Sun Electric Co. had a slogan they put on all their equipment- - - - -"We Test Not Guess". Start with monitoring the voltage at the battery terminals (not the connectors) during cranking. It should not drop below 4.5 volts with the starter cranking the engine. After a cranking time of 20 seconds, with the starter still turning the voltage should not drop below that level. If that isn't possible, recharge or replace the battery. Remove and clean EVERY connection on both the hot and ground sides. "It looks good" isn't sufficient. Until they're all clean and tight every connection must be suspected. I owned a flathead Ford V8 for 13 years, with 11.25:1 compression and a 6 volt battery. It taught me a lot about maintaining an electrical system.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!