Spark plugs

[Gnasher]

sounds like you need to trim your leads a bit. if you trim about 1" off the leads and push the plug caps back on, it should sort it EDIT

1". been smoking the funny fags I think. try just about 5-6mm at a time

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Before you start cutting.  Where is it arching at the lead/cap union or down the plug well?

Aching is normally due to either the ceramic isolator breaking down/cracking on the plug or a cracked cap, it is possible at the lead/cap union in which case trimming the lead should sort it. 

If it's down the plug well or you can see the spark leaping from the side of the cap.  Unscrew the cap on one of the the plugs that is aching and swap from one that isn't, now start the bike, see if the aching has moved it has it's the cap if not and the old plug is still arching it's the plug.

I would avoid cutting unless you really have to, the reason being the leads are bonded into the coils if you cut too much off you have to replace the coil :eek  also as pointed out if you do cut them only cut mm off at a time.