Just a thought - the right hand plug lead can get trapped under the steering column if it is not well clipped out of the way. This will damage the insulation and might cause misfiring at low revs. Misfiring will send unburnt fuel in to the glowing exhaust pipes where it ignites and burns causing the blackening. Maybe???
Last edited by Phil Bleazey; 20/05/21 04:37 PM. Reason: spelling!
