When the gas inside your Stihl chain saw fuel tank heats up it creates pressure inside the tank. If the tank can't vent this added pressure, fuel will stop flowing from the tank to the carburetor due to an imbalance in the atmospheric pressure and the tank pressure. For the carburetor to work, these pressures must be equal. The tank vent cap sits on top of the fuel tank and allows these heated gasses to escape. The tank vent cap needs to be cleaned occasionally for the carburetor to work properly.
Things You'll Need
3 mm drift
Switch the Stihl chainsaw's master control lever into the "Stop" position and set the saw on a flat workbench. Unhook or unscrew the carburetor box cover on top of the saw. Take the carburetor box cover off.
Locate the tank vent on the fuel tank's nipple, just to the left of the rear handle, and beside the starter cover. Pry off the tank vent with a pair of needle-nose pliers, pulling it away from the fuel tank's nipple.
Pull the tank vent out of its seat in the handle housing and remove the tank vent and its hose from the saw. Push the grub screws out of the tank vent hose using the 3 mm drift.
Wash all parts in warm water and dish detergent. Replace the hose and grub screws if they're cracked or damaged.
Press the grub screws back into the tank hose using the 3 mm drift. Slide the tank vent back into its slot in the handle housing. Press it down into its seat. Refit the carburetor box cover.