If I recall....
With the blade at full height, the belt will be the shortest and most tight. If it's too tight, you have to slide the motor mount plate in a bit and recheck it. Absolutely get a new V belt, should be about 42" or 41" ... if I recall....? It's 4L series belt, NOT an automotive type, the V angles are different.
The overall weight of the motor is always supported by the hinged mounting plate, however the hanging weight is what keeps the belt under tension. It will have a spring washer on the mounting plate to keep it from "bouncing" too much by adding a little friction.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)