While this clip is more Tim Conway as star than the recently departed Harvey Korman, it highlights one of my favorite parts of the Carol Burnett show. There's something absolutely hilarious to me in watching a sketch when one of the actors is fighting fighting fighting not to bust up at the antics of the other actor(s).
Here, Harvey's doing his best to pass it all off as toothache, but watch his face (and the shaking of his shoulders) and it makes all the silly little physical stuff Conway's fiddling with almost unbearably funny. Conway's funny enough on his own at this time period, but the two of them together were once an unstoppable force.
More of the same here.