[PPuxley]

I wont break open the champagne yet, but I think I may have solved the issue! I need to balance tune and road test yet.

This morning I installed a new pick up coil for the vertical cylinder.

On start up with the colourtune in place on the vertical I was immediately able to see it was receiving a spark and firing throughout the rev range. I then set up the strobe and again I was now able to see the strobe working at all engine speeds, I was also now able to confirm that the dynamic timing was spot on at both idle and revs.

So it would appear that it was indeed my vertical pick up coil that was causing the issue. Inspite of inspecting the old coil carefully I could see no fault on it , let alone a fault that might only develop at speed.

In summary, for a fault on firing of a single cylinder, it is relatively easy to swop coils, HT leads, plugs, LT wiring to see if the fault changes cylinder. A fault in the carb or pick up does involve a bit more dismantling and rework effort. The fault can be made all the more opaque by apparently seeing the correct resistance in what turned out to be a faulty pick up coil.

I would also now recommend using a Colourtune, (not necessarily as a quantative analysis tool for tuning), but theres nothing like being able to physically see into the firing (or in my case, non firing ) chamber. Although the non working strobe at revs was indicating the same thing the colourtune let you physically see the spark stop working as the revs were raised.

So it may be axiomatic that you find the fault in the last place you look........but its always a shame when its the last place you COULD look rather than the first!