Im struggling with fuel pressure dropping as the motor warms up. 650bhp 533cui. Same setup worked for a year before car came off road for new engine, box etc. Pressure starts about five psi when cold but drops at motor warms up, dropping to to 2-3psi

100gph Carter 5psi electric pump. I replaced it no change.
Half inch line, replicates original line.
Removed sock off old line incase it was being sucked into new line.
Originally ran old line as a bipass for a year with no problem. Removed regulator so no bypass, no change.
Tried new 12volt battery power supply.
Pump has gravity filter before pump, removed filter, no change.
Placed gauge at pump, same reading.

I have a 15/18 psi pump I can use but had planned to use this on a blown motor.

any ideas

 

5psi is low for a Qjet and i believe a Holley uses 5-6psi. But what i see is that since u removed the regulator and bypass the fuel pump dead heads against the neddle vlv and seat now once the float moves to close. I'm guessing that 100gph electric is not self regulating without some flow. U either need a regulator or just use an tiny 0.020"-0.040" orifice in a return line to the tank.
Years ago i tried running an electric pump even with a regulator but no return line to the tank from the Qjet on my GTO. Man, the fuel pressure went all over the place trying to adjust and set it at idle.
Now i just use an orifice restricted return line and my Carter mechanical pump maintains 6-7psi all day long (self regulating) - but this is on a lo po sbc. I plumbed a restricted return line from the Qjet inlet. Also have a Holley electric press gauge (only $70) teed in there so i can read fuel press while driving. U probably need that 100gpm electric pump with 1/2" line for the 533 cubes.
U can install a orifice restricted return line anywhere down stream of that electric pump but the closer to the carburetor then better for cool fuel and reduced chance of vapor lock. I just plugged a brass tee fitting then drilled out a 0.040" hole.

Hope this helps,
cardo0

 

I previously did use a jetted return line, I could go back to that. The main problem is that the fuel pressure is higher when cold.

This is the pump:
http://www.jegs.com/i/Carter/180/P4600HP/10002/-1

I opened the fuel tank lid incase venting was an issue.