Hi new to bike building so need some help please. Project is a Peugeot 531 P frame and sourcing appropriate components. Got 48 T / 36 T chainrings a 6 speed freewheel 12 T - 17 T, rear mech is a simplex SX610 which seems to working ok. The chain tension is correct when on the big chainring but way too slack when on the small ring. Is the rear mech not working properly or is it not compatible with the gearing I've got ? Any advice appreciated.