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bluebus86
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Joined: September 02, 2010
Posts: 11075
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 Posted: Tue Oct 30, 2018 6:41 pm Post subject: Re: Fuel pump pressure is high adding gaskets ? |
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Since pressure is important for correct operation of the motor, buy or borrow a pressure gauge, and use it to determine the gasket stack needed. The Bentley Official VW Service Manual details the procedure in detail, with a pressure gage.
The Bentley book also states upon pump install to start with 2 gaskets under the plastic intermediate flange, then measure rod length above the flange, and add or subtract gaskets under the flange as needed to obtain correct pushrod stick out length. of course many aftermarket pumps require a different rod stick out length than an original specification pump. I have had to add many gaskets to obtain the correct pressure. Sometime below the flange, sometime above the flange.
the Orange 1970 thru 1978 Bentley Manual states pressure should be 3 to 5 psi maximum. I target 4 psi maximum on my 73 motor. The older Blue Bentley Manual for 1966 thru 1969 states pressure should be 2.8 psi at 3400 rpm..
Bug On!
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Zylinderkopf
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Joined: September 09, 2012
Posts: 679
Location: SE Oklahoma
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| Posted: Thu Nov 01, 2018 6:20 pm Post subject: Re: Fuel pump pressure is high adding gaskets ? |
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Bluebus86 and 95nostalgia: thanks. Good contributions. I’m dealing with fuel pressure issues myself on my ‘66 and will need to address one way or another. You gave me some options to think about.
On a side note: I was reading the beginning of this thread and found it very entertaining. Where did these colorful characters go?
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scottyrocks
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Joined: August 19, 2016
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Location: Long Island, NY
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| Posted: Sun Oct 13, 2019 7:45 am Post subject: Re: Fuel pump pressure is high adding gaskets ? |
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I have an aftermarket, or rather, Brazilian fuel pump in my '63. Test pressure is 3.6. I'd like to get it to below 3.
Does anyone know the thickness of these gaskets before I go buy some gasket material?
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wayne1230cars
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Joined: April 05, 2010
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Location: Canada
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| Posted: Sun Oct 13, 2019 3:58 pm Post subject: Re: Fuel pump pressure is high adding gaskets ? |
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I just changed a fuel pump last week. The old compressed top gasket measured out at 0.23 mm or 0.009 inches.
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scottyrocks
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Joined: August 19, 2016
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Location: Long Island, NY
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| Posted: Sun Oct 13, 2019 4:49 pm Post subject: Re: Fuel pump pressure is high adding gaskets ? |
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| wayne1230cars wrote: |
| I just changed a fuel pump last week. The old compressed top gasket measured out at 0.23 mm or 0.009 inches. |
Thanks muchly. Now I just gotta get me some vernier calipers.
What is the thickness of a new gasket?
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wcfvw69 
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Joined: June 10, 2004
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Location: Arizona
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| Posted: Sun Oct 13, 2019 6:15 pm Post subject: Re: Fuel pump pressure is high adding gaskets ? |
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| scottyrocks wrote: |
I have an aftermarket, or rather, Brazilian fuel pump in my '63. Test pressure is 3.6. I'd like to get it to below 3.
Does anyone know the thickness of these gaskets before I go buy some gasket material? |
Before you start installing gaskets, did you measure how much the push rod is sticking up above the fuel pump plastic stand? Is this a generator style pump or alternator style pump?
If it's a generator style pump, the fuel pump push rod should be sticking up 13 mm above the plastic stand at its highest travel. Also, understand the main diaphragm spring is what controls your fuel pressure. Shimming the pump up will lower the pressure a bit but will also lower your volume.
Measure the pushrod and report back your findings.
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pwmcguire
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Joined: January 30, 2011
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Location: Kennesaw GA
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| Posted: Sun Oct 13, 2019 7:00 pm Post subject: |
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| johnnypan wrote: |
Add gaskets till the engine starves,then remove one at a time till it runs good..
there,I posted it again... |
And you will have found that you have not changed the fuel pressure all that much |
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ajsstormer
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Joined: April 19, 2020
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Location: North Saanich, British Columbia
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| Posted: Wed Apr 22, 2020 10:26 am Post subject: Re: Fuel pump pressure is high adding gaskets ? |
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| The Solex 30 PICT-2 carburetor purchased f/ Volksbitz last June tended to flood/run rich @ idle after our 40 hp 1385cc big bore type 1 motor was rebuilt recently. I damaged the old VW fuel pump flange trying to remove it & the shop that did the engine rebuild installed a new flange w/ the old fuel pump. Using a borrowed fuel gauge found the pressure @ 3.7 psi w/ the engine idling. In stages ended up using 6 gaskets under the flange to lower the pressure to 2.4 psi. No more flooding or running rich @ idle & the dune buggy runs and drives fine so far. |
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wcfvw69
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| Posted: Wed Apr 22, 2020 1:28 pm Post subject: Re: Fuel pump pressure is high adding gaskets ? |
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| ajsstormer wrote: |
| The Solex 30 PICT-2 carburetor purchased f/ Volksbitz last June tended to flood/run rich @ idle after our 40 hp 1385cc big bore type 1 motor was rebuilt recently. I damaged the old VW fuel pump flange trying to remove it & the shop that did the engine rebuild installed a new flange w/ the old fuel pump. Using a borrowed fuel gauge found the pressure @ 3.7 psi w/ the engine idling. In stages ended up using 6 gaskets under the flange to lower the pressure to 2.4 psi. No more flooding or running rich @ idle & the dune buggy runs and drives fine so far. |
Just be mindful that lifting the fuel pump up away from the pushrod with gaskets will lower both the fuel pressure and the fuel volume. People have done this in the past and then went for a drive on the freeway under load. Their engines ran into fuel starvation as the fuel pump didn't provide enough volume to keep the carb bowl full of fuel.
