Just reading through this 'historic' correspondence as it has now surfaced again and notice that one thing was not mentioned. Going back to the original posting; was the length and type of the lower supports for the Vincent hydraulic damper checked? There are two lengths, short for Vincent dampers, and I believe Konis, and long for Armstrong (Series 'D') and possibly others. A month ago I was working on a 'racing' Vincent to be used at Goodwood. The Koni damper was fitted to long lower brackets and at full extension the mudguard did not just foul the racing exhaust pipes, which are where the original magneto cover would have been, but totally locked up on them. Steering would have been impossible.
The comment about too much weight on the rear is correct but that is light steering, not a proper speed wobble. I went down to St Tropez on my Knight many years ago with a young lady and lots of camping gear on the back. The steering was so light that some of the heavier gear had to be put on the fuel tank. On the other hand I did have a proper speed wobble at Cadwell Park about 40 years ago while racing there. I wrote about it in MPH after Russell had his 'near death experience'. Believe me, at 100mph in a proper speed wobble you will not be able to hold on to the handlebars. Even while pumped up on adrenalin whilst racing the bars become a blur and no normal human could possibly try to control the bike or open the throttle etc. There is enough kinetic energy in a Vincent twin at 100mph to keep the speed wobble going for about 20 to 30 yards (the marks were there on the track) so at high speed you would need a lot of road. In my case I was not thrown off until I had gone straight on at the end of the Cadwell straight and was on the grass. The bike stayed upright with me sitting bolt upright and staring in amazement at the whole front end going from lock to thumping lock. And no ladies and gentlemen, my scars cannot be inpected for a small, or even a large, fee.