When the sawyer depresses the throttle trigger, a small cable opens up a vent on the carburetor, allowing more fuel into the chainsaw's carburetor. After the throttle is let go, the vent closes and a small amount of fuel is still let into the carburetor keep the engine idling. If the Stihl chainsaw stalls after cutting, it is likely from problems in the carburetor, which will require an adjustment.
Before Adjusting Carburetor
Before adjusting the carburetor,you need to clean the fuel and air systems. If you adjust the carburetor ith these systems dirty, when they finally get cleaned, the adjustment will be off, which can destroy the Stihl engine. Remove the air filter pad and shake off all the dust with your palm. Replace the air filter if it can’t be cleaned. Drain any gas older than three days from the fuel tank and dispose of it according to local environmental regulations. Mix a fresh supply of Stihl chainsaw fuel and fill the tank.
Adjusting the Idle Screw
Start the Stihl chainsaw and allow it to run for about 10 minutes to warm up. Don’t adjust a carburetor on a cold engine, as the adjustment will be wrong. Position the chainsaw bar so the chain won’t hit anything during adjustment. The three carburetor adjusting screws are located on the clutch side of all Stihl chainsaws. The screw marked “LA” controls the idle speed. Insert a small screwdriver into this hole and adjust the idle screw clockwise and counterclockwise to find the highest rpm before the clutch engages and the chain starts spinning. Never use a chainsaw if the chain still spins while idling.
Adjusting the Low Screw
The screw marked with an “L” controls the low speed fuel mixture on the carburetor. Rotate this screw very slowly clockwise and counterclockwise while noting the different noises from the engine. When the engine starts surging, back off the screw until it starts bubbling.
The optimum low speed setting for your Stihl will be between the bubbling and surging. Try to achieve an optimum balance of acceleration and power, but don’t over-adjust the screw so the engine is surging, which can damage the engine. After finding the low speed setting, readjust the idle speed. Leave all high-speed adjustments to a Stihl mechanic.
A cleaned fuel and air system, combined with a properly adjusted carburetor will solve most minor engine and carburetor problems. However, if the Stihl is getting older, over seven years, you may need to remove the carburetor, disassemble it and clean it.
Clean the carburetor parts in a bath of unmixed fuel and brush off any hardened material carefully, so as not to damage the gaskets and diaphragms. Inspect the inlet control lever and needle arm, as they are likely not seating properly and causing the engine to stall out after the jets close. Leave carburetor repairs to a mechanic if you’re not qualified.