Have '67 fsbk with 289 4v, engine starts fine, so quiet sometimes I wonder if it's running. After the initial 15-20 seconds of running, I touch the gas for a second. That moves a piece on the carb(can't think of the that piece name right now) lowers the engine speed a bit. In park and neutral the speed is fine, (approx 500 -525 rpm). Manual says, 525rpm in drive, have turned distributor, left and right to speed up engine. Then turn idle mixture screws. Engine in drive still at 375-400 rpm. Engine dies around 350 rpm.