The plastic gas tank on a Stihl weed eater holds the entire fuel supply for the machine. If this tank cracks, breaks or gets a puncture hole, the entire fuel supply will be compromised. The tank can also get warped to the point where fuel will no longer remain under the proper pressure in the fuel tank. Besides keeping the tank clean, there is little you can do to repair the tank; most tank repairs require a replacement tank.
Open the tank's cap and empty all remaining fuel into the approved fuel container. Start the Stihl, if you can, and let the system burn off any remaining fuel. Unhook the tabs locking the air filter cover over the filter. Remove the cover and the filter.
Reach behind the air filter's mounting box and unhook the fuel hoses from the carburetor's elbow connectors. Unscrew the bolts retaining the rewind starter to the rear of the engine. Lift the starter assembly away from the engine.
Pop out the tank plugs, using the flat tip of the screwdriver. Most Stihl trimmers have two rubber plugs. Remove the screw holding the tank unit to the engine, which is beside the carburetor. Take out the retainer on the rear of the screw. Pull out the sleeve that the screw just sat in.
Pull the grommet off the tank's housing. This rubber piece is attached to the fuel lines. Pull the grommet away from the tank. The fuel lines, which are attached, will come out as well. Discard the old fuel hoses and fuel filter.
Wash the fuel tank in water and dish soap. Use the brush to scrub any dirt from inside the tank. Wipe the tank dry with the rag. Inspect the fuel tank for any holes, cracks, warping or signs of damage. Close the fuel tank's cap and submerge it in clean water and look for air bubbles, which will indicate tiny puncture marks. Replace a fuel tank with damage or puncture marks
Reinstall fuel parts in reverse sequence: Fit the two new fuel hoses to the two slots on the fuel tank. Push the new fuel filter into the main fuel line's hose. Slide the grommet onto the main fuel hose until it is flush with the tank housing. Screw the fuel tank back onto the engine. Reinstall rubber plugs to the tank. Reattach fuel lines to the elbow connectors on the carburetor. Refit the air filter, air filter cover and starter assembly to the engine.