I have an old Specialized Hardrock that hasn't been used in years. I pulled it out the other night, aired up the tires and it rides fine except when shifting into a lower gear on the rear derailleur it skips two gears. The thumb shifter does not shift at the normal position, but goes almost twice as far as normal. Is this a problem with the shifter or cable, or with the derailleur? I am a novice to bike repair and would prefer not to tamper with the deraileur if not necessary.Shifting is just fine going to a smaller gear.
Thanks,
Brandon