so i picked this bike up from a friend, it had been sitting only over the winter. it ran like **** when we got it started. i brought it home changed the plugs and put fresh gas in, cleaned the carbs out. Will NOT start now WTH?! it just cranks and then backfires. any ideas?????? thanks guys, also when we got it started at his house it would only run with full choke! really wanna ride some this year so any help is appreciated. im getting spark on all 4 too, and correct firing order

 

Are you sure the plug wires are on the proper cylinder, there's numbers on the wires, look closely. Maybe a vacuum leak, since it only runs on full choke. There is vacuum ports on each cylinder, just like the one the vacuum operated petcock works from, make sure they're still plugged.

 

Running on choke only is a sure sign carbs are clogged. .phase separation starts as early as 3 months so sitting over the winter yeah I could believe that they can clog up ..you said you clean them are you sure they're clean hundred percent?