Carter p4070 fuel pump. How does it not need a FPR? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 03-04-2016, 02:31 PM
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Carter p4070 fuel pump. How does it not need a FPR?

I have a question about the Carter p4070 fuel pump. I'm using it with a Weber 32/36 carb. In between the fuel pump and the carb I'm using a Holley fuel pressure regulator but I'm curious......the Carter p4070 manual states it doesn't need a fuel pressure regulator. The Weber likes a fuel pressure of 3-3.5psi and the Carter fuel pump pumps fuel upto 7psi. How does it know what pressure to pump fuel at? I can't get my head around how it can tell what psi the carb needs without causing damage.....

Thank you in advance for any answers I receive.

GB

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Old 03-04-2016, 03:15 PM
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set the fuel pressure to 3.25 pounds. if the carb needs 3 to 3.5 that is the factor to set the fuel pressure at. any additional pressure may cause the needle and seat to leak raising the fuel level in the carb throwing off the calibrations and perhaps dumping fuel either out of the boosters.. and into the intake and cylinders or over the top and sides of the carb and on to dangerous stuff like the exhaust manifolds, spark plugs, and starter..
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Old 03-04-2016, 04:11 PM
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I understand what will happen if the carb receives too much fuel pressure. What I don't understand is why would carter say that this pump doesn't need a regulator? The pump itself must have some kind of valve to adjust the fuel pressure on its own right?
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Old 03-04-2016, 04:20 PM
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I assume the pump has some sort of pressure sensor that's tells the pump to slow down or bypass when the carb floats close down. I run a Holley Bypass reg and a pressure reg in mine....... Bad thing happened to mine when the carb failed to stop the fuel flow..... crankcase full of gas! Luckily I caught it before the Kaboom.
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Old 03-04-2016, 04:26 PM
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Thats what i was thinking but it just looks like a basic rotory fuel pump, i mean i dont like the idea of relying on the pump to keep the pressure. Last thing i want is something to malfunction and possibly rebuild the carb......

Damn! That must've really sucked! Did you catch it before any serious damage was done?
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Old 03-04-2016, 05:15 PM
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Yes, I had been to a show and started driving home it started running rough, stopped and checked things out, all things appeared ok, but it wouldn't restart, finally got it fired up but rough........ then it quit, looked under the hood heard the gas boiling in the carb bowls. My thought was the fuel pump wasn't pumping enough cool fuel, due to the oil safety switch..... messed with the oil switch for approx. 50 min ended up running the fuel pump with a direct jumper to the batt (bypassed the oil safety switch and fuse block) and she fired right up.... cool I need a new safety switch or a wire has come loose but I'll make it home.

When I got home I went inside to start my search for parts approx 1 hour, went back out to the garage, fuel pump was still running, odd I thought, I forgot to pull the jumper, checked the oil..... oil looked thin and was a 5 qts high and smelled like gas....... changed the oil/filter etc..... the carb was basically old and had issues, so I ordered a new carb....... happy face.......

Got it installed started checking fittings for leaks with the key on/motor off after a few minutes I smelled gas..... checked the oil..... dang, gas in the oil again. Purchased a Holley bypass reg got it installed, fired right up, let it run and continued checking for leaks..... no leaks which is good, shut it down and heard the fuel boiling in the carb bowls........ checked the oil.....gas again, after some research decided to add a Holley pressure regulator.

Currently no leaks, no fuel in the oil..... good!
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Old 03-05-2016, 12:22 AM
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Quote:
Originally Posted by gypsy_belvidere View Post
I understand what will happen if the carb receives too much fuel pressure. What I don't understand is why would carter say that this pump doesn't need a regulator? The pump itself must have some kind of valve to adjust the fuel pressure on its own right?
i looked at the carter fuel pump site..

i saw nothing about that pump not needing a fuel pressure regulator.. i saw a N on the line if a fuel pressure regulator is included.

as far as i know.. there are no bypass valves in those pumps.. like holleys pump have.. perhaps there are. it did show 8 psi max pressure..

lets see if this link will load for you..

Carter P4070 Electric Fuel Pump | Carter Fuel Systems Online Catalog
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Old 03-05-2016, 04:23 AM
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The Carter pump has an internal fuel bypass loop that limits the output pressure to 7 psi....and with a Holley performance carb would need no fuel pressure regulator...so that is why it is referred to as not needing a regulator.

For your case you have a different fuel psi specification, and would thus need to regulate pressure down to 3-3.5 psi and you would need a regulator to do that....the Carter pump won't just automatically adjust itself to the psi you need, it will happily keep pumping away to it's 7 psi limit.

If the Carter pump didn't have the preset7 psi internal valve it would be able to make even more pressure, to the point of either stalling out the electric motor, going into cavitation, or shortening the life of the electric motor or pump.

Carter does sell a higher pressure Competition pump that does need a regulator even for performance 4 barrel applications.

All you need to do is put an inline regulator between the carb and the 4070 pump and set it to your pressure requirement and your all set, good to go, no worries.
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Old 03-05-2016, 02:25 PM
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They need to delete that from their literature we have been down that road here 100's of times most Holley's are OK with 7psi but Edelbrock Performers which are a later version of the Carter Competition Series run best at 4.5 to 5.5 psi any more than that and they bog and flood at lower RPM's and drive folks crazy. Read what Carter's own tuning guide specifies on page 4 second to the last paragraph. http://www.mymopar.com/downloads/carterafbtuning.pdf
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