Any problem with the clutch will manifest as a difficult gearbox. You mention it almost lurches forward, ie is dragging. If its doing that throughout the box will feel sticky.
I'm guessing you haven't identified what clutch you have:
The 6 spring clutches are notorious and while its possible to get them to work adequately (I put up with one from 1985 until last year!) they would be an embarrasment in a washing machine. The primary issue being the bearings are patheticly budget balls rather than rollers so the baskets typically wobble and flop about.
The latter triumph type (4 spring) is a better bet and still standard and should be able to be made to function well although superior modern replacements including belt drive types are now available too if finances or engineering capabilities permit.
In my race A10 I run a Bob newby racing 6 spring belt drive (loverly
)
and in my super rocket I have a modified suzuki clutch.
my ZB33 now has a four spring triumph type and its a huge improvement, I was starting to think my plunger box was a bit sad, but a decent clutch really improved it.
However a BSA pre unit box will will always be a fairly slow changer (compared to anything modern) unless you upgrade to a TT industries replica box which is filly modern and your pockets are way deeper than mine.
Lovely bike!
