Hi Robchap,
I had an almost identical problem with my mastercut 460 a few weeks ago. While I was using the mower, I could feel the smell of burning plastic. I saw then that the the plastic just above the muffler was completely burned. After removing the mufflet, I could see that It was out of order. There was a hole at the rear leaving the exhaust gases to escape. After replacing the muffler by a new one, the problem was solve.
Hopes this will help you.
Regards,
Serge