VERY happy to give this update. I have figured out my cable slack issue. Basically, not being able to get any tension can have a few causes. 1.) You could have a stretched or broken cable, 2.) Something could have broken in the lever itself, or 3.) Your ball and ramp assembly that attaches to the clutch cable under the transmission side cover can get stuck on the bearings, if they ride up out of the little divots that they slide up and down in. For me, it was number 3. I drained the transmission and took the cover off and removed the snap ring, took apart the little bearings and ramps assembly, put grease on the bearings and put it all back together and reinstalled everything.
Now my clutch is very firm with very little cable played out. Neutral is super-easy to drop into, and I am working through the normal clutch adjustment experimentation to get everything dialed in. Then torquing and loctite-ing everything down.
I am probably going to revisit the throwout bearing and install the Baker Heavy Duty Throwout Bearing Kit into it. After that. hopefully will not have to look at it for a long time. I learned a lot about clutches in general and Rivera Pro clutches in particular and it was totally worth the time.
Thanks for everyone's help! What a supportive and kickass community of chopper maniacs we have here! Grateful!
