I have a Husqvarna YTH20B42 and the snowblower keeps throwing the belt...what size belt do I need to order? I can't see it on the belt...Serial # 122906B002271

Its likly the belt is OK the problem is there is on idler pulley that has an adjustment so when the augers are engaged it puts force on the belt witch tightens and makes every think turn and when disengaged every stops turning. There is always an adjustment involved with this. If you can remove the belt then with the mechine not running of corse engage the drive and watch how it fucnitions then look for the adjustment that would move the idler closer or further away from the belt