The carburetors used almost every Stihl power tool is called an all position diaphragm with integrated fuel pump. The purpose of the carburetor is to mix air and fuel before it's ignited in the chamber. There are three sections to these carburetors: a metering section, where gas and air are measured; a mixing section, where the two are blended before ignition; and a pumping section, where the mixed fuel is sent to the cylinder. Rebuilding these carburetors takes extreme caution--the small parts are easily damaged.
Things You'll Need
Needle nosed pliers
Start rebuilding the carburetor on the fuel metering section. Screw the high speed and low speed fuel mixture screws, with new rubber seals, into their holes, using the small screwdriver. Tighten them until they are fully seated.
Fit the inlet needle onto the carburetor body using the needle nosed pliers. Set the spring into its hole just before the inlet needle with the needle nosed pliers. Push the spindle through the circular hole on the inlet control lever. Using the needle nosed pliers, engage the control lever's clevis with the head of the inlet needle.
Press the inlet control lever down and tighten it in place with the screw, using the screwdriver. Test the inlet control lever by pushing it down lightly, to make sure it engages the inlet needle. Make sure the top end of the inlet control lever is flush with the bottom of the metering chamber floor. If the lever isn't flush, it is bent and must be replaced.
Set the metering section's gasket on top of the carburetor's body. Place the fuel metering diaphragm on top of the gasket. Push the pegs in place with the butt of the screwdriver to secure the diaphragm and gasket in place. Place the metering cover, positioned with the ventilation hole toward the cylinder, over the diaphragm. Screw the cover in place with the screwdriver.
Press the fuel inlet screen into the fuel inlet valve using the toothpick; make sure it is flat and even. Place the pump's diaphragm against the carburetor body. Set the pump's gasket on top of the diaphragm. Tighten the cover's screws, using the screwdriver, in place over the gasket and diaphragm.
Pressure test the carburetor after it's assembled. Place the hose of the pressure gauge into the fuel inlet valve. Submerge the carburetor into a container of fuel. Pump the pressure to 11.6 psi. The carburetor shouldn't leak any air and the pressure should remain constant. If the pressure drops, it is likely due to an improperly seated inlet needle or a warped diaphragm. Replace parts as necessary.
Use only new gaskets, diaphragms, fuel inlet screen and Welch plugs when rebuilding the carburetor to ensure proper carburetor function. The fuel pump on Stihl carburetors, while integrated into the carburetor, is separate from the rest of the carburetor because it is regulated by the pressure in the crankcase.
Rebuilding the carburetor improperly can cause permanent damage.