Thanks for the info. The carbs are clean, float height is good and floats do not leak fuel, will put stock plugs rather than the B9ES that are here. There is an inline fuel filter too.
I read about CDI's that the tach goes funny if a CDI unit is at at fault. Mine does not so I assume its OK.
I will investigate the power valves as the servo motor does rotate the valves but perhaps it is dirty inside. The piston and rings were replaced recently so not a problem.
Any other ideas?
Thanks
John