Just swapped a mechanical fuel pump with a electris pump and now it runs poorly
 

The engine is a 355 carbed engine in a 87 Camaro which did have a mechanical pump on it. I put in a Carter p4594 electric fuel pump (https://www.jegs.com/i/Carter/180/P4...B&gclsrc=aw.ds) and connected the pump to the tank and the existing fuel line and at the engine I by passed the mechanical fuel pump. and connected directly to the carb. I still have the mechanical on the engine disconnected from the tank and I closed off its return line back to the tank. The electric fuel pump is connect to a fuse in the fuse box with a jumper. This car still has the computer in it from when it had a fuel injected 305 in it. The pump stays on all the time , and it runs like hell.
So could this be a computer thing or should their be a pressure switch turning the pump on and off. It says 12lb pressure at the carb , the mechanical was 4 at the most.
I always leave out information that is needed so If You need more information just ask if you would , Thank you.

You probably need to install a bypass or pressure regulator to bring the pressure down to 4-5 psi or switch to a pump that's no more than 5 psi. 12 psi is going to overpower the float shutoff in the carburetor causing too much fuel to flood the engine.

also, if you aren't powering the pump using the wiring that was powering the original EFI pump, you really should install a pump cutoff mechanism in case of accident or the engine stalls.

Holley makes a preset regulator, at 6 lbs. You better put a regulator on it even if that's not the cause.

The regulator is coming tomorrow, I'll let you know how it turns out.

The regulator came it runs great , thanks for the support , it is very much appreciated.