A very good friend of mine who has 30years of experience with bikes and who has had 10years work working at mounttunes working on rally cars aswell as rebuilding the engine on colin mcrages world champion car he has a few bikes which are pre 65 championship scramble winners god knows how many times. he runs his challenger and grifon on 16:1 on castrol rand trials bike a villiers engined Anglian on 20:1 he has also had a champion ship winning bantam and he has advised me that no less than 30:1 is to go through that bike.