The Ford Barn

ford31 · 11-13-2015, 09:10 AM

going to put gas tank behind seat,carb is original zenith,getting ready to buy electric pump,found 1 preasure is 1 to 3 psi,will this work and running 12 volt

Tom Wesenberg · 11-13-2015, 09:33 AM

That should work fine.

I'm running a 12 volt Holley on a 6 volt battery, and I still need to use a resistor to slow the pump down to get the pressure lower.

MikeK · 11-13-2015, 10:06 AM

The question is what is the maximum pressure a Zenith float valve will seal against.
Stock the A would see a gravity feed, thus the pressure with a full tank would be the height times the density of gas. The stock setup never delivered over 0.6 psi, even with a full tank.

OK, so how much pressure to shut the float valve at 0.6, 1.0, 3.0 psi? Not all valves have exactly the same size hole, I have some that are ~3/32” and a few that are 1/8” with a ball. Let’s use the larger size. The surface area of a 1/8” opening ( 0.012 sq” ) times the pressure = force needed. Converting psi to g/sq” you get 3.3g for 0.6 psi fuel, 5.4g for 1psi, and 16.3g for 3psi. Yea, I know grams are mass, not weight or force but we are all stuck on the surface of earth with it’s 9.8 M/Ssq., so that many grams levered upward will close the valve.

Now, The float does not act directly, it acts through a lever with unequal length legs. Guessing

I’d say the float arm is about a 2:1 lever. If you measure how many grams it takes to make a Zenith float sink about halfway in gas then multiply by 2 you will get a rough estimate of what would be applied to the float valve to close it.

Looking at it percentage wise, 1psi is a 67% increase over a full stock tank with gravity feed, 3psi is a 500% increase. When Ford went to the B carb with a pump a helper spring was added to the float.

So, the answer is I don’t know, but I would avoid anything much over 1 psi. Another consideration is that the fuel bowl height (amount the float is submersed) will increase as more float valve pressure is needed as the fuel pressure goes up. Expect the carb to need the float level reset.

Tom Wesenberg · 11-13-2015, 10:45 AM

Way to go Mike. You just smoked my last brain cell.

They don't like to run in overdrive so early in the morning.

BTW, he could also run a recycling line back to the tank to lower the pressure, plus it would keep the fuel cooler to avoid vapor lock from crap gas.

ford31 · 11-13-2015, 12:02 PM

confusing,reg that i have looked at has a range 1 to5 2to 6 ect, i did see 1 awhile back on a car had a dial i think 1 to 5 maybe dependent on fuel pump preasure,so i am looking at 1 max,and not having test equipment to know for sure,and auto parts store for fuel injection only, they hardly know what a carb is

MikeK · 11-13-2015, 02:15 PM

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You can easily measure and set a low fuel pressure regulator without a gauge. All you need is a 6+ foot long piece of clear tubing. With the pump in place attach the hose to the output and hold it vertically. Start with the regulator at zero and turn it up slowly. With an average fuel density of ~0.75, you get 0.32psi for every foot of lift. Thus 2' up the hose is 0.64psi, 3' 1/2" is 1.0psi, 6'1" is 2.0psi. If you want to be really accurate measure the actual vertical lift above the carb level. If the pump put out 3.5psi it would go 11' 7 3/4" up a vertical column.

11-13-2015, 09:10 AM	#1
ford31 Senior Member Join Date: May 2010 Location: Rocky Mount,MO. Posts: 410	another doodlebug question going to put gas tank behind seat,carb is original zenith,getting ready to buy electric pump,found 1 preasure is 1 to 3 psi,will this work and running 12 volt

11-13-2015, 09:33 AM	#2
Tom Wesenberg Senior Member Join Date: May 2010 Location: Mpls, MN Posts: 27,582	Re: another doodlebug question That should work fine. I'm running a 12 volt Holley on a 6 volt battery, and I still need to use a resistor to slow the pump down to get the pressure lower.

11-13-2015, 10:06 AM	#3
MikeK Senior Member Join Date: May 2010 Location: Windy City Posts: 2,919	Re: another doodlebug question The question is what is the maximum pressure a Zenith float valve will seal against. Stock the A would see a gravity feed, thus the pressure with a full tank would be the height times the density of gas. The stock setup never delivered over 0.6 psi, even with a full tank. OK, so how much pressure to shut the float valve at 0.6, 1.0, 3.0 psi? Not all valves have exactly the same size hole, I have some that are ~3/32” and a few that are 1/8” with a ball. Let’s use the larger size. The surface area of a 1/8” opening ( 0.012 sq” ) times the pressure = force needed. Converting psi to g/sq” you get 3.3g for 0.6 psi fuel, 5.4g for 1psi, and 16.3g for 3psi. Yea, I know grams are mass, not weight or force but we are all stuck on the surface of earth with it’s 9.8 M/Ssq., so that many grams levered upward will close the valve. Now, The float does not act directly, it acts through a lever with unequal length legs. Guessing I’d say the float arm is about a 2:1 lever. If you measure how many grams it takes to make a Zenith float sink about halfway in gas then multiply by 2 you will get a rough estimate of what would be applied to the float valve to close it. Looking at it percentage wise, 1psi is a 67% increase over a full stock tank with gravity feed, 3psi is a 500% increase. When Ford went to the B carb with a pump a helper spring was added to the float. So, the answer is I don’t know, but I would avoid anything much over 1 psi. Another consideration is that the fuel bowl height (amount the float is submersed) will increase as more float valve pressure is needed as the fuel pressure goes up. Expect the carb to need the float level reset.

11-13-2015, 10:45 AM	#4
Tom Wesenberg Senior Member Join Date: May 2010 Location: Mpls, MN Posts: 27,582	Re: another doodlebug question Way to go Mike. You just smoked my last brain cell. They don't like to run in overdrive so early in the morning. BTW, he could also run a recycling line back to the tank to lower the pressure, plus it would keep the fuel cooler to avoid vapor lock from crap gas.

11-13-2015, 12:02 PM	#5
ford31 Senior Member Join Date: May 2010 Location: Rocky Mount,MO. Posts: 410	Re: another doodlebug question confusing,reg that i have looked at has a range 1 to5 2to 6 ect, i did see 1 awhile back on a car had a dial i think 1 to 5 maybe dependent on fuel pump preasure,so i am looking at 1 max,and not having test equipment to know for sure,and auto parts store for fuel injection only, they hardly know what a carb is

11-13-2015, 02:15 PM	#6
MikeK Senior Member Join Date: May 2010 Location: Windy City Posts: 2,919	Re: another doodlebug question Sponsored Links (Register now to hide all advertisements) You can easily measure and set a low fuel pressure regulator without a gauge. All you need is a 6+ foot long piece of clear tubing. With the pump in place attach the hose to the output and hold it vertically. Start with the regulator at zero and turn it up slowly. With an average fuel density of ~0.75, you get 0.32psi for every foot of lift. Thus 2' up the hose is 0.64psi, 3' 1/2" is 1.0psi, 6'1" is 2.0psi. If you want to be really accurate measure the actual vertical lift above the carb level. If the pump put out 3.5psi it would go 11' 7 3/4" up a vertical column.