CatBird

Recommended fuel pressure regulator for gravity feed system??

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Recently acquired prewar car that has a gravity feed gas tank. It has an electric (12v) fuel pump with a cheap old fuel regulator that was leaking badly. I pulled the old regulator  and replaced it with a piece of tubing. Now the carb (upflow) is overpowering the needle valve and leaking. 

I want to get a new regulator, but don't know the proper pressure for my system. I have looked at a few, but are made for much higher pressure. Probably for fuel injection. I need something that will work with my carburetor.  I could go back to original and just remove the electric fuel pump, but may be better to stay with it and get the pressure properly.

 

Any recommendations? What pressure should I need? 

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Obviously fuel "pressure" in a gravity feed system will be negligible - depending only upon the vertical distance between the fuel tank and the carburetor.  Unless you can source a pressure regulator that can be dialed down to almost zero, your best bet may be to resurrect the gravity system. The common regulators in use today can be set at 2-3 psi for carburetors, but even that may be too much for a carburetor designed for gravity feed.

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The Airtex External Electric Fuel Pumps E8016S is rated at 4.5 psi.  I have used it on Volkswagens and Corvairs without a regulator and it works fine in those applications.  

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Measure the difference between the tank bottom elevation and the carburetor inlet. 

Then, multiply by .433 and .74 to covert for the specific gravity of gasoline to closely determine the inlet pressure from the original gravity system.  

SG of gasoline is .733.

 

Feet of head pressure to Psi

Converting Head to Pressure. Converting head in feet to pressure in psi. Pumps characteristic curves in feet of head can be converted to pressure - psi - by the expression: p = 0.433 h SG (1) where. p = pressure (psi) h = head (ft) SG = specific gravity of the fluid.
Edited by Mark Shaw (see edit history)
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Yep, If I remember right thats the psi on Airtex pump I used with my Packard 8 to assist the oem pump.  Good unit, but if OP has say, a Model A it is likely too much for it.

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3 hours ago, CatBird said:

may be better to stay with it and get the pressure properly.

 

????

 

It would be better to use the gravity system than to add an unneeded pump, my opinion. Has the tank location or carburetor location changed sine it was built by the manufacturer? Was the system known to be difficult when it was newly purchased? If no to both, why reinvent the wheel with added parts to break?

Edited by Frank DuVal (see edit history)

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This is a relatively expensive Holley regulator that I use on my 1929 Cadillac, which was originally gravity feed from a vacuum tank. Have been driving it for nearly 10 years on this system. I would prefer to have a correct vacuum tank, but the one I found, restored, and was ready to install was stolen at Hershey, so until I find and restore another one, the electric pump is working. You can get this regulator at Summit Racing for about $100. These are old photos, I've since replaced the rubber hose with hard line, which I recommend.

 

Regulator3.thumb.jpg.796435e5101170aec06ef6c0025a7db0.jpgRegulator1.thumb.jpg.2c6745957385724df86a6249f085f957.jpgRegulator2.thumb.jpg.62b20319e98d6fa313c80c5e89b3fa38.jpg

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For supplemental electric pumps on mechanical fuel pump cars, I use a Holley no. 12-804 regulator similar in appearance to Matt's, adjustable between 1 and 4 psi.  Last one I bought was 10 years ago and was about $65.  For a gravity system, I'd try a setting of 1, not more than 2, psi.

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British cars have an electric pump AND a float bowl overflow pipe. Once they started putting catalytic converters on the cars they got to shying away from that.  The specific gravity of gasoline is ,8 or so. That drops the pressure another 20%. I have even found gravity systems with a pressure regulator between the chamber and the carb, nothing better than making the original system work.

Bernie

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The pressure that may be used with a carburetor is dependent on: (A) the buoyancy of the float, (B) the mechanical advantage of the float (due to placement of float pin, and type of hinge), and (C) the orifice size of the fuel valve seat.

 

Generally speaking on pre-1932 carburetors, the mechanical advantage/buoyancy is such that 3/4 to maybe a maximum of 2 psi (if the fuel valve seat is modified) is a reasonable range.

 

Some of the better carbs (in alphabet order) such as the Carter BB-1, the Stromberg SF and SFM series, and the Zenith 63 and 263 series from 1932 and newer can get to MAYBE 4 1/2 psi with a modified fuel valve seat.

