Gazza,
After reading what grumpy was saying I realised that what I used to do and still do to clean the carbon build up is cook the mufflers.
In the old days I used to cook them on the stove but these days I use the oxy by heating the muffler to a bright red all over. This makes it easy to clear the carbon as all you need to do it tap the muffler in order for the carbon fall off.
Not many people do this procedure as it is common sense and easier to just replace the muffler.
