Belt is loose? Something is driving the wheels, the belt can't be loose enough.
Could you have a broken return spring on the drive plate?Working on a Husqvarna 6-23 and I can not get the wheels to stop turning when in gear. I backed off the cable adjustment all the way. I also turned the belt guides away from the belt. This helped but as the shift lever moves forward to the end of it's gear slot the wheels start turning, while not using the clutch. The belt looks good and is loose.
What's going on?
you should look at the pictures again. that is how this machine works. the drive system works the exact same as the auger system. you pull the handle it tightens the belt. the drive plate does not move.if I'm not mistaken the wheel drive pully is always engaged and your problem is that you drive disk is not disengaging the drive platter. Look for a problem at the drive disk not the belt which should always be engaged spinning the platter.