 

BE CAREFUL WITH THE REGULATOR YOU SELECT (IF YOU OPT TO USE A REGULATOR) ! The inline dial-type 0~5 psi for $34.95 at the FLAPS is excellent..................IF YOU HAVE A RABBIT PROBLEM IN YOUR GARDEN AND A STRONG RIGHT ARM ;)

 

Jon. 

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Nothing like an electric fuel pump on a gravity system to burn your car to the ground.......and occasionally the garage also. The cars were driven millions of miles with vacuum tanks and there were no issues back in the day. Yes, modern fuel can make a very small difference in the system functioning......maybe two or three percent. Fact is, the old way was the best way, and most of the time the modifications people make are due to ignorance, laziness, cost(many think it’s cheaper.....it’s not.) Why not just dump a crate motor and fuel injection in..........if your car doesn’t challenge you, are you really having fun? I recently spent quite a bit of time sorting a 100 point V-16 Cadillac, keeping it stock wasn’t easy, as I had to remove all the trash and great ideas people installed on the car. I put the car back to stock, and jumped in it and took it down Interstate 95 in Souther Florida..........no problems, no issues, no worries. The carbs will hold gas without shutting off the petcocks for about two weeks before one side seems to bleed down for some reason. Since we drive our cars just about every two or three weeks, I just make an effort to drive the 16 a bit more than the others........not a bad fix if you ask me. I have probably seen and know of two dozen cars that burned to the ground with pusher pumps on these early and crude systems. Don’t chance it. By the way, I carry FOUR two pound Halon extinguishers in the Caddy even though it’s stock.......they burned a lot back then without the extra help. Ed

Edited by edinmass (see edit history)
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I agree with Ed, I've almost lost two cars to electric fuel pumps that fed too much fuel to carb, backfire, engine fire.

 

If you insist on using an electric pump, 1)have a big ON/OFF switch easily hit when your engine catches fire, 2) use a fuel pump delivering 1 to 2 PSI mac, 3)don't trust those cheap damn chrome regulators you buy at any parts store and aren't worth a tinker's damn, 4)spend just a LITTLE effort getting your original fuel system to work, it worked when new, it'll work now, just FIX the darn thing.  Do you really think the buyers back then drove out of the dealer saying "Gee, wish I had an electric fuel pump".

 

I have a 1927 Dodge with a vacuum tank, and getting into car with a DRY vacuum tank, it works so well that 5 or 6 revolutions of the engine suck gas to the tank and start the car.  It's been rebuilt properly..

 

Almost lost cars?  1928 Packard 443 coupe and my much cherished 1931 Pierce phaeton...

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All very good advice previously given!

I note you did not ID the car or carburetor.

Many of the previous responders, I am thinking, have brass carb  bodies, if yours has a white metal "pot metal" carb,   I would not pressurize it.

As example, many Marvel carbs are a little better than "pot metal" but not by much.

Modern gas seems to be able to erode or soak into the metal and deteriorate this type of carb.

Please provide info to this Forum about your vehicle and the fuel system components you have remaining, and I feel certain the Forum can help you.

 

 

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I think I will refit the gravity feed system. Not sure why the previous owner had fitted an electric pump. The original carburetor and gas tank are in the same places. Should work as original.

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43 minutes ago, CatBird said:

I think I will refit the gravity feed system. Not sure why the previous owner had fitted an electric pump. The original carburetor and gas tank are in the same places. Should work as original.

 

Good thinking, 1.5psi was the acknowledged standard for vac feed systems back in the day.

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Recently I was at a gas station and someone was filling up their big classic and left it to run while filling the tank. For some reason right at that moment the car pushed fuel past the float, although it had never done so before. About two gallons of fuel landed under the car, I managed to see it and got the car shut off and the gas pump off also. Problem is there were still a bunch of people walking past the car with a lit cigarette.........I was standing there with an extinguisher for ten minutes before we were able to push the car back and put speed dry down. I notice in a Florida that there are many more smokers than I see up north. Anyway, down here I am ten times more concerned about people tossing cigarettes in and around cars..........not too much of an issue on modern stuff, but many cars hot soak and dump fuel on the ground from updraft carbs.......the last thing you want is a puddle of gas under your parked car in a lot down here. My 14 Caddy had a factory pressurized fuel system, and was famous for dumping gas after you shut it off and didn’t bleed down the pressure. It turned into a habit every time I shut the car down to let the pressure out......even if I only stopped for five minutes. After many, many pools of fuel in the garage, on the driveway, and in parking lots I sold the car...........it seemed I was always smelling fuel in the garage and house whenever that car was at my home. I just didn’t want to deal with the chance of fire........Ed

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2 hours ago, hchris said:

 

Good thinking, 1.5psi was the acknowledged standard for vac feed systems back in the day.

