A muffler modification is the fastest and easiest legal way to get more power out of your Stihl chain saw. Drilling small holes into your muffler will allow more exhaust to escape out of the port. This opens up your engine, allowing it to breathe a little better by letting more of the gases out. The saw will also run cooler and have a noticeable increase in power. But this process will often void any manufacturer's warranty, so you shouldn't perform this modification if your saw is still covered under the warranty.
Use a wrench to take off the bar nuts on the Stihl saw. Remove the bar cover, bar and chain. Set them aside in a safe place, and lay the saw down with the clutch assembly facing down.
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Measure the surface area of your exhaust port at the cylinder, using a tape measure. Multiply this number by 0.8 to determine 80 percent of the hole's size. This is about the largest hole you can drill into your muffler without decreasing the power.
Select a spot on your muffler for the hole. This hole should not be pointed toward or located near the saw operator, rubber or plastic parts on the saw.
Measure the exhaust port hole's size again to make sure you've made no errors.
Choose the proper-size drill bit that is smaller than 80 percent of the exhaust port's size.
Drill the hole through the chain saw muffler. File down any rough or jagged edges with a hand file.
Cut a new spark arrestor screen with metal scissors so the screen is slightly bigger than the new muffler hole. Install the screen by welding it with filler over the hole.