As a final add to this thread, I wanted to add this info to this thread as some may be wondering why there sprockets are dished or non dished/flat.
Here is to two different shims for the sprockets that come on the derbi flatreeds oldstyle. This shim is behind the sprocket.
Here is a thread in the b/s about derbi sprockets. Check your shim/sprockets.
The thin shim came on mopeds with a dished rear sprocket. .550-.552ish
The thick shim came on mopeds with a flat rear sprocket. .748-.750ish