Have you tried smaller jets just to see?
Is there no way to adjust the floats at all?
If no on the above, it may just be what you suggest... too low of compression with too big a cam. What is you lowest compression reading?
I had one do that same exact thing, and tried everything. Then found the compression was terribly low, so I tore it down and found worn out dished pistons. After boring and rebuilding with flat top pistons, and using the very same exact cam and top end without changing anything else, the rich condition went away... and wow how it came alive!