Sirius Black's motorbike was a motorcycle owned by Sirius Black. It has a large engine (suggested by the noise made when active and that it can carry a sidecar). Book references to "its exhaust pipe" in the definite singular show that, if, as likely, it had more than one cylinder, they all blew into one exhaust pipe, which is likelier for a vee-twin then for a parallel twin engine. It also appears to have a kickstart.