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Discussion Starter #1
I've acquired an in-line fuel pressure gauge. I have a Malpassi Filter King filter/regulator and dual Webers. Dumb questions following:

1. Should the gauge be installed before the filter/regulator (i.e. between the pump and the filter/regulator) or after the filter/regulator (i.e. between the filter/regulator and the carbs)?
2. I understand the pressure should be between 3-3.5 p.s.i. Is this with the engine running or not and, if running, at what RPM should be used to set the pressure?

Thanks,

Ken
 

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Should be placed after the regulator, so you can see the pressure it has been regulated TO. As for PSI, it should be when running and should maintain that PSI regardless of the engine speed.
 

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Discussion Starter #3
Should be placed after the regulator, so you can see the pressure it has been regulated TO. As for PSI, it should be when running and should maintain that PSI regardless of the engine speed.
Many thanks for your quick response. K
 

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3.5 psi maybe on the high side, it depends upon what size needle jets you have. The bigger the jet, the lower the maximum pressure. I have mine set at 2.5 psi on an engine that makes 170+ HP so it should work for you. You will get flooding and a too rich mixture at low rpm and hard starting when the engine is hot if the pressure is too high.
 

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Lets explain Ed's comment. #200 N & S with BRASS float Webers, 3.5# may ( or not) overwhelm 26 gram float weight. #200 N & S with plastic float Webers will probably tolerate 3,5#, (but not 4#). Ed has this DEAD ON right.
 

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For amusement....

Generally, a float type carburetor wants the same pressure that would flow via gravity from an overhead fuel tank in the top wing of an old biplane, traveling downward to an engine approximately 3 to 4 feet below it. This takes into account that the engine may be higher with respect to the fuel tank when the aircraft is climbing, and lower with respect to the tank when diving.

Pressure really isn't the critical element. It is flow rate sufficient to feed the engine's requirements. As a car's fuel tank is generally below the engine carburetors, it needs a pump to get there. If the flow is sufficient, even a few ounces of pressure would be sufficient. However, 1" fuel lines and fittings sort of get in the way, so some tiny pressure to make sure there is sufficient flow through a 1/4" fuel line (and 2.00 needle and seat) gets the job done.

Things change if you intend to operate inverted. This is rarely desirable in an Alfa.
 

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The needle valve in the float chamber has two opposing forces - the buoyancy of the partially submerged float which is trying to close the valve and the fuel pressure x seat area which is trying to open it. If the product of fuel pressure and seat area is greater than the buoyancy then the valve will be open. If it is greater than the buoyancy when the float is submerged then the carburetor will flood. That is why larger seats will allow flooding at lower pressures than smaller seats.
 

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Richard Jemison
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Fuel pressure regulators

Rather than relying on a regulator to control fuel pressure choosing the right pump is key to a fail safe installation, with or without a regulator.

Alfaholics sells a fuel pump with way too much pressure targeting Weber installations(?). Strange, as the same manufacturer sells a correct low pressure pump.

There are pumps for sale on the internet, designed for use on Onan gas generators for motorhomes, that produce only 3-3.5 PSI (w/o any regulator) Price +- $30.00.

I bought a couple for my MH, and since I was questioning some carb leakage on my race car and the Montreal "coupe" with 4 side draft Webers, I installed one in place of the vane pumps which were internally regulated (shortened bypass springs)to 3.5 lb and then reduced to 3 psi at the carbs with a remote mounted regulator on the firewall.
Issue was that the system had no fuel return line to prevent pressure rise from the pump " deadheading" and raising pressure at the carbs at "no demand intervals" causing fuel overflow.

At a minimum I suggest having a return line from the delivery side of the regulator back to the "feed side of the regulator (or hose) so there is no pressure increase at the carbs.. All that is necessary is a very small opening (.020 (.5mm) to restrict the return line.

Some pumps have thees built into the pump body (Holley and other vane pumps) but the spring controlling pressure will need to be reduced in length (lowering output pressure) but the volumn of fuel created by these pumps can overwhelm the small size of the fuel return passage, resulting in higher "deadhead pressures".
 

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Rather than relying on a regulator to control fuel pressure choosing the right pump is key to a fail safe installation, with or without a regulator.

Alfaholics sells a fuel pump with way too much pressure targeting Weber installations(?). Strange, as the same manufacturer sells a correct low pressure pump.

There are pumps for sale on the internet, designed for use on Onan gas generators for motorhomes, that produce only 3-3.5 PSI (w/o any regulator) Price +- $30.00.

I bought a couple for my MH, and since I was questioning some carb leakage on my race car and the Montreal "coupe" with 4 side draft Webers, I installed one in place of the vane pumps which were internally regulated (shortened bypass springs)to 3.5 lb and then reduced to 3 psi at the carbs with a remote mounted regulator on the firewall.
Issue was that the system had no fuel return line to prevent pressure rise from the pump " deadheading" and raising pressure at the carbs at "no demand intervals" causing fuel overflow.

At a minimum I suggest having a return line from the delivery side of the regulator back to the "feed side of the regulator (or hose) so there is no pressure increase at the carbs.. All that is necessary is a very small opening (.020 (.5mm) to restrict the return line.

