Faulty ignition from plug to distrib.
Valves not closeing fully from debries or worn/broken valve spring or bent.
Poor fuel distirbution from dirty/worn carb.
The list could go on.
but having those wouldnt my car run rough? cuz my car runs liek a dream so quiet and smooth. i know my started has some worn teeth cuz the odd time that i go to start it, it grinds.