29 Chev· A Little Off Plumb
If it is a points and condenser ignition the condenser may be intermittent and providing a weak and intermittent ignition spark. If you had a timing light you could clamp it on the plug wire and look at the flash of the timing light to see if it is steady or intermittent. Could also be a bad spark plug, bad fuel, ignition timing, low compression ore another problem - just suggestions to check that may help. Did the problem occur all at once or has it been hard to start for bit and getting progressively worse?