Is the belt routed as per Owners Manual?

What Bill posted and check all the idler pulleys and tensioner mechanism.

Also, is the replacement belt a real sears belt or aftermarket substitute?

Yes, you must use the proper belt. Also, the brake not releasing as quick as it should will make a rider launch like that. too tight or binding.

FWIW, to me, that's a classic signal that the old belt has stretched and time for a new one. However, as pointed out above, is it OEM, as some of these things are not integral numbers, but have fractions added on. e.g. 80.5" rather than 80" If your new one is not OEM, that could be the answer. Also, as pointed out above, check linkages, routing, are idler pulleys free to spin, spring, binding, etc. Said another way, if you needed a 80.5" and your neighbor got an 81" or the generic cross reference took them there.....

Don't understand "bucks you off?" It is a manual trans and if the rpm's are up and you "pop" the clutch, it's going to jump. Again, as stated, check for free travel on the clutch/brake verifying smooth operation and spring movement.

I've attached a link to a Belt Cross Reference giving the length and width by part number. Using that, if you want to go aftermarket, you'll at least have the right size. Also, havinig the proper length, but wrong width can also lead to excitement.

Good Luck,

Ev

Here is a link that might be useful: Belt cross reference