It's like Monjoho said that light is your reserve, I did the samething you did way back when I first got my zx750, looked all around for the petcock and was wondering why the hell it had a button and no switch. If it ever starts to run real crappy all of a sudden and wont idle, check that vaccum line running to to the primer, mine came off and just about made me shoot the bike trying to figure out what went wrong.