If you plot rpm vs time you’ll see the line go flat horizontal for a short time then go up, that’s your stall speed.
For where to set it you need to know the peak torque of your engine. Fortunately the KT100 has been around forever so we know it’s somewhere in the mid 8k area for the new sportsman pipe set up and low 10k range for full pipe (L2).
The thinking is to get the clutch locked up 200 rpm before max torque is applied to the drive system. This helps the clutch stay locked as maximum twisting force is applied to it. If the lock up is after peak torque the twisting force will make the clutch slip longer and build heat which is wasted energy.
You see all different numbers being discussed because the engine comes up to peak torque then there’s a slight drop off then it rises to another peak torque so some try to ride the second wave. IMO this pushes the limits of your clutch for no benefit however, if you have a track with no tight turns and you’re only on the slip very briefly you might consider it to keep the rpms ups. For most tracks the lower number is best for clutch life and good pulling power off the turns.
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience