My Sp20 SC has 14mm plugs (I believe some run 18mm ). This are not directly in the combustion chamber but connect via a c5/16" dia flame tube. - I believe the head is speed 20 with square ports and high compression chambers Although absolutely fine on a run, Its always suffered from plug fouling (particularly no's 2 and 6) if it spends too long ticking over or in thick traffic. When I got it in 2010 it had ancient Champion J6's (Very short 3/8" reach) which are the most reliable plugs it has ever had - Since then we have tried NGK B6's and now Champion J6C's. - Both of these are readily available but both are absolute buggers for fouling - Pointing to plugs that are a grade too cold. However, as far as I can establish, no one makes a hotter 3/8" reach plug (both NGK and Champion only make 6's in this size). I can also lean the carbs but then starting and power both suffer. My initial though was to fit NGK B5HS (1 grade hotter but 1/2" reach) along with a 3mm copper index washer but when I took a torch and vernier to the plug holes I found that the threads are 15mm long - i.e. if I clean the threads it will take 1/2" reach plugs with no fear of piston interference but with the electrode 1/8" closer to the flame tube throat. This would give access to the full range of plug heats including iridiums (which have cured all the issues on my '24 Sunbeam 350 racer) Does anyone else run a 14mm plugged Speed 20 and if so what sparkplugs do you run? Are they 3/8 or 1/2" reach and do you have fouling issues? Thanks in advance Shaun Posted by shaun matthews 20/04/2018 08:39:57
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