To start its a 69 with a 540 4spd 4.10 12B. I have a stock tank with a mod to the pick up tube, I replaced the stock pick up with a 1/2 alum line bent to match. The pick up is on the bottom of the tank the sending unit works. The pump is a Comp 140 with -8 an line from tank to pre-filter & pump. A -8 to BG rail filter with check valve, which is in the right direction for flow, to carb which is a BG vac 850, with a psi bypass regulator past the carb set up with a -8 return line to tank. My problem is my car will run strong in 1st-2nd and noses over in third? I look at the gauge and before I make a run it's set @ 7.5 psi at idel, rev it up no change, but hammer it on a run and the psi drops to 3psi? would gas slosh case this? even with a full tank?
I'm thinking I might have to add a rear sump into the tank for a pick up point, has anyone done that fix?

I had the same problem. I also have the same type of fuel system you have.
I ended up adding a second pump and filter. I would say that your large displacement engine requires a lot of fuel, just as my ethanol powered BBC does.
I am currently running 2 Holley Black pumps. That solved my problem while racing, but is overkill for the street......

I don't know how it compares to yours but... Buddy of mine had a 9.teens with a N/A 540 in a 70 Nova with a Aeromotive SS pump and no return line from his fuel cell.

I'm going to fill the tank & try another few runs if the motor runs out of steam. I'll relocate the pressure lead off the log to the pump, to see if the pump is flowing or getting air due to the pick up. If so I'll rig a cell to see if that cures it, if so Then I'll have to sump the tank. Any pointers on getting a tank flushed clean enough to cut & weld?

You can take your tank to your local radiator shop and have them flush/boil it out. Dry ice is another option.

Your problem kind of sounds like you are not pumping enough fuel. You might try to block off the return port and see if that helps. I ran a Mallory 140 a long time ago and didn't use a return line.

blocking the return will only overheat the pump.
which will fail soon after.
i think the pump is also on the small side.
heck most performance mechanical pumps can pump that.
I think you need a bigger fuel line -10 maybe
you should at least have 5 psi minimum at WOT.

A local welder told me about a simple trick to make it safe to weld/work on a gas tank

hook up the tank ( I assume you have dropped it before this) to the exhaust of another running car for a few minutes

dries out the tank in a jiffy

I have not tried it and I am not a welder - so you will have to verify yourself - it kind of makes sense though

Dawg,
Not all electric pumps are designed to run in a return style system. I suggested blocking the return to see if that would help with his problem. "Troubleshooting" the problem is usually cheaper and smarter than throwing money and parts at it. I suspect the pump is probably on the small side for his application.

If I remember correctly when I ran a Mallory 140, it came with both the pump and regulator, the regulator was a dead head (non return) style.

To start its a 69 with a 540 4spd 4.10 12B. I have a stock tank with a mod to the pick up tube, I replaced the stock pick up with a 1/2 alum line bent to match. The pick up is on the bottom of the tank the sending unit works. The pump is a Comp 140 with -8 an line from tank to pre-filter & pump. A -8 to BG rail filter with check valve, which is in the right direction for flow, to carb which is a BG vac 850, with a psi bypass regulator past the carb set up with a -8 return line to tank. My problem is my car will run strong in 1st-2nd and noses over in third? I look at the gauge and before I make a run it's set @ 7.5 psi at idel, rev it up no change, but hammer it on a run and the psi drops to 3psi? would gas slosh case this? even with a full tank?
I'm thinking I might have to add a rear sump into the tank for a pick up point, has anyone done that fix?


Your feed and return lines are fine. -8 is plenty big.
The Comp 140 pump is also fine. I run one and go high nines.
The pickup shouldn't be an issue...if the fuel is sloshing, I'd think it would crap out in 2nd.....not in 3rd gear. Depends on the 60ft though.

I'm puzzled with the BG fuel rail and bypass.

I run the Mallory 4309 bypass regulator. The return goes right from the reg to the tank. Two -8 lines feed the carb bowls.

Where are you measuring the pressure? What set's the pressure?
If there is excess pressure in the return system, you get a false reading in the main system leading you to believe all is well at either idle or even not running. The return side should show little to no pressure.

Also, you did remove the pressure bypass spring from the pump, right? When running the Comp 140 with a return system, the in pump pressure reg needs to be deleted. They include a solid spacer to replace the spring.

Got a photo?

Ron

I'm useing a BG carb & fuel log, with a comp return regulator past the carb, I'm pulling a psi reading from the beginning of the fuel log. I'll make ya laugh now, the whole problem was of all things, My vent on my stock gas cap was not working causing a a vacume problem, I replaced my cap, and she runs great. It's always the little things that seem to throw ya a curve.

Glad you got it fixed...thanks for posting the root cause.

That will do it everytime. Nice easy fix.

I'm useing a BG carb & fuel log, with a comp return regulator past the carb, I'm pulling a psi reading from the beginning of the fuel log. I'll make ya laugh now, the whole problem was of all things, My vent on my stock gas cap was not working causing a a vacume problem, I replaced my cap, and she runs great. It's always the little things that seem to throw ya a curve.


I was going to say check the gas cap vent next.......kidding!
Glad you figured it out!

Ron