I would do a simple test...
Take out the spark plug, drip some gas directly into the second cylinder, put all together and try to start the bike. If the spark is OK, it will start. If it starts OK and keeps running, try to warm it up. Then stop the engine and try to start it again. If it starts OK and runs without backfiring, leave it to cool down a bit. Then try to start again... What happens?
There are two things that come in my mind. First, check the ignition coil. Second, if it's backfiring, something's wrong with ignition timing.
Try to change the spark plug in the second cylinder one more time. (Simply change from one cylinder to another.)
Last edited by tehnis; 04-04-2017 at 04:59 AM.