tuning a carb

Slant 6 with a 1 bbl carb. I think its original, the PO said he recently had it rebuilt. He also told me it needed to be tuned.. any tips? For example, which screw is which?

When its in gear its very rough, very shaky. As if I was lifting up on the gas and then stomping on it over and over. It's better at around 60+ mph.... And its very hard to start.

Any ideas and advice would be great.