If this is the first time that it has been started this year, I would say that the carbs(pilot jets) need to be cleaned. It most likely isn't running on all 3 until you get off of the pilot jets.
is the clutch tight can you see it wabble when it is idling or put a gage on it to see if it runs true.when the 98 cranks were bad they would run fine but you could see the clutch wabble when at a idle.sounds like it happens before engagment.