My 02 500HO was starting very hard and would even backfire during starting once in a while. Changed the plug, it was black carbon. My question is does the battery have to be fully charged to get a good spark? I was not getting much of a spark off the grounded plug but the engine was turning over, I charged the battery and the spark was good and the machine started right up.

 

Sounds like plug part of problem. Either time to change plug or running rich ( May be time to check carb components OR do you do a lot of SLOW driving?) EVEN if battery low I usually can pull start. Plugs when sooty/carbon caked can cause backfiring on the 500,have had a bunch just like you described. Good luck