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[Harry Potter] Hagrid's Motorcycle

If Hagrid's motorcycle can use magic to fly, why does it need a working motor?
gpt
Magic is finicky and weird in some cases. A lot of enchanted muggle objects operate on the property that their input is mundane, but the output is magical. The Deluminator is clicked like a lighter, but it puts out lights. The street entrance to the Ministry is a telephone booth, with a code that is dialed in. Maybe the flying motorbike is operated like a regular bike, but the enchanted engine makes this one fly? 

Alternatively, since it's actually Sirius Black's bike that Hagrid was borrowing, Sirius painstakingly preserved the original engine sound to piss off his mother.