Tighten belts on my Yard machine ride on lawn mower

There is rarely any adjustment to tighten the belts on a riding mower. The belts are tensioned by an idler pulley which is controlled by a idler pulley spring. Almost all riding mower belts must be accurate to the hundredth of an inch or no less than accurate to 1/8th inch. It is very rare for a belt to be in full inch increments. This is why automotive or general purpose belts will not work on a riding mower. I know it is tempting to try to get by with a $10 belt rather than spending $35 or more for the correct belt. Trust me, you will only waste your money trying to get by with the inferior belts. Riding mower belts and extra heavy duty and generally will not stretch at all. Generally they will have 5 or more strands of a Kevlar type material so that stretching is virtually non existent. If the belt will not tighten then it is most likely not the correct belt. If the belt is in fact the proper belt and still is too loose, then you should look for pulley wear or weak tension springs. Metal pulleys do wear and tension spring do loose tension.