Bedknobs and Broomsticks is a Disney film released in 1971. It focuses on Eglantine Price, a middle aged woman who lives alone and practices witchcraft. When we are first introduced to her she is seen riding a strange looking motorcycle. It is a modified version of the 1923 Triumph Ricardo. It has a fairly large sidecar which is later used to transport the three main children to her home.
The most recognisable trait of this bike is the exhaust fumes that spill out from it. This gas is a thick yellow smoke. When two secondary characters are commenting on it they discuss how it smells of sulphur. They then disregard this fact when it becomes clear that this element cannot be used as a fuel source.
However, it is heavily implies that this is the case. Eglantine uses a variety of spells throughout the film and can easily transform states of matter. It therefore makes sense in the world of the movie that she could create a bike that runs on sulphur. This makes the vehicle in Bedknobs and Broomsticks one of the most unique motorcycles in cinema.