1) Carbs have to be squeaky clean, pilot jets get clogged at the mere mention of dirt, replace pilot jets with new ones or clean the living #$%^ out of the old ones
2) Ignition is very voltage sensitive, works erratically at lower battery voltages. Causes backfiring while cranking if battery not fully charged or up to snuff.
3) Carbs have to be beyond clean, pilot jets get clogged if dirt is found anywhere near the bike, replace pilot jets with new ones
4) Check float height, easy to do with carbs on bike and a bit of clear plastic tube.
5) Clean the carbs again after you have completely emptied the tank of old gummy fuel and filled with fuel/SeaFoam mixture
6) Clean and gap plugs
7) Thoroughly clean the carbs especially the tiny little holes in the pilot jets, or put in new pilots and spend a day cleaning the old ones and them putting them away neatly for the repeat of step 1)
I have two sets of pilots and keep a clean set in the spare parts box, these bikes are a just a teeny weeny little intolerant of gummy old gas and or dirt in the fuel. They HATE sitting still and after two years the pilots will be completely blocked.
After all that, clean the carbs again
'99 Vulcan "Victoria"