Okay, so I bought a 1986 Pontiac Trans Am the other day.
It originally came from the factory with a T.P.I. 305 and 700R4.
The guy I bought it from took off the T.P.I. stuff and dropped
on a Edelbrock Performer RPM intake & Edelbrock carb with a Holley
fuel pressure regulator. The stock in-tank pump was left in as well
as the computer. (I have ALL the T.P.I. stuff to convert it back, if
I was so inclined. I'm not.)

I removed the computer, pulled the T.P.I. wiring out as it was still in
the car. So anyway. Here's my issue. The car is idling @ 1500rpm, when
I put it in gear it will drop down to 300rpm and unless I keep my foot on the gas it will die. The car is getting tons of fuel with that stock pump, obviously, but would that be the reason it's dropping from 1500rpm down to 300rpm when put in gear?

I plan on pulling the stock sending unit/pump out of the tank this weekend
and pulling the pump off, run some sort of line and then use an in-line fuel pump. Also, the trans being a 700R4 and it no longer having a computer to tell it to lock-up the convertor I plan on putting in a switch to make the convertor lock up. Would this cause the rpm/idling issue?

I'll probably have more questions, I'll post'em as they come to me.
Sorry if this post is confusing.

 

I trust the stock distributor was changed out (it only works with the computer).
Do a search for TPI to carb swap. It's been pretty well covered.

 

Ditto the distributor. The stock computer unit is wrong for this set-up.

Same for the Holley regulator. You need a return-style regulator, and dollars-to-donuts that Holley reg ain't return-style. Probably not causing the idle issue you're talking about, but not good for the pump. An in-line might mean you can retain that reg, but IMO that's replacing the wrong part. Keep the in-tank and get a proper regulator.

The TCC isn't causing your idle problem, either. The lock-up system doesn't get any pressure until 2nd gear. So, it can't engage until then, even if commanded to.

I have yet to see an Edelbrock carb set-up that impressed me. Again IMO, they're cheap, shiny junk. Probably not your idle problem, but not the route I would take (okay, I would have kept TPI, but there I go again. . .).