As far as I can tell from the parts assembly diagram, the upper belt is tensioned by the jockey pulley, not by moving the gearbox (which would be used for adjusting the chain, not the pulley). In your first picture you can see the swinging link that the jockey pulley moves on. The parts list calls the moving arm the "clutch arm", suggesting it is part of the clutch mechanism. When you push the clutch, does that tighten the top belt? If it doesn't tighten it enough, the clutch adjustment may be the solution (it is a locknut adjustment on the back of the clutch pull-rod). I find it difficult to see how the mechanism works from the parts diagram, since I can't find the connection between the clutch pull-rod and the clutch arm (they are on two different diagrams) but that is my first guess as to how it works.