 To get 1.5 psi to the carb with a gravity feed system the vacuum tank would have to be up on the car's roof.  https://www.convertunits.com/from/psi/to/foot+of+head

 

With the average firewall mounted vac tank, fuel pressure measured at the carb is only about 1/2 to 3/4 psi. I have a lab grade pressure gage that reads in 1/4 psi increments and have measured it. But, if the system is put back the way it came from the factory, it has plenty of volume, so it doesn't need much pressure.   As  pointed out by forum member Carbking a number of times in posts, gravity fed carbs used a different float needle and seat. That's because it needs a larger opening for flow volume, not a smaller opening to help control  fuel pressure balanced by the float.  

 

I've driven vac tank cars around our long, steep CNY hills and never had a fuel starvation problem if the vacuum tank and carb are in good shape.  The problems tend to show up when it's thought that the fuel system was rebuilt, but it wasn't rebuilt properly. About half my work load for 40 years has been repairing what others "fixed".

 

Paul

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2 hours ago, edinmass said:

Recently I was at a gas station and someone was filling up their big classic and left it to run while filling the tank. For some reason right at that moment the car pushed fuel past the float, although it had never done so before. About two gallons of fuel landed under the car, I managed to see it and got the car shut off and the gas pump off also. Problem is there were still a bunch of people walking past the car with a lit cigarette.........I was standing there with an extinguisher for ten minutes before we were able to push the car back and put speed dry down. I notice in a Florida that there are many more smokers than I see up north. Anyway, down here I am ten times more concerned about people tossing cigarettes in and around cars..........not too much of an issue on modern stuff, but many cars hot soak and dump fuel on the ground from updraft carbs.......the last thing you want is a puddle of gas under your parked car in a lot down here. My 14 Caddy had a factory pressurized fuel system, and was famous for dumping gas after you shut it off and didn’t bleed down the pressure. It turned into a habit every time I shut the car down to let the pressure out......even if I only stopped for five minutes. After many, many pools of fuel in the garage, on the driveway, and in parking lots I sold the car...........it seemed I was always smelling fuel in the garage and house whenever that car was at my home. I just didn’t want to deal with the chance of fire........Ed

 

For those very reasons, I've installed a few of these electric fuel shut off valves in customer's  driver cars with gravity feed systems. They work very well. Then, the original hand operated fuel shutoff valve is only used for long-term shut down as a safety back up.  

http://www.snydersantiqueauto.com/ProductDetail/A-9230-B6_ELECTRONIC-SHUT-OFF-6-VOLT-30-31?fromCategory=SearchByKeyword

 

They come in 6 and 12 volt models.

Paul

Edited by PFitz (see edit history)
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Paul - thanks for posting that link. I was somewhat surprised by the price, but since I haven't personally purchased one in over 20 years, my surprise might be related to the time-lapse.

 

For years, I have been suggesting these to customers with leaky updraft carburetors. Have always suggested they try a motor home supply house, as many motor homes were equipped as dual fuel (gasoline/propane). One of these valves is necessary in a dual fuel environment.

 

Jon.

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We did work on a '31 Caddy 12 some years ago. The owner insisted on keeping the electric fuel pump. A few years later he did in fact barbeque his engine compartment. Only luck prevented the whole car from being destroyed.

 

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18 minutes ago, carbking said:

Paul - thanks for posting that link. I was somewhat surprised by the price, but since I haven't personally purchased one in over 20 years, my surprise might be related to the time-lapse.

 

For years, I have been suggesting these to customers with leaky updraft carburetors. Have always suggested they try a motor home supply house, as many motor homes were equipped as dual fuel (gasoline/propane). One of these valves is necessary in a dual fuel environment.

 

Jon.

 Jon,

Yes, the price has jumped - and in recent years. It was about 2/3 that for the last one I bought only a few years ago.

 

But, I think it's still worth it because it not only makes stopping the engine safer, it's a lot more convenient not having to open the hood and shutoff the fuel line each time the engine is stopped. And if there is a sudden fuel leakage problem at the carb, fuel flow is positively stopped as soon as the ignition is shutoff.    

 

Paul

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