I have a 350 smallblock holley 4150 carb mechanical fuel pump. I get the car started but I have to keep my foot on the pedal at 2000-3000 rpm to keep in running at idle. As soon as I release the pedal the car dies. Any suggestions?

 

Have you fiddled with your idle jets and curb idle screw (linkage arm rest) yet? Sounds like the carb needs an adjustment.
That, or maybe your ignition timing needs to be advanced more, which will speed up your idle.

 

I had the same problem a few weeks ago. I ran quite a few tests and in the end, I rebuilt the carb. This fixed the problem with the idle.

Some things that I checked to try and figure out what was going on:

Checked the timing. It shouldn't affect this, but something worth verifying.
Checked the vacuum advance. Make sure it is in the right range and steady.
Checked the distributor cap and rotor. Mine was old and worn out.
Checked the plugs. You can sometimes identify problems by how the plugs look.
Checked the fuel pressure. I have a mechanical pump and it is running with too much pressure. I'm planning to put a regulator in this week.
Verified the choke is opening properly as the motor warms up.

 

how is it running at 2k rpm? is it rough?

I would definitely check the timing, orientation of plug wires and then go for the carb

 

is this motor a recent install or did it just start doing it? check for any vacuum leaks, hoses damaged or missing.