not enough info to help
model # and more info will help someone figure out what is going on.
Most problems are fuel related, but who knows for sure without some history and what you have done to it already
Maybe open carbs with throttle control and check to ensure throttle plates are wide open. Drain each carb float chamber(drain plug) for water or debris. You may have a dirty/plugged up carb/jet causing loss of power on that cylinder. Have carbs ever been cleaned? Check/clean/replace spark plugs for fouling/carbon. Good Luck!
