Hi Bruce, I had a good look at the muffler and couldn't find anything in it. Blew through it with a vacuum cleaner that has a blower outlet and felt the air coming out reasonably strong.
The muffler is quite large, approx 20x10x10cm and the outlet is a series of holes in a circular pattern punched in one end of the front. It has a plate that bolts over that with a pipe attached at about a 30 degree angle. There is nothing in the pipe. While it was running, I noticed a lot of sparks shooting out of the exhaust port. Could it just be a carbon build up in the muffler?
cheers2 Gazza