Aye Nigel, Eldert and Graeme,
Nigel,
The issues with the original pressure plates are to do with the thread often not being central or square to the plate face.
I suspect that there is a wee bit of this, with my pressure plate, as it doesn't spin true, but has a wobble of between 1mm and 2mm in and out from the plates, depending on where it is mounted in the basket. This doesn't help when there is only 1.5mm lift in the clutch from engaged to disengaged, hence the dragging clutch and warped 2mm steel plates.
Eldert, thank you for the advice to remove the friction material from one of the friction plate faces. Having found that one of my 5 x 2mm steel plates had warped, I replaced it with a sound 2mm steel plate. I removed the friction material from a single face and then put the plate first into the clutch basket, with the metal face towards the clutch basket friction face, then replaced the remaining plates and finally mounted the pressure plate where it had minimal "wobble" in the basket.
Adjusted the clutch arm clearance from the gear change cover to 2mm, using the central clutch adjusting screw. I re-routed the clutch cable as shown in Graeme's images and on Elderts 450/500 scrambler and adjusted the slack at the handlebar lever.
It is still not a "light" single finger pull clutch and requires a good grip, but is better than it was, using my old routing above the cylinder head and light years different from the disintergrating nylon liner cable.
With the half friction plate in place the clutch now disengages, allowing easy gear changes and finding neutral, with no snagging or creeping. Also the biting point has changed, with more travel before full engagement. And, no clutch slip when rolling the power on in 5th gear above 55mph, or when changing down to 4th and giving the motor throttle for an overtaking manouvre.
Now to see how long the truce lasts!
Thank you for your help and good health, Bill