Do you have any pictures of a plug (or several) after it has been used in the machine?
Is it possible that the new replacement plug works as the machine has cooled down sufficiently, after it has stopped between changing plugs, that it may be hiding a different issue to what it appears to be?
I am working on a similar engine that is running that hot, it has melted a hole in the cowling above the muffler. I am looking at changing the position of the Poppet Valve from A to C to richen it up and have just uploaded a photo of the spark plug to the Post "Power Torque muffler getting hot. Melting Cowling".
Not being a small engine mechanic myself, just a few thoughts from reading about different issues I have had on various mowers I have been working on.
Daryl. :-)
