For those of you interested, I bought the SF2 out of a short hibernation beggining of March. It had started hibernating mid January. In the mean time, clutch had been checked, primary changed, oil changed of course, fork oil changed, instrument holder ring changed having broken again, clutch cable changed, tuning and fitting of straight through exhaust pipes doing away with the under engine box. Very quickly, the engine started to stutter briefly accelerating mid range and would fluff cruising at constant throttle. This was of course attributed to the neccesity to rejet with the straight through pipes. So a whole load of experimentation with various jets folowed until the fracture of the weld of the collar at the head end of the stainless pipe before a long trip prompted me to put the old system back on. Strangely, the problems attributed to carburation with the new exhaust persisted.
Previous dismantling of float bowls to change jets had revealed rust dust coming from the tank due to it having been rinsed out with water the previous year to weld it. So that was why the engine would falter! Cure: treat the tank or put inline filters in.
Thursday evening, I thought I'd try one last thing. A finger under the cut out in the points cover revealed the usual drop of oil. Off with the cover and wipe the points clean with a rag soaked in acetone.
Rode to work Friday, all symptoms gone.
The seal behind the advance mechanism leaks. It always has done even if changed and it's not for nothing that later points covers have a cut out underneath to drain oil. I know the problem and identify it immediatly if the engine starts running lopsided or on one pot. Just whip the cover off, clean the points wth acetone if at home or petrol if on the run, and off we go.
This time, the fact that the symptoms appeared at certain revs having just changed the exhaust misled me.
Stay simple and back to basics. These big twins, at least for road use are not very sensitive to tuning, be it carbs or ignition.
Ciao
Paul