The battery needs to be at full charge with a points ignition system. I would start by checking grounds of the wiring harness and engine. Use and OHM meter and measure resistance from each ground terminal and the engine case to the negative battery post. IF you connect to the Positive post you will fry the meter. Ohm reading should be no more than 5 which isn't very good. Preferably it'll be .5-1 ohm or less.