Tom---Black plugs after "running aroud town" could be an indication of
either too cold a plug OR too rich on the mix. (I'd be more concerned
if they were black after a highway cruise.) Still, if this bugs you
enough to want to do something, raise the jets on both SU's about 1/4
turn on the adjustment screws (counter- clockwise) each. Or more, if
the engine doesn't stumble.
I don't know the heat range of the SplitFires that you're running. The
RN9's Champions that you also have are a tad cold for a '74. A hotter
plug would be the RN12's that many owners find ideal in the Champ brand.
The spark plug choice has little if anything to do with the engine's
coolant temperature.
Dick T.
'73
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