I had a 76 Trailduster Sport, rebuilt or replaced everything. Ran great at idle, around the neighbor hood, get it on the road, sputter, cough, finally die. Wait a minute, Ok, than back on the road same thing. Finaaly found a piece of rubber hose in feul line, moving back and forth, when there was demand, it would move up to carb, and start pluging. No demand when at idle.