My situation is a little unique. I am running quad Weber IDA carbs. These require fuel pressure of less than 3 psi. They are also susceptible to vapor lock and fuel boiling due to heat soak. My fairly thoroughly researched plan was this:
Walbro 255 lph in-tank fuel pump feeding the fuel rail (through two filters) and regulated on the back end of the fuel rail by a Fuelab return-style regulator. I have no problem regulating to the correct pressure. My problem is this - the Walbro is cavitating badly at no or low load. I can turn the pump on and it will pump just fine for a few minutes, but will then act like it is running out of fuel. The pressure begins to fluctuate a good bit and I can hear the pump revving higher. I'm not sure what is causing this. My guess is that the pump is designed for an EFI application which would typically have a PWM speed controller that would be throttling the capacity down during low fuel demand. Since the pump is running at full speed all the time, it is creating vapor lock inside the pump and is cavitating. This will eventually destroy the pump. Although I was told that the pump would be fine in this type of application, Im starting to doubt that. I could install a speed controller, but a programmable one costs over $300. So, I think I will replace the pump with an external pump. I am looking for suggestions. The pressure rating for the pump is irrelevant, since virtually every pump on the market will be able to supply the 3 psi. It needs to be a pump designed for a return-fuel system. There's a lot out there, and I am looking for suggestions.
On my GT40, I ran twin Carter 7 PSI fuel pumps (2 fuel tanks) regulated to 2PSI with a Holley low pressure fuel regulator. I never had a problem
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I recommend you ping Hot Rod Bill on the Coupe forum. He's running webers on his Coupe with a 347 with the FFR fuel cell which has an electric in tank pump and knows how this set up works. If memory serves me correct he added a return line with his. You can ask him the specifics about where the fuel pressure regulator is in relation to the return line and what size return line he uses. He has a beautiful Coupe and knows weber carb set ups.
I believe that Walbro pump is a high pressure pump and you are correct, that is a FI style high pressure pump.
The pressure rating on the pump is important, you need a low pressure carb style pump.
Actually, the pressure rating on the pump is only important if you need the high pressure. The pump is capable of producing good flow at any pressure up to it's maximum pressure. The amount of pressure the system produces depends only on the regulator, not on the pump. The pump will be just as happy to produce 3 psi as it would be producing 75 psi. In fact, it should be happier since less pressure means less heat and stress on the pump. I confirmed this when I spoke to the folks at Aeromotive and Fuelab.
I'm beginning to wonder whether the pump is defective. It is "new", but I bought it several years ago for a different project and never ended up using it. It should not be cavitating under these conditions. Cavitation is typically caused by too much restriction, not enough flow and heat build-up as a result of the back-pressure. I have the pump running in the exact opposite conditions. There is very little restriction, a lot of flow and nothing to cause heat build-up. The pump shouldn't be cavitating under these conditions, but it definitely is. I'm starting to think that I may have accidentally run it dry while doing the wiring. I don't remember doing it, but with no fuel in the tank, it would have been very quiet and I may not have noticed if it was on while I was testing other circuits. If it ran dry for too long, it is ruined.
So, I'm going to replace the pump regardless. I'm just not sure I want another in-tank pump, or whether I should go external. There is no reason why an in-tank pump shouldn't work, and there are several guys who use this setup for low-pressure carb applications. Most of the external pumps are designed for a deadheaded (non-return) application and are not designed for continuous duty. I believe that is the case with the Carter pumps, though their literature does not make it clear of they are rated for continuous duty or not.
I'll ping Hot Rod Bill.
2FAST4U - Was your GT40 set up with a fuel return?
You may have a regulator problem. I've seen this happen on natural gas. You may be trying to take too big a "pressure cut" at low flow, causing the regulator to "hunt". Think about putting regulators upstream and cutting your pressure to 10-15 psig with the first, then letting a second cut from there to 3psig.
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Most of the external pumps are designed for a deadheaded (non-return) application and are not designed for continuous duty. I believe that is the case with the Carter pumps, though their literature does not make it clear of they are rated for continuous duty or not.
I'll ping Hot Rod Bill.
2FAST4U - Was your GT40 set up with a fuel return?
Thanks for the help.
Actually, most of the external EFI pumps are designed for a return system and do run continuously. I'm using a Walbro in-line 255l/hr pump on my roadster with a '93 Mustang based EFI system. Whenever the engine is running, the pump is running. The computer doesn't regulate the pump at all other than on or off. If the pump is supplying more fuel than is needed, the rgulator returns the fuel back to the tank.
Have you ever run the pump dry? The walbro pumps will eat themselves in a hurry if they suck air.
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