Not sure if my very recent experience will help, but I had the bike running but knew the battery was on it's last legs. Tried to get out for a quick ride before I replaced the battery and it would not start. Got a new battery and fitted and still would not start. Would turn over, stutter, backfire, but not catch. Based on lots of good input I ended up replacing the plug in the rear/vertical cylinder as the thought was the trying to start had fouled the plug. New plug in, battery fully charged and it started first go. Ran perfectly smoothly. Other thoughts were the charcoal canister may have had fuel in it, with lots of posts about removing it. My bike is new to me so went exploring and found the canister had already been removed.
