fuel pressure question

[jamie2370]

H/SA 350 sbc with quadrajet(Deano carb). Car has a Magnafuel Prostart 500 fuel pump(rated 26-38psi) 12an line to front, Product Engineering Regulator rated 4-12 psi. I am pretty sure that the carb is letting fuel by the needle and seat. It is very rich, sometimes floods out, but then other times at an idle it is perfect. I can not get pressure to go below 6 psi. What is too much pressure for that carb? I know the pump is overkill but it was on the car when I got it. I tried a barry grant regulator that I took off a holley black pump system that was set at 7 psi, it went over 12 psi when I turned the pump on?? SO... do I have a carb issue, too much fuel pressure, or Both lol

[rdlong1961]

The Holley "Black" pump would make more sense with the BG regulator (providing it is still working properly). The volume is sufficient (12 an) to the regulator and reduced to 8 an to the carburetor. Always flush the fuel line prior to hook up. The needle and seat is an area that "must" be clean at all times. The float setting of a 1/4" down is a good starting point also.

[SSDiv6]

Originally Posted by jamie2370

H/SA 350 sbc with quadrajet(Deano carb). Car has a Magnafuel Prostart 500 fuel pump(rated 26-38psi) 12an line to front, Product Engineering Regulator rated 4-12 psi. I am pretty sure that the carb is letting fuel by the needle and seat. It is very rich, sometimes floods out, but then other times at an idle it is perfect. I can not get pressure to go below 6 psi. What is too much pressure for that carb? I know the pump is overkill but it was on the car when I got it. I tried a barry grant regulator that I took off a holley black pump system that was set at 7 psi, it went over 12 psi when I turned the pump on?? SO... do I have a carb issue, too much fuel pressure, or Both lol

What needle and seat size do you have?

With a 0.125 seat you should be able to go as high as 8 PSI. With a 0.135 seat or larger, I stay around 6 to 6.5 PSI. Check for seat and needle assembly wear or dirt. With the hard to find Grose-Jet needle/seat assembly, I have gone as high as 12 PSI.

[ss3011]

Could be too much pressure from the pump feed the regulator . Try reducing it down to 15 psi . Also I have had problems with debris getting caught under the regulating ball inside the regulator , pressure to the carb goes way up . I can usually recover by shutting off the pump while the engine is running then after most of the fuel gets used up in the carb , turn the pump back on , and usually the flow will flush out the debris .

[jamie2370]

Originally Posted by ss3011

Could be too much pressure from the pump feed the regulator . Try reducing it down to 15 psi . Also I have had problems with debris getting caught under the regulating ball inside the regulator , pressure to the carb goes way up . I can usually recover by shutting off the pump while the engine is running then after most of the fuel gets used up in the carb , turn the pump back on , and usually the flow will flush out the debris .

Pump will not go lower than 26psi. Thats the lowest it can be internally regulated. I really think it is too much pump and causing spikes at the guage that I can't see(gauge under hood)while car is under a load. As fuel demand goes up I think the internal bypass is causing it to just push by the regulator. I can take the adjustment screw on either regulator all the way out, in my hand, and it still has the same issues. I also just discovered that there is NO fuel filter in this system anywhere

[Dissident]

Sounds like the regulator is creeping pressure (leaking by the internals). Bummer but no surprises there. Maybe you should proof your regulator's condition.

A very good rule of thumb on QJs is to use 5.5 to 6.5psi and that will fit with all N&S sizes.

Regards,
HB2
Dissident

[ss3011]

There has to be some sort of regulator in the fuel pump . Just like a Holley Blue which has a spring that can be shimmed or cut down . Does your pump have a bypass that gets plumbed back to the fuel cell ?

[jamie2370]

Originally Posted by ss3011

There has to be some sort of regulator in the fuel pump . Just like a Holley Blue which has a spring that can be shimmed or cut down . Does your pump have a bypass that gets plumbed back to the fuel cell ?

Yes, I have adjusted it as far as I can. This is off their website

This pump, like all MagnaFuel pumps, is compatible with all racing gasolines and alcohol
fuels. Fuel additives are not recommended and may void the warranty. The MP-4401 and MP4450 have and external by-pass is adjustable from 25 to 35 psi. They come preset at 28 psi.
The PRO STAR 500 Series has enough flow capacity to support 2000 HP on gasoline or 1200
HP on alcohol. All MagnaFuel pumps require a #8 return line and a #8 vent or rollover valve.
Return line and vent must be above the fuel level. The Pro Star 500 series pump is designed to
work with a dead-head style regulator

[Jeff Stout]

Had the exact same problem on 2 cars. Installed a -6 return and problems solved. Went this way in a street car that had a -4 and a car with no return. Solved both cars right down to rock solid 4 lbs. One with quadrajet and one with 700 Holley

[ss3011]

It might be a good idea to stick a big fuel filter on the outlet side of the pump . Reduce the fuel pumps external regulator pressure down to around 15 psi by cutting down the regulator spring a bit at a time .