If the engine suddenly stops running with little or no warning, you may have a failed rotor in the distributor shorting the spark to ground on the the drive shaft. First disconnect one HT wire from a spark plug, hold it near the spark plug or near a head bolt, and crank the engine to determine if you have spark or not. If you have spark out from the plug wires you are "barking up the wrong tree" and need to look elsewhere for the problem. When you have no spark at the plug wire, then check for spark from the coil.

Disconnect the coil HT lead from the distributor cap. If you have the side entry type cap with a screw holding the HT leads in place, this could be easier if you have a spare HT lead handy. Hold the coil HT wire close to a head bolt and crank the engine to verify that you have spark out from the coil. Then hold the coil HT wire close to the intended connector hole in the distributor cap and crank the engine to verify that you have spark going into the distributor cap. If there is spark going into the cap but not coming out, it is a very good probability of a failed rotor.

Then remove the distributor cap, pull the rotor out, and look inside of the bottom of the rotor. Sometimes you may find a dark powder smear that is indicative of spark leakage through the rotor to the shaft. Quite often visual inspection of the rotor reveals nothing. In that case, reinstall the rotor, and turn the engine until the points are closed. Switch on ignition, but do not crank the engine over. Hold the coil HT lead near the tip of the rotor, and flick the points open. You will get spark as the points open. If the spark jumps from the HT lead to the rotor, then the rotor is bad and is grounding the spark to the rotor drive shaft. Solution is to install a new rotor.

Beware that due to function of the condenser, voltage at the points as they open may spike briefly around 300 volts, which can give you quite little tickle, so you should use an insulated tool to open the points. Also keep in mind that the HT spark can be many thousands of volts, so you may want to hold the HT wire with an insulated tool, and keep your fingers well away from the end of the wire.