When a Stihl chainsaw idles or runs at low temperatures, the jets on the carburetor open a little to let enough fuel in to sustain combustion. If the amount of fuel drops below this threshold, the saw will stall out and die. Also, a drop in fresh air can also cause a loss of the proper fuel to air ratio necessary for combustion.
Why It's Stalling
A dirty fuel and air system most often causes a decrease in gas or air supply. Any restrictions in the fuel and air systems will force the ratio of fuel and air into an imbalance, throwing the combustion off. These systems must operate in a continuous cycle, and a dirty carburetor, filters and hoses will suffocate the supply of fresh gas and air. The carburetor must continue to pull in fuel and air, measure their balance for the required speed and mix them together. A carburetor adjustment and cleaning can solve most stalling problems.
Before Adjusting Carburetor
However, before you adjust the carburetor properly, the fuel and air systems will need a thorough cleaning, especially if it's been more than 50 operational hours since their last scheduled maintenance. Adjusting the carburetor with dirty fuel and air systems will cause the adjustment to be incorrect. Pull the air filter out of its housing area. Clean the housing area and around the carburetor with a brush. Shake out the filter pad. Dump all bad gas or gas that's older than one week. Replace the fuel filter and fuel hoses if that hasn't been done in 50 hours.
The carburetor on a Stihl chainsaw operates with three small screws, which allow the operator to set the amount of fuel, and therefore the engine speed, at the high, low and idle settings of the engine. Small adjustments in these screws can have a big impact on performance, so only move the screws one-eighth of a turn at a time. After letting the engine warm to operating temperatures, rotate the idle speed screw clockwise until the chain spins. Adjust the screw to the highest rpm setting before the chain starts spinning. Move the low speed screw clockwise until it gets to the stop, and adjust it back slowly one full turn counterclockwise. Fine-tune the low speed up and down in small one-eighth turns.
Warnings on Adjustments
Don't attempt to reset the high speed screw without a tachometer and professional training. Improper carburetor settings can destroy a good engine in a matter of minutes. Therefore, make only small adjustments as necessary. When adjusting the idle speed setting, position the bar and chain so it won't hit anything while it's turning. If these stalling problems persist, take the Stihl chainsaw to a professional to clean the carburetor thoroughly and install an upgrade kit.