Eric and Bob---Good as the Bentley manual is, I don't think it was
rewritten for the later model TR6's. The N9's was the plug for the early
engines that had the higher compression. When my '73 was new, the
dealership said they received a service bulletin recommending the hotter
N12 plug, for its better anti-fouling heat range.
Sometimes just changing to any type of fresh plugs can improve
performance.
The vacuum retard installed on the "newer" ('71 & up?) all were supposed
to have the timing set at 4 deg. ATDC at idle. However, this feature
does not cause hot engine running once off idle, so shouldn't affect
performance for the majority of the spark plug's life. Those who have
converted to a high energy ignition most likely found they can run just
about any plug without foul-out.
Dick T.
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