I had a c4 in the past.
The two pinions on the axle where the spur is mounted can be too loose.
You can tighten it and check now and then.
You can adjust the friction so it acts like a oneway on the belt.
I made a small hole in the left and the right pinion, where they come together, and then put a small (diff)ball in between so when you press the pinions together there is no more movement because the ball blocks the two pininons.
Hope you understand what I mean, my english is not good enough to explain more