not sure what reeds your runnin but i had the same thing happen to me. turned out it was a dinged reed cage and everytime i put my carbs back on my reeds would fall out of adjustment, got new reeds and havent had a problem since.
mine did the same thing last spring. I changed the cool head gaskets and it didnt solve the problem so I pulled the heads and noticed it was the rubber plug on the bottom side of the jug that was leaking. Slapped a new one in and it fixed it.