Low fuel pressure

[Cnlgolfer66]

Hi everyone, could use your expertise here. A few month back, my 69 Datsun 2000 was stalling on my drive to gas station. Had to get it towed home. Replaced two fuel filters (one upstream of fuel pump and one after). Started car and drive OK, then it started hesitating a bit going up hill. Got a fuel gauge installed just prior to carb and it read about 1 - 1.5 psig after initial start with choke, then it goes to 0 after driving a bit. Replaced the fuel pump, got about 3 psig cold, then after driving (10 miles), pulled into garage and it went to 0 psig again. I am baffled, could use some opinions on how to address this. It drove fine, seems a bit better than before but concerned with low fuel pressure. Thanks!

[keith0alan]

Block off the return to tank fuel line. If this brings the pressure up then your thermo valve of the rear carb float bowl needs attention.

[redroadster]

Has it a S bend in the line 8-10 inches from the tank ? inside it calcium s up it 1/8" inside the line to start so 1/16" which struggles to pull fuel, it you put a HF vacuum pump it should pull fill up the catch cup easily ,

[Cnlgolfer66]

Block off the return to tank fuel line. If this brings the pressure up then your thermo valve of the rear carb float bowl needs attention.

Hi Keith0alan,
Thank you for the suggestion. Finally got around to some parts and installed a shutoff ball valve on the fuel return line. Even after shutoff, it didn't help with the fuel pressure. Any other suggestions?

Thanks,
CNL66

[Nissanman]

You might have a blockage either in the fuel supply line or at the fuel pickup area in the tank. Reverse the flow by using compressed air at the fuel line in the engine bay and see if that clears it. However, it will only be a temporary fix. The tank/lines will need to be removed and thoroughly cleaned to prevent the recurrence of the problem. It isn't that difficult to manufacture new lines which the car probably deserves by now

[Cnlgolfer66]

You might have a blockage either in the fuel supply line or at the fuel pickup area in the tank. Reverse the flow by using compressed air at the fuel line in the engine bay and see if that clears it. However, it will only be a temporary fix. The tank/lines will need to be removed and thoroughly cleaned to prevent the recurrence of the problem. It isn't that difficult to manufacture new lines which the car probably deserves by now

@Nissanman, that'a a great and fairly simple way to varify supply line is clear. I will definitely do that next. Thanks!

[redroadster]

No it's not , I did that on mine ,yeah blows air thru line @40psi but barely to none pulled fuel thru -5 psi and if it's debrie instead of calcium, it will clog again in 15 min
There's a restrictive S bend or a vertical loop in the feed line to stop the siphoning of the complete tank in a wreck where the frt of car is lower than the back which is common many times