misfire? timing? i dont know please help!!!

First of all everything I know about engines I have learned over the past 10 days. I bought a 85 chevy silverado, 305 small block, 165,000mi. . I knew it needed work, the carb was missing linkage and choke housing. I had it rebuilt at a performance shop they put everything missing back on it. They set both mixture screws at 4 turns out. I changed spark plugs and spark plug wires, dist cap, rotor, changed oil, flushed radiator and put new hoses on, put new pcv valve and vaccum hoses on. set timing to 4degrees btdc.
Starts fine even under 30degrees outside, idels ok. Drives fine at low speeds. But if i need to accelerate quickly(going up on ramp to highway) it bogs down spits out a lot of black smoke. I have to back off accelerator to get it to stop.? If i am parked and in nuetral it will rev up to high rpms no prob..???
I have adjusted timing to 6, 8, 10. Still same problem...Very frustrating..

You probably have dirt inside or blockage somewhere in the small holes and jets in your rebuilt carb plus a vaccum leak somwhere on the carb and it takes your engine under load make things happen. You might be better off with a purchased rebuilt carb.

Question is this vehicle a computer controled carb?

If i remember correctly 4 turns out on each screw is too much. I think it is supposed to be 2 or 2 1/2 turns each. It may be getting too much fuel when you stomp on it like that.