With the positive meaning, if it possible, just so I didn't have to use adverbs like happily. The only thing I came up- buzzed. Thank you.
Two-cycle, or four? Anyhow, consider brattle, a verb that means “To rattle; to make a scampering noise”, with etymology “Apparently imitative, probably under influence of break + rattle”. I don't know what a scampering noise is, but believe brattle suggests the right sort of sound.
To rev up an engine is “To increase the speed of an engine”, so revving up may be appropriate too, along with the less-well-known poppity-popping.