Popping thru the exhaust when decelerating is common. Many motorcycle carbs have a bolt on part (forgot name) that adds fuel under high manifold vacuum (decel) so it doesn't pop, which suggests it is from "too lean" misfire, putting unburned mixture into the exhaust. Many remove them, to save fuel or because they like that popping sound. I suspect if you don't totally let off the throttle in a low gear, it won't happen.
