6 sounds pretty late. I haven't seen that spec'd anywhere before. Everything I've read on here says initial timing should be 10 degrees BTDC. I've heard mention of 8 before, but never as late as 6.
How much advance are you getting? If your springs are rusted up and you're only getting marginal advance combined with late initial timing, this could explain your RPM problem.
As mentioned above, the only "limit" on rev in neutral is at the throttle control. If you've disconnected that, there is no limit at the throttle body.