strange arcing

[squirrel]

hi. started the bike this morning and it sounded like it was only firing on 3 cylinders ran ok if i upped the revs a bit .but the strange thing was the arcing from the rubber spark plug cap on l/s rear plug to metal and this was a constant arcing . not sure what to look at first .( word of advice if this happens to you do not touch the rubber plug cap you will get a shock ) weird any advice welcome thanks
.

[bluewing2010]

HI there. The most likely culprit in this is the spark plug cap itself. It has probably cracked and with a bit of moisture collected overnight it will spark to the cylinder head in preference to the spark plug. I don't know how old all the plug wiring is on your bike, but if everything is a few years old, all the rubber boots and the Bakelite caps could be ready for replacement. Your diagnosis of running on 3 cylinders is correct for this type of fault.
Hope this helps.