once it's been running for a minute, brush your knuckles on each side of the y-pipe. If it's hot, that corresponding cylider is running.
If it is not at least pretty warm, it's not running on that cylinder. In order to fix it, you need to diagnose what is wrong. Do a compression test to see if there is sufficient compression. Make sure there is spark. It has both compression and spark, it's not getting the air/fuel mixture.