anyone know what could cause the motor to shake at idle? it smoothes out if you give it some gas... almost like the idle is too low? anyone had this problem?

anyone know what could cause the motor to shake at idle? it smoothes out if you give it some gas... almost like the idle is too low? anyone had this problem?
i had one in the shop missing. check the spark plugs, these engines seem to eat them up pretty quick. also if its only on one cylinder could be a bad injector or ignition coil or wire.