then removed the "a" tool, installed the pump, and checked pressure with TWO 15 psi gauges, one fluid filled, the other not. Never got to a 2 psi reading but who cares? Car goes like schnell (now with a pump from Sierra Madre) and I'm done with it. Process was generally a PITA; pump off, on, off, on, repeat.

I'm curious, what is the thickness of the OEM "insulating spacer", part #616.108.440.00 (without the two paper gaskets).
Stoddard's is ~7.6 mm. The one I received with the first rebuilt pump was ~5.6 mm.
Paul Ahnell
'60 Normal Coupe

Steve Kram
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Re: A/B Fuel Pump Pressure

#14 Post by Steve Kram »

I measured mine (while assembled in the car) and the spacer looks to be 10 - 11 mm wide. Have two gaskets and the pressure is 2 psi.

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Paul Ahnell
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Re: A/B Fuel Pump Pressure

#15 Post by Paul Ahnell »

I told myself I'd put this in the bottom drawer and forget it. But, we've already ID'd 3 different thicknesses for this spacer. Are there more? Three or four or five mm makes a big difference when you're setting up the pump. Seems to me too thin and you'll push the actuator rod through the pump. Too fat and you won't get enough pressure.

To satisfy my curiosity, what's the true OEM thickness of this bugger and why are there variations?
Paul Ahnell
'60 Normal Coupe

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