87 Firebird, 350 tpi.
Just installed a new fuel regulator again, at prime it reads 42 psi and drops to 20, at idle it jumps from 38-42 psi, is this normal?
I tried an older regulator from a spare rail and it primes at 45 psi and runs at 43 psi and doesnt seem to move much at idle and holds pressure when the car is off.
Whats the norm for this car?

 

Don't base your decision on what it reads when on the stock 2 second prime cycle and certainly not on what it drop to after that prime. If it primes up anywhere near the stock 43 PSI setting, you're fine. If it drops after that, don't worry about it. They were designed so they wouldn't back-flow and loose pressure but the system wasn't perfect even when it was new.

Go by your running readings. Should be 43 PSI running with the vacuum line to the FPR disconnected and pluggged. With the vacuum line attached to the FPR it should be around 35-36 PSI at idle on a stock engine.

Alll the regulator does when it sees vacuum is drop the pressure DIFFERENTIAL across the injectors to maintain the stock 43 PSI. When there is less pressure in the intake manifold (i.e. vacuum) it drops the fuel pressure exactly the same amount. Remember that vacuum is measured in inches and fuel pressure is measured in PSI. The ratio betwen the two is almost exactly 2:1. So, if you have 15" of vacuum at idle that equals about 7.5 PSI. 43 PSI - 7.5 PSI = 35.5 PSI. Your observed fuel rail pressure at idle with the vacuum line attached.

Make sense?

 

Makes sense although the pressure pulsates, like the needle on the gauge is bouncing from 38-42 psi rapidly and I can feel the same on the fuel hoses, like its pulsating.
I have never felt anything like that on a fuel line, doesnt seem normal, or is it for the tpi?