the 4010 does not have a wet clutch.
One thing that comes to mind, i would check the rear axle splines. the hubs are splined to the axel, if those splines pullout, or strip, it could cause a low speed issue like you're having. you should be able to check this by remove the rear tires, and axle nuts.
it could also be an issue with one of the drive pulleys not fully engaging, This is the first thing i would do, remove the belt cover and make sure the pulleys move in their full range of motion.