Spark Plugs for Spitfire

[BruceB]

I am cautious about asking, like oil questions about things like plug types are a recipe for an argument!
Anyway, 66 spitfire but running lower compression ration and concentrics.
It was fitted with N4C plugs when I got it and the BSA manual says these are correct.
However the Haynes manual says from 66 all twin carb bikes should be running N3C's
There is also a little bit of info that suggests the hotter early Spitfires should run N3C's and the later LC versions N4C's
This has me slightly confused as to what plug is best to use.
I have a stock of plugs for all the bikes and have both N3C's and N4C's in the cupboard.
I assume that the bike should be treated like a 66 onwards Lightning or a 68 Spit because of the detuning so what do folk use from these two spark plug types?

[Bess]

Hi,
     I have this guide, hope it helps:


Best wishes...