I bought a 85 project c4. Switched over to carb. Its a 4 speed car. Even though its been switched, does it need the ecu to operate the car. I also heard the manual has an electronic controlled computer for overdrive.

 

Have you already switched it to a carburetor or are only thinking of doing so? The car does not need the factory computer to run if you change to a carburetor (or switch to aftermarket EFI.) You will lose the fuel range and MPG function in the dash though. The 4+3 manual needs the EFI to switch the overdrive on. You can wire a switch to do that, as well.

 

Its been converted over to carb. By doing that, does it increase the hp? What kind of switch is required for the transmission?

 

That depends. If the carburetor and ignition curve is properly tuned, you might make slightly more power as well as raising the peak power rpm of the engine if everything else is stock. (Not much though.) The stock TPI intake is rather restrictive and limits the peak horsepower rpm. The right intake manifold will allow you to make power higher in the RPMs. Odds are that the previous owner removed the factory EFI system and intake because of a problem that they couldn't figure out or because they couldn't find a tuner that knows how to tune that old system. (I have aftermarket EFI on my 85 because it is much easier to tune plus it can control boost from my turbocharger, methanol injection, and almost anything else I need to control.) If the heads, camshaft, and headers have been changed as well, you can make much more power.
Is the engine stock other than the carburetor?
You can use the factory overdrive switch if yours is in the console. You will have to change some wiring to do it.