By primary belt you mean the engine to mower deck belt correct? Did you adjust tension as per manual? You should tighten all the way, then back off one or two full turns.
Also, did you get the right belt? There are several different sizes depending on tractor serial number & mower deck serial number.
Did you buy the JD belt or an aftermarket belt?
If you got an aftermarket belt and went by the size of the old belt is it probably too long. The old belt was probably stretched quite a bit.
There are also two different length belts depending on what the deck serial number is.
Take a look at this PDF of parts. Depending on tractor SN & deck SN it will tell you part number. You didn't say if you had 38 or 47 deck but both are listed there. Sounds like you have 38 deck as they are about 80" long. View attachment 39-parts.pdf
Beware of "After Market" belts. I thought I'd try one one evening after the dealer was closed and I wanted to finish up mowing that night. I made about 125-150' of grass, (not very heavy) before the new belt completely shredded into nothing but a bunch of useless strings. Original belt the next day lasted the remainder of the mower's life.
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