Well I just swapped 305 TBI to 350 Carb in my 91 RS and have a problem with my fuel pressure. After about 5 minutes of the car running the fuel pressure will slowly drop to 0. I have the Mallory 7309 Regulator set at 6 psi. I have never changed my fuel pump, and I didnt mess with fuel pump relay or wiring. Is my pump pooping out on me???

one more thing, as the pressure is dropping I can turn the pressure adjustment screw on the regulator and keep it between 5-7 psi until there is no more adjustment then it finally just drops to 0 it jumps up to 1 every now and then, so the car doesnt go dead sitting there idling.

The fuel gauge says I have 1/4 tank left.

 

Sounds like it's time for a pump.

 

Thats what I thought but I was just hoping it was something simple that I forgot to do before I have to change the pump. I can go out there after everything cools back down and start it up again and it will hold 6 psi for a while then drop slowly to 0.

Any pump recommendations, do I really need a high flow pump in the tank for a Carb???

 

Before you go and replace the pump, what are you using to read fuel pressure?

Just curious, because if you using a fuel pressure gauge mounted in the engine compartment, it may just be the gauge being affected by the underhood temperature. My Summit Racing FP gauge does this, it reads 11 PSI cold and slowly goes down to 7-8 PSI once the car's heated up.




No, you don't really need a high flow fuel pump for a carbed motor. But, if your going through all the trouble of replacing it anyway, it definitely wouldn't hurt. Most people here seem to really like the Walbro fuel pumps, the 190 lph and the 255 lph. The Walbro pumps cost about the same as a replacement GM pump.

 

I had a liquid filled FP ga and it gave erroneous readings with fluctuations in temp underhood. Exactly as you're describing.
Removing the plug and draining the liquid out solved the problem.

 

I am using the Summit Racing liquid filled fuel pressure gauge. It is sitting on top of the manifold right in front of the carb. You think it could really be reading that far off when it gets hot???

 

I think so, mine does it and always has. I should have asked this in the beginning, but when the FP gauge reads 0 PSI, does the engine still run fine? If there truely is no fuel pressure, the engine wouldn't run.

 

It will sit there and idle but I am scared I will hurt the motor driving it with low fuel pressure. I have since moved it over next to the regulator on the fender well and it hasnt dropped all the way to 0 it went down to 1 1/2 and motor runs fine, hasnt went dead on me. I went and got some gas also just to rule out the fuel gauge not reading right. So if I drain the liquid out of the gauge it wont fluctuate with temp like that anymore???

 

It did with mine. As the engine warmed, the reading on the FP ga dropped right before my eyes. When I drained the liquid out and then left a small breather hole there, everything was perfect.
Pressure builds up in the gauge if it's all sealed up, and it offsets the pressure sensing mechanism, causing a false reading.
I'm sure you must be wondering why they even sell them if they do that. I don't know. Neither do any of the countless guys who've had the exact same problem.
Drain it and leave a breather hole in there and your problem will be fixt.

 

Well that was the problem, I went out and drained it when I got home from work and drove about 10 miles and came home popped the hood and it was exactly where I set it at when I first started it up. THANKS!!! I had already planned on spending half the weekend changing the pump. Streetiron85 you the man!!!

 

Glad to be of assistance.
When the next guy posts that same problem, it'll be your turn to reply