If the engine was only idling then the black on plugs would denote a rich idle mixture, which is adjustably, also choke might be partly on. When you do a plug check on racing engines, the engine is cut at running speed and knocked out of gear, brought to a halt, then plugs are checked. That way you get a true picture of the plug and can access the mixture or whether the plug needs changing to a hotter or colder type.
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Richard
1943 Bedford QLD lorry - 1941 BSA WM20 m/cycle - 1943 Daimler Scout Car Mk2
Member of MVT, IMPS, MVG of NSW, KVE and AMVCS
KVE President & KVE News Editor
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