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The Boggy Saw Blues

The Boggy Saw Blues
Location of the spark arrestor inside the muffler. Photo by Brett McLeod.

A friend recently showed up at my workshop complaining that his chainsaw was “bogging” and wouldn’t run at full throttle. He assumed the issue was related to fuel delivery and asked me to give him a hand rebuilding the carburetor. While the symptoms he described certainly fit the description of a dirty or clogged carburetor, there is an equally common issue that’s often overlooked. Internal combustion engines must be able to breathe. This means being able to draw in clean air and to expel exhaust.

The Boggy Saw Blues
Left: By holding the spark arrestor over a sheet of white paper it’s easy to see that nearly the entire screen is clogged. Right: The cleaned spark arrestor should be easy to see through.

By law, a chainsaw is required to have a spark arrestor. This screen of fine wire mesh sits inside the exhaust and catches or “arrests” sparks as they attempt to exit the muffler. Spark arrestors are essential in making sure that internal combustion stays internal, thereby preventing wildfires. Because the spark arrestor screen needs to be fine enough to capture sparks, it is also prone to clogging with carbon deposits.

The Boggy Saw Blues
To clean the spark arrestor, use a propane torch. This will burn off the carbon, which can then be scraped away with an old toothbrush once it cools.

To fix the clogging, first remove the muffler cover on the chainsaw. Once you do this, you’ll be able to access the spark arrestor, as well as to inspect the condition of the piston through the exhaust port. In this case it was instantly obvious that the spark arrestor was clogged with carbon deposits. Depending on the make and model of your saw, there may be either a screw or retaining tabs that need to be bent before you can remove the spark arrestor with a pair of plyers.

Once you’ve cleaned the spark arrestor screen, use a rag to clean the inside of the muffler before replacing the muffler cover. Your saw can now breathe, leaving behind the Boggy Saw Blues!

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