91 Miata running rough at idle.

Hi I have a 91 mazda miata with a 1.6 manual transmission that I am in the process of making a Scca racer out of it, and when I bought it the car always ran rough at idle. The number 4 cylinder had no compression and the car blew out a very large cloud of black smoke when revved up. I have since then swapped in a new 1.6 engine with rebuilt injectors and flow matched injectors. Fired up the car and the miss is still there but the smoke is not. When you step on the gas 1/4 of the way it feels like a cylinder kicks in and takes off like a bat out of hell and runs fine till you get back to idle. I checked the compression and leakdown and everything checked out fine. Checked the cam timing and it was fine. I have a bunch of engine sensors and parts laying around so I then started swapping them out, I tried maf sensor, throttle body and all sensors on it, cam sensor, new temp sensors, new injector harness, computer, swapped out different injectors, new exhaust from header back, no cat, new fuel filter, new air filter, and nothing has changed a thing. Also the inside of the exhaust pipe is very black, alot more than the other miatas we have. I am now thinking that this is wiring related. Any other ideas???????? :banghead: