the engine won't run if it is completely bad. you won't have spark. if it is just starting to go bad it might run good for days or weeks but can cut out at any time and the engine will die. it might restart right away or you might get it to start after some time. if you are getting intermittant ignition problems it is a good idea to get it replaced. carry a spare with you too so you won't be stranded in the future.