With this ridgid clutch set-up the proper basic set-up FROM THE BEGINNING is much more important.
On the old chassis there was an inch of rubber slack in the motor mounts. Start the belt a little tight and at full throttle it was too loose.
Mine came too tight and the belt IMO was too short. So I found longer ones and installed them.
If you start with your belt too tight on this rigid set-up, in 20 miles of part throttle you will have already started seperating the top of the belt, even if you don't see a cord yet. You just lost a ton of grip and performance.
All the things that I have read here, pulling a cord within 20 miles, chattering at low speeds, large changes in deflection in a short period of time, black steak 2/3 rds up the primary, track speed stalling, rpm dropping are classic syptoms of too tight a belt.
I don't think there is anything to fix. You just have to set it up right. Read any belt manufactors web site or any clutching book or any shop manual, and the deflection adjustment is the same.
What does the proper deflection allow? It allows the fractional release of grip that a belt type CVT needs to allow the belt to change from going up or down on the sheaves (and your clutches are always doing that as the load changes from the front or the rear). Without that when one clutch starts to change direction (backshiftm or upshift) and the other doesn't it puts huge loads on the belt. Loads no belt can take.
IMO there is nothing to fix just proper set-up. With this rigid set-up (drag racers know this already 'cause they bolt the engine up solid) you will need to check deflection every time you change a belt and re-adjust when broken in. And, that deflection needs to be the full 1.125 to 1.250 of an inch.
The days of put it on tight and wait for it to wear in are gone. There is no longer a rubber connection for some give for the belt. If you do it this way you will pay the price in belts and performance.
One other thing. Clutches, gears, and chains take some time to break-in too. Be nice to them as they heat cycle and bushings wear in to help the sheaves run true in every shift position.