had cut-off problems with my '67 250. drove me nuts tinkering with carb controls, checking points, etc. then decended into cut-off immed. after start-up. problem was the ign. switch would not stay in 'on' position. found/solved this by running a wire to coil from '+' term. of batt. with on/off switch in line. as this is very easy to do, you may want to try first to help isolate ignition from fuel problem.