Westwood Deck Belts burn out?

Hi,
I have an old Westwood mover S1200, with a 30inch cut deck.
The deck runs on two pulleys, one on the end of the B&S engine shaft and the other on the deck.

When I put on a new belt today and started mowing, lots of blue smoke from the pulleys and the belt burned through.

Any suggestions?
I'll be checking the two pulleys tomorrow in the light of day......

Cheers