I have a 5.7 350 1988 that was throttle body, I switched it to carbureted and put a edelbrock 1406. Motor ran perfect before, but now it will not idle below 900 rpm and the rpm's will jump from 900 to 1500 about every 30 seconds. Can anyone help me please?

Did you change the distributor?
How are you regulating fuel pressure?
What caused you to make the swap?

Adjust the elec. choke and the fast idle.

Adjust the idle mixture screws using a vac gauge to get the highest vac reading.

First question did you change the fuel pump or install a fuel pressure regulator? The Carb only needs 5 to 8 lbs of fuel pressure the TBI uses a lot more. You could be flooding it.

Tested the fuel pressure its at 6 psi. It has a mechanical fuel pump. It was a throttle bodied motor in another car. All the vacuume lines should be hooked up correctly. The only times the motor sounds like it's running well is at 2500 or greater.

Did you put in a new distributor? The TBI one wont work with a carb

Yes is has a mad ignition.

Are you using vac advance?
What is the base timing?
What is the timing at 3k rpm?

Have you checked for vacuum leaks?