Check the air filter isn't blocked, the engine breather is in good condition, and the oil level isn't right up to the top of the plug. A little over the mark is ok but not FULL. Simple i know but easy to overlook...
If no other obvious reason is found then i'd be looking at the valve stem seals as the most probable cause. It can be done without taking the head off BUT....it's not a job for the faint-hearted!
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Just going for a short ride on my bike....
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