Stock fuel pump pressure.

[oldsmobile joe]

They did, but only for F bodies. It was a low psi in tank electric fuel pump to pressurize the long suction side of the main mechanical fuel pump to prevent vapor lock. They weren't high pressure like EFI electric fuel pumps and are long discontinued. However, they could have used higher psi mechanical pumps than G bodies and part manufacturers consolidated the pumps? Just can't find the original specs to know

The l69 engine in F bodies would run out of fuel near redline. I worked at chevy at this time and installed one these pumps to alleviate this concern. It was a manual trans car and i had to get to redline to duplicate the concern.
As for the 9psi mechanical pump, it sounds too high. I would believe the gm service manual before haynes.
As for your car running rough and your high dwell reading , if the pump is at fault, that would mean the float needle is not seating or is being pushed off of its seat.

 

[fleming442]

Go ahead and throw 9psi at it. I'll be watching....

 

[Clone TIE Pilot]

Does this car still have the EVAP system hooked up? I remember way back when diagnosing over rich condition on OBD1 equipped Chevy Qjet vehicles that we would often find a defective purge valve. This is is a black plastic item tword the front of the intake manifold with 3 or 4 vacuum lines with one of them being 5/16 or 3/8 in diameter, and connected to the carbs float bowl vent. A ruptured diaphragm would allow fuel to be drawn into the the intake, preventing the ECM from controlling mixture. The first time I found this, it really kicked my a$$. After that we replaced dozens of them.

It still has EVAP, I did perform the tests outlined in the GM manual on the purge valve and it is ok. Both diaphragms hold vacuum. So that is ruled out. Sadly I think its just too much fuel pressure. The same GM manual says the fuel pump should be 5.5 to 6.5 psi while all the replacement pumps advertise 7.5 to 9 psi.

 

[Clone TIE Pilot]

Go ahead and throw 9psi at it. I'll be watching....

I already did, the Delphi pump is putting out 9 psi. Could be most if not all of the aftermarket is going off the wrong specs.

 

[Clone TIE Pilot]

The l69 engine in F bodies would run out of fuel near redline. I worked at chevy at this time and installed one these pumps to alleviate this concern. It was a manual trans car and i had to get to redline to duplicate the concern.
As for the 9psi mechanical pump, it sounds too high. I would believe the gm service manual before haynes.
As for your car running rough and your high dwell reading , if the pump is at fault, that would mean the float needle is not seating or is being pushed off of its seat.

I tested the needle valve with a Mityvac and it holds vacuum. It serms the needle is being pushed opened by excessive fuel pressure. Most of the replacement derp can fuel pumps advertise 7.5 to 9 psi. As I said, Jegs and Summit advertise the Carter M6101 pumps as having 6.5 psi while Rockauto and Carter themselves say the M6101 has 9 psi.

 

[butch1024]

I have a 1980 Malibu with a 383 stoker and a 670 cfm Holley dual feed. My new mechanical fuel pump was putting out 7-8 lbs of pressure and
was a bit too rich. I installed a fuel regulator between the pump and the carb and adjusted the pressure to between 4-5 lbs and the car runs
great.

 

[Clone TIE Pilot]

I found a PDF scan of a old Chevy High Performance article concerning increasing power for a L69 3rd gen Camaro. They tested the fuel pressure and it was 6-7 psi stock. They also mentioned that the 350 conversion kit GM used to sell for 305 Camaros included an in tank, high pressure, EFI electric fuel pump that completely replaced the 305 mechanical fuel. This electric fuel pump also came with a return style fuel regulator preset to 6 psi. This means that stock fuel psi is around 6 psi and that 9 psi is much higher than stock. Looks like I will need at least a dead head regulator.

 

[fleming442]

 

[pontiacgp]

I think with the problem of finding a pump that will give you the pressure you require you have no other choice but to install a regulator....

 

[69hurstolds]

As others have chimed in, nominal 5-7 psi for fuel is widely considered the "safe" range for Q-jets. I'm not an expert on anything. But uninterrupted fuel flow is more of a concern than pressure. As long as you can keep the fuel flowing to the carb and be able to shut it off when the bowl is full, exact pressure isn't necessarily that important. The bowl is vented, so pressure isn't as important as maintaining proper fuel level throughout the rpm range. When I used to run Holleys and Thermo-quads on non-stock engines I had, I would usually run the fuel line block with the gage in it just to be sure, or if electric pump, I'd run the gage off the regulator to help me maintain correct pressure.

They weren't cheap gages, but some were cheaply made, apparently. This one time...at band clamp...Had one gage get out of whack and read higher than it should, and trying to adjust the regulator down without knowing cost me fuel on the upper rpm range and I incorrectly fiddled with the ignition timing ad nauseum until I figured that crap out.

Maybe find a cheap regulator and put it in line to the carb, set it to 6 psi thereabouts, and see if your problem disappears.