Check clutch alignment. Check belt deflection. The belt length can vary a little- too small, too big. I don't know what the acceptable tolerance is, but 1/4 to 3/8 is common. The engine stop is not something to play with, unless you are prepared to check clutch alignment. If it's too loose, the engine is going to move under load, and throw off your alignment. Can it really be too tight? Clutch alignment - belt deflection.