Replacing the Drive Belt on the Kubota Mower
In order to fix the issue I had getting my mower to drive up a hill two years ago, I replaced the drive belt, tension pulley spring, and fixed the tension pulley. Unfortunately, the mower is having trouble moving again. So I decided to try replacing the drive belt again.
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I removed the mower deck.
The pulley in the left side of the picture is attached to the gearbox that turns the rear axle. The pulley in the right side of the picture is the tension pulley. You can see the arm to which the pulley is attached that provides tension on the belt using the spring attached to the tension arm.

I removed the rear pulley in order to remove the 2 year old drive belt.
I replaced the 2 year old Kubota belt with a new Kubota belt. After installing the new drive belt, the mower still had problems moving, especially up a slight hill. Upon closer inspection, it appears that the tension spring I replaced 2 years ago didn't appear to be stretching hardly at all. The spring looks to be too long to do its job. The belt feels much too loose.
So my wife found the old spring I replaced 2 years ago.
The shiny one on the left is what I put on the mower 2 years ago. The rusty one on the right is the original spring. The replacement spring is definitely longer than the original. It appears that the Kubota dealer sold us a spring that is too long to put proper tension on the drive belt. My wife is going to go back to the dealer to see if we can get a spring that matches the original. Our mower was manufactured in 1989. I wonder if Kubota no longer makes the original spring. Finding a replacement may become a headache.