Nick-
Yes, having the idle screw in more than it should normally be is the same as sitting at a stop on a perfectly running moped and then giving some gas while you have the brakes on. In other words, the perfect moped just sits there at a stop with the clutch "disengaged", the brakes not applied and the engine just simply putt putt putts normally. On such an old bike as a Batavus you might want to do a compression check.To do this screw the end of the comp hose into the spark plug hole with its other end connected to the gauge.Then twist the throttle to full open and hold it there and then crank your ped over. Decent comp should be approx
120 psi or better. There are a lot of other causes too: loose cyl head nuts, a carb that is loose on its inlet pipe, bad ignition timing, etc. etc. Just remember, don't keep wearing out the clutch since parts for a Batavus engine are tough to find. If you find that with the engine off and ped up on the centerstand you then try to turn the rear wheel in a forward rotation and the result is that the engine is turning over (without the clutch lever being pulled) you then have a clutch drum that is not disengaging. Anyway, try some of these things, but be patient. Good luck