Can anyone please suggest why my 20 year old Yamaha continually oils up it's plugs? I have checked coil resistances, points gaps and condensers as well as mixture,and fitted new plugs but it will only run smoothly at high revs and then only when the plugs are new.The plugs are always oily and quickly soot up. The cylinder pressures are good. The petroil mix is 1/100. The engine came with a 20ft. Jeanneau Flirt I recenly purchased. Any suggestions would be greatly appreciated, I am at a loss.