Raiders of the Lost Ark
Looks like a pre-war British machine. Triumph? BSA? Brough Superior? Who knows. -Dave The bike appears pre-war. Unfortunately, the above photo is about the best view of it you are ever going to get. My guess is that its either a BSA Bantam, a BMW R2 or something they built specifically for the movie.
its a c 1946 BSA M20..identified in an article, 2008 by a member of the Military Vehicle trust.