Some pumps have thees built into the pump body (Holley and other vane pumps) but the spring controlling pressure will need to be reduced in length (lowering output pressure) but the volumn of fuel created by these pumps can overwhelm the small size of the fuel return passage, resulting in higher "deadhead pressures".
If you are converting a SPICA system to carbs, keeping the existing fuel flow loop back to the gas tank helps cooling the fuel immensely. Dead headed fuel systems can be problematic for pressure regulation.

Note that you need to provide a small restriction for the regulator to 'work against' in the return line for best pressure regulation.

A fuel pump must produce more pressure than the desired regulator output in order for a regulator to maintain pressure under conditions of low to high fuel flow.
 

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Dead headed fuel systems can be problematic for pressure regulation.
That depends upon the type of regulator. I have never heard of this problem with Filter King regulators and my Carter vane pump/Filter king regulator works perfectly. I have a gauge on the output of the regulator and it never moves from 2.5 psi even when the engine is not running and the pump is on. I have no direct experience with Holley regulators but I have read that they will not hold the pressure steady under these conditions. I had a disc type regulator which is sold as a Mr Gasket 9710 and it was next to useless.
 

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That depends upon the type of regulator. I have never heard of this problem with Filter King regulators and my Carter vane pump/Filter king regulator works perfectly. I have a gauge on the output of the regulator and it never moves from 2.5 psi even when the engine is not running and the pump is on. I have no direct experience with Holley regulators but I have read that they will not hold the pressure steady under these conditions. I had a disc type regulator which is sold as a Mr Gasket 9710 and it was next to useless.
My point exactly. Even the disc type regulator will work well in a guaranteed flow setup (i.e. a fuel loop).
 

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My point exactly. Even the disc type regulator will work well in a guaranteed flow setup (i.e. a fuel loop).
Mine did not. The pressure dropped off at high flow. My AFR's went high at WOT in 4th gear and I discovered that the regulator was the cause.
20yearoldspider has a good setup that does not require a return loop.
 

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Mine did not. The pressure dropped off at high flow. My AFR's went high at WOT in 4th gear and I discovered that the regulator was the cause.
20yearoldspider has a good setup that does not require a return loop.
Sounds like a nasty regulator for sure :)

There's good reasons pretty well all modern cars went to fuel flow loop type systems, fuel cooling being number 1, and ease of pressure regulation at varying flows is made easier with a "pre-load" of a return flow. Pumps in general do not like to be dead-headed.

If a dead-headed single line is all you have, it isn't worth adding a second return line - but don't abandon the return part of the loop on a former SPICA-based car when you change over to carbs - it has significant advantages.

My 40 year old spider featured two lines, and works great with a pressure regulator and a small restriction in the return line, from idle to WOT.
 

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When I bought my Spider in 1999 it had a Shankle conversion with the original Bosch pump, disc regulator and return flow line. A couple of years later I spun the car in the rain, landed in a ditch and almost lost the car due to fuel being pumped out of the broken return line. I decided there and then to use a Carter P4060 low pressure pump and no return line. It has been running like that for over 15 years. I experimented with a Facet bendix pump for a while but it was problematic and I went back to the Carter. Then I dumped the disc regulator and installed a Filter King.
I don't need no stinking return line on my 170+ HP motor with 45DCOE's. KISS!
 

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Discussion Starter #15
3.5 psi maybe on the high side, it depends upon what size needle jets you have. The bigger the jet, the lower the maximum pressure. I have mine set at 2.5 psi on an engine that makes 170+ HP so it should work for you. You will get flooding and a too rich mixture at low rpm and hard starting when the engine is hot if the pressure is too high.
I originally set it to between 3 and 3.5 and, just as you said, at low rpm it ran badly and I could smell gas. I decreased it to between 2 and 2.5 and it runs beautifully. Thanks.
 

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I just finished installing the Filter King, I installed the temporary Fuel Pressure Gauge between the regulator and the carb and set it to 3 PSI at idle as advised by Gordon in another thread, then I removed it and installed to test between the two carbs and still got 3 PSI on idle, the psi will drop a little when the RPM goes up, but not below 2.7 PSI. No more fuel smell and the car runs smoother.
Rich.
 

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Hi All,
I have 2 Carter P4070 pumps installed. One from the fuel tank to the swirl pot and the other to the carbs from the swirl pot. The SP has a return to the tank and I haven't yet checked the pressure from the regulator to the carbs, however what is your advice on a return line from the tank side of the regulator to the tank???
Jim
 

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Return lines are useful, but not necessary for some applications. Ferrari used them for years with a 2 pump, mechanical and electric system. The idea was to fill the system using the electric pump (Autoflux) with this "pusher" type electric pump, then start the car, turning off the Autoflux, running it with the BIG FISPA mechanical "puller" type pump. An FRB 11 was fitted in the feed line ahead of the Webers for pulse dampening, filter, and pressure regulation. Now running, the "return" line was from the back Weber, and pumped excess fuel back through the electric pump into the tank. In theory, this kept fuel cool, and further moderated pulses from the mechanical pump. A non-dead-headed system. The electric pump was mounted in back near the tank. Some used the Autoflux pump on, in racing, or for mechanical pump inadequacy.
 

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I can see why you might want a return line if you have a pulsing pump or if you have a high pressure pump that cannot handle being dead headed. But don't the AR mechanical pumps make pulses and aren't they handled by a Filter King regulator with no return line?
 

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Thanks Gordon & Alfaparticle,
Could I have too much fuel pressure as I am having problems with the car seeming to run out of fuel with plenty in the tank.
Jim
 
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