Strange vibration noticed on motorway. Gentle rumbling vibration that builds over 5 seconds and then fades away again over another 5 seconds. Does this continuously and same at any motorway speed. Can't notice it much on average motorway road surface, but obvious on good smooth tarmac. When vibration worst, it seems to take a bit of power away. Not sure if this is engine or increased drag to rolling on road due to vibration.
Also does anyone know the spark plug spanner size for this engine?
Rockingrabbit