Les,
From what you have said you have to do, it sounds as if something in the gearbox is too tight, causing the internals to stop rotating as soon as the clutch is lifted. The problem is most likely to be on the layshaft or input shaft. Checkout the shim behind the front layshaft bearing - there is a choice of two thicknesses. When fitted, this shim should still be free to rotate - that is, not pinching on the bearing. Also check the input shaft to see if that is rotating freely - sometimes the oil thrower on the clutch coupling gets pushed back and drags on the casing. The kickstart mechanism could also be faulty - make sure the kickstart gears are timed correctly, so that it disengages the layshaft when the kickstart lever is raised. Overtightening of the nut on the rear of the layshaft can cause the bronze sleeve for the kickstart ratchet to bulge slightly, causing the gear to drag.
I dont think the problem lies in the selectors or springs as you are able to select the gears once the dogs are in alignment.
If you have to take the gearbox out of the frame, try removing the toolbox and exhaust pipes - you can then tip the engine/gearbox unit forward on the front engine bolt and then remove the gearbox - you may need to undo the swinging arm bolt to let the swinging arm drop back for more clearance but it's still easier than removing the whole unit from the frame.
Good luck with the repairs,
Regards,
Eddie.
