You shouldn't use those sprays at all. They most often do more harm than good. Also, you should check the condition of the belt. If there's cracks in the rubber, if it's stretched too far or any of the grooves are broken off, replace it.
Before you install any belt, check the pulleys for anything clogging the grooves or any pulleys loose/wobbly/misaligned. You can do this by turning each pulley by hand. Finally check the tensioner for wear. Once you install the belt, make sure it's not too tight/too loose.
Wednesday, February 16th, 2011 AT 12:46 AM