Kept foiling the bottom plug & dealers couldn't find any problems. Eventually I traced problem to water getting into the lower crankcase. Engine was rebuilt (new bearings, gaskets, seals, etc.) but also split & rebulilt crank due to one of big ends being water damaged. As Yamaha don't sell con-rods (only complete crank assemblies) for model, fitted Suzuki motorcycle con-rod (identical other than 1mm longer) & skimmed 1mm off the pison crown to compensate. Engine is now running fine.
