**Always follow the instructions in your repair manual when doing repair or maintenance work on Outdoor Power Equipment. Manuals can be found at the manufacturers website.**
Small engines used in outdoor power equipment often suffer from carbon build up in the cylinder head, exhaust port and muffler. The is more common in 2 stroke engines than 4, but it does occur in both. In this video I inspect the exhaust port and muffler on a Echo SRM-230 trimmer. The muffler was very clean, but the exhaust port had carbon build up that blocked half the port. To clean the port, rotate the engine until the piston skirt is covering the exhaust port, and take a screw driver and lightly scrape the port to remove the carbon. Than take compressed air and blow out the port to remove the carbon pieces. Mufflers on small engines are usually sealed units, so cleaning them is very difficult if not impossible. Its easy and less expensive to replace a sealed muffler with a new one. Some mufflers have screens, which can be cleaned with a wire brush and compressed air.
This trimmer was run by the previous owner without a trimmer line knife attached to the deflector. This allows the trimmer line to extend beyond the manufacturers recommended length, and this causes a excessive load on the engine. I believe this is primary responsible for the carbon build up on the port.