Abstract: A belt tensioner for tensioning the belt of a belt drive system includes an eccentric adjusting member, a pivot structure eccentrically mounted on the adjusting member for pivoting around the adjusting member, a belt-tensioning pulley mounted to rotate on the pivot structure, a biasing member that resiliently biases the pivot structure in a belt-tightening direction, and a coupling structure. The coupling structure temporarily couples the pivot structure to the adjusting member to pivot with it during a belt installation procedure, and the configuration keeps the coupling structure unloaded except for a limited period of time during the installation procedure such that the coupling structure can be installed and removed easily. The tensioner is rendered operative after the pivot structure is uncoupled from the adjusting member, thus allowing the pivot structure to pivot with respect to the adjusting member.