BTW, from the looks of the pic I can see that the spark arrester in the muffler might be clogged with carbon.
If he wants I can bring some tools and disassemble the muffer and clean the spark arrester, or just remove it which will certainly free up the muffer, make it a little louder and allow it to breath better which will free up some power.
The job of the spark arrestor is to stop any glowing carbon pieces from escaping out the muffler. The spark arrestor is important if the engine is operated in areas where there is danger of a spark causing a fire.
