This morning I was watching an episode of Truck U and they were advocating the use of a fuel return line to the tank on a carburetor based fuel system. Their theory was to add a return line near the carb fuel inlet to keep the fuel cooler and to prevent vapor lock.

I have a sbc in my F1 with a new Summit carb, Summit hiperf manual fuel pump, and a 4psi Holly electric fuel pump near the tank to push fuel to the mechanical pump. I also have an adjustable fuel pressure regulator between the pumps.

I have a gauge at the mechanical fuel pump that bounces between 4 - 12 psi at idle to 2,000 rpm. Gauge is not liquid filled which may account for the fluctuations.

Engine runs great ( New crate 350).

Should I add a return line to the tank or leave it as it is?

If I add a fuel return line - where should I tee off the line?

Your opinions are valuable. Please hit me with them.

Thanks

Dan

 

Dan, unless you are having a vapor lock issue you shouldn't need a return line. If you decided to use one there are pressure regulators available that are return type. They look like yours but have inlet, outlet and return ports. Once the set pressure is reached the excess fuel returns to the tank. This would also require some type of return fitting at the tank. More trouble than it's worth unless you have vapor lock problems.

 

Agreed, no need to add a return line. Only time I can see it with a carb is if you were running an electric fuel pump and an external fuel pressure regulator. Then the return system is better than a dead head system.

 

Thanks guys.
When I laid out the fuel lines I used the low volume electric pump to push fuel to the mechanical pump.

I used this type of system to cure vapor lock in a mid 70s Bronco. Fixed the problem.

Didn't think a return line was necessary, but my fuel pressure regulator has a return line port and my tank has a nipple that I can useat as a fuel return.

The guys at Truck U made valid points. Good thing is that if I decide a return line is needed, I can add one fairly easily.

Thanks - I appreciate your feedback.

Dan

 

Dan,

Normally, unless the fuel line sees crazy heat, vapor lock shouldn't occur with an electric pump "pushing" fuel. That said, I do have a few comments to share should they be helpful.

Where is your Pressure Regulator in the fuel circuit chain? It looks to me as if it might be between the Electric Pump and the Mechanical Pump?...

The Regulator should be last in the chain with a pressure gauge after (or with) it before the carburetor. Regardless of whether your gauge is liquid-filled or not, it should not fluctuate wildly.

The SBC Diaphram Pump is a "Puller" from the Tank. The Summit "performance" pump is listed at 8 psi. output. If the Electric Pump is rated at (or regulated to) 4 psi, I'd think there would be drama inside the fuel line between them. If the pressure settles down after 2000 RPM, the Mechanical Pump may be without enough fuel to provide its full pressure capacity.

The Holley (Summit) Carburetor is designed to receive 5-7 psi with 6 psi being ideal. The Edelbrock design should not exceed 6.5 psi. So if the Regulator isn't last in the chain (again, I'm only going by my interpretation of the photos), then the Mechanical Pump output at 8 psi. is too high for the Carburetor. The new Carburetor may hold the pressure back initially, but it will fail as it ages causing flooding.

Lastly, for a street driven vehicle, I see little reason to have both Pumps in Series. Either one should serve you well on its own.

Best Regards,

 

Eman92082

My pressure regulator is mounted on the frame between the electric fuel pump and the mechanical fuel pump.

This was done to keep the pressure to the mechanical fuel pump low.

I have been worried that the pressure at the carb is too high.

Looks as though I should add a fuel pressure regulator between the mechanical pump and the carb.

I agree that the carb is handling it now but once on the road - I don't want problems.

Thanks for the advice.

Dan

 

My take would be to remove the Regulator and re-locate it before the Carburetor. The Mechanical pump diaphram is pulling the same 8 psi it's pushing out. If you must have the Electric pump, I'd make sure it's proving at least 8-9 psi to the inlet of the Mechanical pump.

Cheers

 

Sounds like you guys are on the right track.

Regarding a return line. These were added when cars went to fuel injection and computer control because the injectors need consistent pressure over the full range of flow (idle to WOT) to deliver the right amount of fuel. If the pressure drops then the amount of fuel drops and air-fuel ratio increases and you are kaput. I suppose they could have made a system without a pressure regulator and return line and built a progressive curve into the computer to compensate for the pressure drop as flow increased (well except for dealing with the problem of the fuel filter gradually restricting flow) but I'm guessing is was more precise and easier to add a regulator return line.

But a carburetor is not as sensitive to the pressure as long as there is enough to keep the bowl full but not too much pressure to blow out the needle valve.

 

Thanks Dan

I have decided to add a second fuel pressure regulator between the mechanical fuel pump and the carb to controlled the pressure to the float bowls.

I have a regulator between the electric and mechanical fuel pumps mounted on the frame rail. A second regulator is cheap enough and I don't want to replumb the fuel line at the frame rail.

No return line in my future.

Thanks for all the feedback

Dan

 

my SBC350 is from a 98 and has the casting bosses for the fuel pump but it's not been drilled out or tapped for mounting a pump. So I had to put an electric pump on it. I'd much rather have the good old fashioned mechanical pump. I went too cheap on the first pump and about a month ago it stranded me. Then I bought a
Fascet posi-flow model 60106 that's rated for 32 gph at 4 - 7 psi. NAPA had it on the shelf and I was on my way with only an hours delay. Pretty hard to replace an in-tank pump on the roadside.