How many miles on the belt, what is it's exact width compared to the specs in the manual and what is the exact deflection.
Ok, belts slip during low speed engine braking because of contact loss between the two primary sheaves. The #1 cause is the deflection is in access of 27mm. The best place for any Kawie belt is 22-24mm deflection. The #2 cause is belt glazing from slipping. This not only reduces the width and changes the side angles, but it also makes it too slick to maintain friction at low pressures. The #3 reason is an extension of #2 in that constant high stall engagement and slipping at slow speeds wears the angle on the sheaves to a flat profile just off the spindle which is no good for belt contact. This also means that if bad enough, it can not be repaired by machining. The 4th reason is binding flyweights. If you can not spin the pins with your fingers, they need cleaned...all if it. Use NO wet lubricants on any part of a primary clutch.
On the spring, if its a primary and you have done nothing with the secondary, put the stock spring back in. Secondary first, then primary, not the other way around. Increasing resistance on the primary reduces the force transferred to the secondary which is adjusted for the load requirement. Leaving the stock spring in it can allow for slippage under high loads...and that means belt destruction and clutch damage in extreme conditions. A few lbs isn't a big deal but 10+ is.