Crash, how did you set the Centax clutch? Does the clutchbell have a lot of play when installed?
I think your problem is that the clutch retaining nut is rubbing against the clutchbell which causes the retaining nut to self tighten after a tank or two.
In order to keep the retaining nut away from the rotating clutchbell, you need to put shims between the retaining nut and the normal bearing that goes directly behind the clutchbell.
There are 2 benefits from doing this... Firstly, this may avoid the clutchbell rubbing against the clutch shoe at idle and prevents the engine from stalling. To check, your clutchbell should rotate freely when installed and turned on the engine shaft with the clutchbell upwards facing you. Secondly, your thrust bearings will thank you for this. By putting shims in between the retaining nut and the normal bearing that goes directly behind the clutchbell, you lessen the clutchbell free play you get once installed. This will be easier on the thrust bearing and extends it's life. How many shims? That you need to find out yourself.
The rule of thumb is that the total shim thickness added in between the retaining nut and the normal bearing that goes directly behind the clutchbell must be 0.2 to 0.3 mm less than the end float (i.e. distance of the clutchbell and the clutchshoe). Good luck !