On hot days, when the engine is hot, your battery can't produce enough juice to give you a good spark and turn the starter motor. It's a known issue with underpowered motorcycle batteries. Can be a bran new battery, but if it's borderline on what is required to do the job, then on hot days with a hot engine, it is just weak enough to give you problems. I see it all the time, literally every day on the Vulcan 750 forum, where someone complains of hot starting problems, especially now. All the experts will chime in asking what kind of battery they have and recommend a MF battery with more cold cranking amps, and once they try it, it works every time. Of course, if your wiring is old and corroded, you might be adding to the problem. You probably have already checked your voltage at the coils compared to at the battery with key turned on, bike not running. If not, you should check that. It should be at least 12 volts at the coils. If it is less, then when the bike is hot, it will add to the problem.
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