My 64 dart wont start

Had this happen to me before, turned out to be the rotor in the distributor cap (metal tang on rotor lost its shape and wasn't making contact in distributor cap). Bought a new rotor, installed it and all was fine.

Also when you have the distributor cap off, check the reluctor air gap adjustment, this is the distance between the reluctor (its the metal piece the rotor goes in) and the pick-up. Measurement shoud be 0.008".
Rotate the distributor shaft by hand. It should rotate freely, but it's normal to feel "pulsations" as the reluctor prongs pass the pick-up. Check measurement using a brass (non-magnetic) feeler gauge.
This can be adjusted by the loosening the screw which makes the pick-up move and setting the distance to 0.008" then tighten down that screw again.

Hope this is clear and helps you.