Ok, cool. I was hoping you weren't using a different plug as suggested. I thought it was weird that a new plug kind of helped.
That makes sense about the pulley then. The belt should be low on the drive pulley when you take off (max torque) and start to rise as the kart starts to move up in speed, thus creating more speed than torque.
So if you're belt is starting to ride up on the drive pulley, and then engages the driven, the load may be too great for the engine to handle and could stall.
So once again, the kart on the stand (no load on the engine except for just the weight of the tires and axle) it probably spins fine. Once you add the weight of the kart and yourself, that load is too great for the engine to overcome and stalls.
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