What needs to be done is to have someone familiar with the SOHC's to try it out. MAybe that wil clear things out. And who knows.. perhaps it is just the forks needing a thicker fluid. I need to chec the air pressure too
I think that would be a good idea. When my friend here in Denver first got his CB650 he had almost the exact same issue with the jerkyness in 1st gear and abrupt slowing down when letting off the throttle. I rode it around and it seemed fine to me. I told him to upshift from 1st to 2nd sooner, and to let off the throttle more easily when in 1st, and that solved the problem.
On the ride home from work last night I messed around with riding in 1st gear and was able to almost exactly reproduce what you show in your video by getting it up to around 4500-5000 rpm and quickly closing the throttle. Not quite as much dive in the front, but like you said that could easily be a fork problem.
A couple of things that will make this worse are a loose drive chain and loose throttle cable.
If it were an issue of the front brake binding badly enough to cause the abrupt slow-down and nose dive then your bike wouldn't be able to coast in neutral more than a few feet.
