I run O.S. CVR 15 motors myself, and I have had problems with the motor free revving when I back off. I tried everything, making sure there were no leaks anywhere, cleaning the carburetors, etc.. The only way that I have been able to fix it, is by using the original 10E-R rotary carb. I prefer it if I could use the 10C slide carb, but I get the problems with free revving if I use one. I have also tried a slide carburetor from another manufacturer with and had similar problems. What is the nature of your problem?
Cleaning the carb is easy, just pull it apart and clean it. There is an exploded view of both the 10C and the 10E-R carburetors at http://www.rctech.net/forum/newreply...te=1&p=3359343
pages 29 and 30.