I might be a dummy. My son and I are working on getting our '72 850 spider going. We can't figure out how the gas cap comes off for the life of us. It is a locking type and we have the key. Key turns and seems to unlock it, but it just spins and doesn't want to come off. We have tried pushing down while spinning and gently prying up while spinning but nothing seems to be working. Is there a trick or tip in the collective knowledge? Any help would be much appreciated.