Fuel pump pressure issue (carbs)

[GT1978]

I'm using the one of the box shaped facet teeth chattering fuel pumps? I have one on my ra28 but it won't maintain pressure for more than about 5 minutes. Initially I get good pressure ~4psi then it drops off to zero on my guage. If I open the line the pump flows fuel but if I close off the line it won't build any pressure. If I leave it off for 30 mins I get pressure for 5 minutes again. Does anyone know if these need constant flow (return line) to keep cool or if they can be plumbed directly to the carbs with the floats controlling the flow?

[lexsmaz]

What are you using to regulate pressure & what pressure do they run if left without a regulator in line, I'm guessing the carbs would not handle any more than about 7 psi give or take a bit, if they are not flooding or raising the fuel float level without a regulator in line then you are lucky & you can leave it as is ..

[GT1978]

I have a holley fuel pressure regulator which has a gauge on the outlet side to assist with setting the pressure. I probably shouldn't need the regulator as this type of pump should max out about 5-6 psi but as I'm running webers I put it in to drop the pressure to just over 3 psi. Problem is only stays at this pressure for short amount of time then gauge drops to zero. Initially though I might of had an air leak between the tank and the pump so I dropped the tank and replaced the hoses and put a new filter in line before the pump. Even put a little fuel resistant gunk around the hose connections in case I had a slight leak that was enough to let air in but not fuel out. The only other thing I can think of is that the pump is heating up and vaporizing the fuel preventing it's valves sealing properly. The pump is however mounted on a slight angle already to try and prevent this (inlet - outlet on approx 40 degree incline with outlet hose continuing upwards). Noticed today that I'm still getting bubbles come through to the engine bay filter. Can't work out how they are getting in to the system though. Positive everything before the pump should be sealed and everything after the pump should be under pressure so should leak out not in.

[woggin]

seems odd, is the breather for the tank operating correctly?

i dont have many other ideas...you seemed to have covered the main things.

[ToyTA22]

I'm thinking same lines as woggin, try undoing fuel cap and try. If the vacuum line to carbon canister is not connected properly may cause vacuum in tank when running. Had trouble with TA22 doing similar.

[GT1978]

I do always seem to have a lot of vacuum when I remove the fuel cap. Hadn't really considered that. Any suggestions on the best way to check the line between the canister and the tank. Do I just pull it off at both ends and try to blow through it? Is it likely that the canister could be gummed up or blocked some how? It would be about 38 years old now.

[woggin]

cant comment on the canister being gummed up, as i dont run them, i usually disconnect them to fit intercoolers and such (they are usually in the way)

but you can try blowing from both sides, alternatively, u can leave the canister side unplugged, undo fuel cap, blow through it (should have flow) and then start her up and see how it goes!

if it doesnt do it, u know its a vacuum issue, then u can put the fuel cap back in, if it starts doing it, you can assume its the line, if it doesnt do it, its the canister!