updated March 11, 2021
Thanks to Bill Stoll for this helpful tip.
Rarely, the muffler can get clogged with soot When this happens, the engine will not go to full output because of the increased pressure in the system which, in turn, restricts movement of the fuel air mixture into the cylinder The engine acts like it is starved for fuel.
The exhaust system should not have any loose parts inside. If it rattles when shaken, replace or repair it.
Below is a normal exhaust from a Top 80 (other engines similar). The holes are well defined and clean. This engine uses AVGAS with the TCP fuel additive and 100% synthetic oil (Amsoil Dominator).
The engine can run rich from a misadjusted or defective carburetor or a defective ignition system including a bad spark plug. Unburned fuel (soot) will slowly build up in the cooler parts of the exhaust system and, particularly, in the muffler. Using cheap oil (rather than 100% pure synthetic oil) or too much oil in the fuel mix can also cause the formation of excessive soot.
The telltale sign of a clogged exhaust system is oil leaking out at the joints. Leaks can also be a result of damaged or missing gaskets so it is important to check for this before assuming that the muffler is clogged.
It is easy to check the muffler for the presence of soot, including how bad it is. Get a 1' (30cm) piece of #10 bare copper wire. With some pliers, bend a small el in the end of the wire about 3/8" (1cm) long. Work the tip of the wire into one the holes inside the output pipe of the muffler. If it does not go right in, the hole is clogged or, at least, restricted, as in the photo of this muffler which was nearly completely blocked.
It is a tedious, time consuming job to clean all of the 200+ holes in the pipe with a piece of copper wiring. Bill Stoll noted that a brush used to clean automotive battery terminals worked for him. Alternatively, the muffler can be cut in two to the right of the main weld, the holes cleaned with a wire brush, and the halves welded back together. A new muffler costs upwards of $600 so it worth the effort. Heating the muffler to 300ºF (150ºC) in an oven for an hour or more will help burn some of the oil out of the soot and make it easier to remove. Do NOT heat it any hotter because it will affect the metallurgical properties of the aluminum. Note: Get your woman's permission to use her oven because it will make a smoky mess.