Jumping motor

[Downwindtracker2]

Now,that I've cleaned and mounted my new to me lathe, I tried switching it on. It's a '91 DF1224g, one of the generic Taiwanese back geared ,belt drive lathes. When I start it up, the motor bounces around. The belt is tensioned by motor weight, it sorta has to move as the levered jack shaft moves back and forth for belt changes. This is a design fault. The original belt was in very bad shape, and the sheave is worn. Luckily, the sheave is iron and not pot metal. Has anyone come up with a easy on/off tension solution? thx

Interestingly the original belt was a First Rope B-32, the same length Jason is B-30. We used Jasons at work, they not bad belts.