It's a backfire, chap. The airbox is under the tank and the entry, or exit if it is full of smoke, comes out under the front of the seat.

A bit less choke next time and job's a good one, IMO. Happens about 1 in 3 starts on the Buell.

Are you sure that was oil on the can? Could it have been soot particles and water?

Sir Topham Hatt