Run it for a couple minutes and then if you have a mist bottle (like something you'd use to spray on windex/window cleaner) or something similar, spray a little water on each header pipe, one by one. It should make a puff of steam on each. If one doesn't, then that cylinder is not running right (ie its not hot).
Could also be bad gas or water in the tank.
If you're comfortable taking the tank off, do so, and drain the carbs/float bowls, and drain the gas tank. Put in new gas and see if its better.