New to me 73 Cuda, didn't run when I got it. I replaced the damaged fuel pump on the 340 with a no-name pump from Auto Zone. Tank, sender, fuel line, and filter are all new. I have a gauge on the Earl's feed line to the Holley 650 DP. When I start it up cold, the needle is averaging 7psi. Of course it's bouncing about +/- 1psi on either side of 7, which I assume is from the diaphragm type pump. Car idles OK but not great, with a miss here and there. If you put it in gear it won't idle, wants to die unless you wait 5-10 minutes for a full warm up. At that point the idle has smoothed out and when in gear it idles fine and drives fine. At that point, the gauge is showing about 5 psi. Go for a drive and it's fine at stop lights and everything runs pretty decent, get home 15 minutes later and the gauge is at about 2.5 PSI. But still idles fine in gear and out. WTH??

I know my carb adjustment isn't perfect but it's pretty close. Timing is at 14 intitial, probably could go a couple more degrees. But what worries me is the high pressure at start, which I think is affecting the idle and then the drop off as the engine warms up. Or is this normal with a mech fuel pump? What should I be seeing? I have a Holley fuel regulator I was considering installing to see if it smooths out the pressure, but it can't boost it if it's dropping so much.

The idle and miss have nothing to do with the fuel pump. Those are carb and choke adjustment issues.

Stumpy is right.

As for your fuel pressure, it "sounds" like you either have a vacuum building up inside the gas tank (blocked tank vent or blocked vented gas cap). When the fuel pressure is down at 2-1/2 psi, take off the gas cap & see if the pressure immediately jumps back to 5-7 psi.

OR you have a vapor lock issue (fuel line/carb getting hot). Look for fuel lines/filters close to exhaust (pass side) and maybe add a "thermal gasket" under the carburetor (thicker, fiber-board carb/intake gasket). Is the carburetor just about too hot to touch once the motor is warmed up & pressure is down to 2-1/2 psi?

Is the fuel pressure gauge liquid filled and mounted under the hood? If yes the heat from the engine will build pressure within the gauge and the readings will be off significantly.

Your fuel pressure is normal and the low pressure after warm up is fine and normal also Look else ware for your problem.

7# won't hurt a thing and 2 is normal also hot.

Gauge is a relatively cheap non-fluid Mr. Gasket mounted to the hard lines coming off the carb bowls. I have a vacuum/fuel pressure test gage that I can plumb in to verify if needed, but I think this happens with the hood open if left to idle for a while.

I'll check the vent line. I thought I blew it out. I did have an issue with the check valve at the 4:1 connection for the vents from the tank. Easy enough to pop the gas cap off. It's older than hell and supposed to be non-vented.

So a vent problem would cause the pressure drop?

Normal to see the bounce? Will the regulator smooth that out? i.e worth the effort to install it?

heres your answer.i know your new here but this is one of the guys to listen to. welcome

Thanks. That's good to know, I won't focus on it now. On to the next issue....