When you rebuild your C-4, try adding some additional down thrust to to the engine mount. Add a bit of weight to the nose to increase the hang angle by a couple of degrees. Double check the angle of the rotor with the plan to make sure it is not sitting at too positive of an angle. Finally, add a 1/64" ply shim to the trailing edge of each blade. Negative incidence will decrease the lift, and increase rotor RPM. Don't go overboard here because to much can cause more problems than not enough!

I'm not suggesting you do all of these at once! These ideas are just some I have used to get my gyros to fly better. Hopefully you can adjust your C-4 to fly as well as mine does!
Randy