My bike has always had a bog when you first start it up and it wont go away untill you shift into 2nd gear. Once you shift up into 2nd it is fine and will run in all gears normally, why?!?!
That's just because the bike has oil builup on the spark plug from warming the bike up at low RPMs, and you have to ride the bike under a load to clean it out. First gear doesn't put a huge load on the engine, so consequently it doesn't clean out until you reach second.