I concur with Roelof. I've had several painful and expensive experiences where my very new pistons looked just like the ones you've posted.

Causes : Not enough lubrication + mixture too lean.

Exhaust port section of the piston heat expands more and couple with a lack of oil, jams itself against the port opening on the liner. It has nothing to do with a 'dirty' manifold or pipe.

Since my experience, I stopped using fuel with less than 11% oil and more specifically full synthetic lubrication fuel. Castor oil gives better high temp and RPM lubrication. A good mix of synthetic and castor oil is imperative for these ultra high revolution on-road engines until technology gives us better synthetic oil for our micro engines.

Also, once I feel my engine is peaking, I back off the HSN a little s that I can be sure my engine is if anything slightly on the 'rich' side.