countax garden tractor smoking belt!?

I have pulled the lever to de tension the belt but its the top belt that goes around the four wheels that is loose and when i try to engage the cutters it just clicks but nothing happens apart from smoke from the front engine pully area, which is not good! how do i re tension, or indeed replace, the top belt? cant see how to get at it! its a small countax , not air cooled, has no gears just forward and back, dont know its model or age.