John,
Generally, one installs colder plugs when one hops-up an engine. Engines
prepared to racing spec use colder still. The black soot you are seeing on
your plugs at idle may be caused by the hotter cam. Often hotter cams, due
to increased overlap, don't allow optimum flow at idle. I believe that
Triumph's use of colder plugs in later models was to help mitigate changes
in the engine due to emission regs.
Hugh Barber
Hollister, CA
'73 TR6
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