CV-6 is right. We need more info. We need to know what freewheel you are running and how many speed. Not all shimano or suntour rear derailleurs are index compatible. If you run suntour index shifter you will need suntour rear derailleur or vice versa. BTW, that not a mid 70's motobecane. It is pre-73 and not a grand jubile or grand record.