Thanks for your replies Trevor and Bob
Yes exhaust valves run very hot. The engine was last touched valve wise a good many years ago, before flea-bay got into its stride. Valves were fitted by my mates father, an ex RAF trained technician who I believe was a most fastidious mechanic. Probably done about 20 k miles since then so material is not too bad.
But in the extremely unlikely but not impossible event that the exhaust valves were really inlet material would this explain a light red crust over the steel of the valve head?
Bob, do you have a reference link? Sounds like you may have found something relevant!
Must admit we were expecting this to be blamed on ethanol in the fuel, though personally I think it may be down to international terrorism or the new world order.
But seriously there are one or two mentions of manganese additives specifically causing the red colour. Then I got to thinking that I commented when stripping down the engine the oil had a strong smell (akin to the sulphurous pong of gear oil) but the bike always runs with Silkolene mono-grade. Curious.
Any other ideas folks?