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Decades of VW and VW parts restoration experience.
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**Now rebuilding throttle bodies for VW's and Porsche's**
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bnam
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Location: El Dorado Hills CA/ Bangalore, India
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| Posted: Sat Apr 25, 2020 12:12 am Post subject: |
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| Aussiebug wrote: |
A lot of that is horsepucky!
You said it yourself, the SPRING determines the pressure.
The pump rod stroke determines the volume, and that stroke is controlled by the volume being consumed by the engine. More fuel burned means the needle valve stays open to refill the float bowl, means the pump arm descends further as the spring moives the diaphragm, means the push rod can push it up further = longer stroke. All that is VOLUME, not pressure.
The ONLY time the two start to come together with this style of pump is when the engine is starving of fuel because the pump has been shimmed so much the VOLUME of fuel pumped is insufficient for the engine - that is, when the engine is starving for fuel. Of course at THAT point the pressure drops, because the float bowl is taking every drop it can get - the needle valve is fully open and the pump VOLUME cant keep up so the PRESSURE drops too.
That's NOT the way to adjust the fuel pressure. The correct way is to get a pump with the correct rating on the spring, since it's the spring which sets the pressure, not the pump rod stroke.
Johnnypan, Bernouli has nothing to do with it - his formula relates the SPEED of flow affecting the pressure inside the fluid ("The pressure inside a fluid is inversely proportional to the speed of movement of that fluid"... which means that as the speed of a fluid increases the pressure in that fluid drops) - but the fuel pump is working with a fixed pressure and variable volume - it has nothing to do with the speed of the gasoline flowing through the pump.
Just for info (no relevance, but interesting), at full throttle, a 1600 bug engine consumes about 200cc of fuel per minute, and so the speed through the fuel lines is so slow that it's completely irrelevant. The VW fuel pump's capacity (max pumping volume) is around 400cc per minute - twice that which the engine can consume, which is also irrelevant for this paragraph. The VW fuel line is about 5mm internal diameter, so the cross section is about 4 sq mm (piD/4), so at 200cc per minute (the fuel flow of a bug engine at full throttle) the speed of fuel flow is about 50mm (2 inches) per minute through the lines and MUCH slow through the much larger pump chamber. And that's at full throttle - most of the time the fuel flow is much slower than that. |
Just read this older post. One correction I’ll make - spring does not exert pressure. It exerts a force. Spring rate is the force needed to compress the spring one unit length. So, force applied by spring is proportional to compressed distance - stroke (Spring force = spring rate x stroke) This force translates into pressure when acting over the area of the diaphragm - (Pressure = spring force / eff diaphragm area)
So static pressure is dependent on 3 variables - stroke, spring rate, and area of diaphragm. The dynamic fuel pressure will also depend on fuel consumption rate.
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Zundfolge1432
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Joined: June 13, 2004
Posts: 12467
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| Posted: Sat Apr 25, 2020 8:36 am Post subject: Re: Fuel pump pressure is high adding gaskets ? |
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| wcfvw69 wrote: |
| ajsstormer wrote: |
| The Solex 30 PICT-2 carburetor purchased f/ Volksbitz last June tended to flood/run rich @ idle after our 40 hp 1385cc big bore type 1 motor was rebuilt recently. I damaged the old VW fuel pump flange trying to remove it & the shop that did the engine rebuild installed a new flange w/ the old fuel pump. Using a borrowed fuel gauge found the pressure @ 3.7 psi w/ the engine idling. In stages ended up using 6 gaskets under the flange to lower the pressure to 2.4 psi. No more flooding or running rich @ idle & the dune buggy runs and drives fine so far. |
Just be mindful that lifting the fuel pump up away from the pushrod with gaskets will lower both the fuel pressure and the fuel volume. People have done this in the past and then went for a drive on the freeway under load. Their engines ran into fuel starvation as the fuel pump didn't provide enough volume to keep the carb bowl full of fuel. |
A dear friend of mine that owned and operated VW repair shop showed me his solution. I presented my car which had too much pressure and was overcoming the needle valve and flooding the engine.
He removed the fuel pump push rod walked over to a grinder and shortened it. That old bugger got it right and I never had another problem. This the kind of thing those old timers knew and just because it’s not in the book doesn’t mean it shouldn’t be considered. He went to the grave with decades of experience. I’ve still got that pump and rod and spacer, they are a team.  |
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