I didn't see this model #. Is this one of the Snapper made rotary mowers? Return springs can break sometimes. Does it seem as though the cable may be binding?

The model number you listed is for the Briggs & Stratton engine. It's a 3.5hp 9.0CID. I think the correct p/n for the mower is 20-3.5-7.

Anyway, I would check and see if you have the new belt threaded properly. Also, make sure spring is properly installed. On older McLane the spring provided tension on the belt; on newer ones like yours the spring is a return to release tension.

Do you have the manual? Although not real clear it gives some idea of relationship of belt, spring, pulleys, and levers.

Here is a link that might be useful: McLane 20 and 25 manual