To slow your engine down, here are some other things to try:
# Remove the hose were the red arrow is pointing above. You should not be able to blow air through it (should be sealed by the diaphragm).
# You can try a lighter spring in the carby.
# You can place a shim or shims under the rotating cam inside the carby.

Others here may have more tips, or expand on this post.

Happy is he who penetrates the mystery of things.