Carb problems

[redroadster]

It has a S bend ( if 1970 ) in the line that needs to be eliminated ,at the tank as mine had only 3/32" open with some calcium build up

[johnscf]

I replaced the fuel return valve and problem solved

[dbrick]

Awesome. Here's a question, does anyone know how that valve should function in concept? It would seem it sends all the fuel to the rear carburetor until the needle valve closes and pressure rises a little, then diverts to fuel tank?

[JackAuffeltrades]

Awesome. Here's a question, does anyone know how that valve should function in concept? It would seem it sends all the fuel to the rear carburetor until the needle valve closes and pressure rises a little, then diverts to fuel tank?

Hello everyone. Just bumping this thread to see if anyone had/could answer the question. I've been looking around and not found much info on these valves. I'm wondering if mine is working properly. I seem to get some back flow into my fuel filter after shutting down and when running, my fuel filter runs really low. I was thinking that if the valve wasn't closing that it might send too much back to the tank and allow reverse flow when shut down. Or am I completely wrong?

[spl310]

The return valve?

[JackAuffeltrades]

The return valve?

Yes, the return valve on the rear carb. I'm just trying to figure out how it functions so I can try to figure out if it's operating properly. I get the feeling that mine might be open all of the time.

[spl310]

I believe that it has a spring inside that regulates the return flow. I have a couple that I'll never use.

[redroadster]

The S bend off the tank in the line calcium up its 3/16 new , put a HF vacuum gun on the line and see ITF it pulls gas easy it's good , but mine was nearly blocked.
This bend stop the tank from siphoning in the event of a wreck and the hose in frt. opened

[JackAuffeltrades]

I believe that it has a spring inside that regulates the return flow. I have a couple that I'll never use.

Thanks. I may have one to take apart in the near future due to some pending additions (roadster shaped piles of parts and rust) to the stable. I assumed the internal mechanism is based on pressure but I've assumed incorrectly many times before.

[JackAuffeltrades]

The S bend off the tank in the line calcium up its 3/16 new , put a HF vacuum gun on the line and see ITF it pulls gas easy it's good , but mine was nearly blocked.

During my previous battle with fuel supply I replaced the supply side with brand new line. The return line was aggressively purged with multiple solvents and compressed air, and seems to be flowing well, and the tank cleaned twice. I'm pretty confident that the circuit flows well now. It's just that the fuel filter seems to run at a minimum level of supply while driving but after I turn off the car, I believe that it's back flowing, filling up the filter to the top again. I thought that if the return valve was closed that it might prevent any backflow.
Lots to learn about these little beauties.

And as an aside, I think that some of the "calcium" deposits in the fuel lines released after I filled the tank with race gas last winter. Our winter lasted about two weeks so I did a fair amount of driving afterwards. That's when my fuel issues returned, showing up in my tank and filter. I'll probably run some more through from time to time for, ummmmmm "system maintenance". Plus, she really seemed to like it.

[Gregs672000]

If your engine seemed to like the increased octane then you may want to consider some Boostaine. It's real deal octane booster that's pretty economical. One ounce in 92 fuel raises it to 96 or so in 10-11 gallons. It does turn your plugs orange. I carry a can with me and use it on hot days or times when I'll be driving harder than normal.

EDIT: It's actually 2 onces of Boostane to take 93 to 95/96 in 10 gallons... I was a bit optimistic and found my measuring bottle mark was 2 ounces, not one as I thought... so about one ounce per full octane number number in 10 gallons.

[david premo]

So according to the shop manual the fuel pressure should be between 3.4 to 4.26 psi while running. Also it recommends that the air be purged from the system to insure that the fuel line has minimal air in the system. Based on this my guess because I can’t find the information in any shop manual that the return line should be operating at about 3.5 to 3.7 psi so that it will continuously return fuel to the tank to prevent vapor lock. Probably the best way to test the valve would be to clamp off the return line and see if fuel pressure goes up. The mechanical pumps are typically self regulating so as not to over pressure the system. If there is no increase in pressure then most likely the return pressure valve is not working.
Dave

[JackAuffeltrades]

So according to the shop manual the fuel pressure should be between 3.4 to 4.26 psi while running. Also it recommends that the air be purged from the system to insure that the fuel line has minimal air in the system. Based on this my guess because I can’t find the information in any shop manual that the return line should be operating at about 3.5 to 3.7 psi so that it will continuously return fuel to the tank to prevent vapor lock. Probably the best way to test the valve would be to clamp off the return line and see if fuel pressure goes up. The mechanical pumps are typically self regulating so as not to over pressure the system. If there is no increase in pressure then most likely the return pressure valve is not working.
Dave

Thanks very much for the tips. With regard to the air purge idea, I figured that the system took care of that on its own after a short period of time. There's always an air pocket in the fuel filter, it just varies in size in my car. I'll give your suggestions a shot and see what happens. Have a great holiday weekend.