Chainsaws require regularly scheduled maintenance for proper operation. Clean your chainsaw after each use and refer to your owner's manual for information. Follow these troubleshooting guides in the event of starting difficulties.
Make sure you follow specific chainsaw starting procedures. This typically looks like depressing a decompression valve, pressing and pumping the priming bulb, closing the choke, and pulling the cable to start. There may not be anything wrong with the saw and may simply have an open choke, throttle, or decompression valve.
With every gas engine, three important ingredients are necessary for operation: air, fuel, and spark. In a chainsaw, the ambient air is pulled into the carburetor through an air filter. Gasoline (mixed with oil) is pulled into the carburetor by vacuum from the fuel tank. The spark plug creates a spark as the engine turns over and pulses the ignition module. When the saw is running lumpy, your issue lies in one of these systems.
Dirty Air Filter (air)
The air filter feeds your carburetor with clean, ambient air. If your air filter is full of oil or sawdust, you can try spraying with compressed air or washing with soap and water. In the event your filter is too destroyed, replace the filter completely.
Clogged Spark-Arrestor Screen (air)
Running too-rich fuel over time can clog the metal screen in the exhaust. Remove and inspect the screen. Put the screen into a vise, and use a torch to heat and burn all of the carbon on the screen until it becomes ash. Use a wire brush to remove the ash, then replace. Inspect the spark plug for carbon soot buildup, and clean with a wire brush.
Bad Gas (fuel)
Without a fuel stabilizer, old gas accumulates moisture. If your gas is more than a month old, or has been stored between 3-6 months, drain the tank, primer bulb and fuel lines. Bad fuel needs to be removed from the entire fuel line before replenishing with fresh fuel.
Plugged Fuel Filter (fuel)
Old or poor quality gasoline along with sawdust and dirt can plug the filter in the gas tank. If your saw isn't getting enough gas, remove and clean or replace the fuel filter.
Cracked Fuel Lines (fuel)
If fuel cannot get to the carburetor, or air is sucked into a cracked line, then your saw won't start. If you notice any of the fuel lines are looking old or developing cracks, replace the old and cracked lines with hose rated for use with fuel.
Dirty Carburetor (air+fuel)
The carburetor is a mechanical computer that feeds the engine a ratio of fuel and air. If fuel sits in the system for too long, the carburetor (and the engine and fuel lines) can become sticky and clog. Remove the air filter and spray the carburetor with carb cleaner—an aerosolized acetone spray made to remove the gunk.
In the event that the carburetor is too dirty or cannot be cleaned, it likely needs replacing.
Carburetor Tuning (air+fuel)
Your carburetor has several adjustment screws that change how your saw performs at idle and at wide open throttle. Below are a few conditions where adjusting the screws may solve your problems. Note: chainsaw must be warmed up for 20-30 seconds before making any 'H' Screw adjustments.
- If your saw has trouble idling and will die shortly after starting, then adjust the ‘L’ screw by unscrewing. This will allow more fuel through the carburetor during idle.
- If your saw has trouble idling and then runs lumpy with a lot of smoke, adjust the ‘L’ screw by screwing in. This will limit the amount of fuel to the carburetor at idle.
- If your saw idles fine, and then bogs down when pressing the throttle, then adjust your ‘H’ screw by unscrewing. This will increase the amount of fuel to the carburetor under load.
- If your saw idles fine, then runs lumpy with a lot of smoke when pressing the throttle, then adjust your ‘H’ screw by screwing in. This will limit the amount of fuel to the carburetor under load.
Fuel Tank Cap Clogged (fuel)
There is a small hole with a valve at the top of your gas cap which may become clogged or fail with time. Inspect, and clean or replace.
Faulty Spark Plug (ignition)
Remove your spark plug with a scrench — screwdriver wrench — or spark plug wrench and check its condition on this chart. If the tip is "normal", you can reinstall the plug.
If the tip is "fouled", clean the tip of the electrode with 220-grit sandpaper or a file, and clean the threads with a wire brush. Alternatively, a torch can be used to burn carbon deposits. Lock the plug into a vise and heat. A wire brush can then clear the ashes off. Spray carburetor cleaner on the plug to clean all deposits off, and check the gap on the plug with a tool before re-inserting.
If the tip is mechanically damaged, glazed, detonated, or otherwise destroyed, replace the spark plug.
Faulty Ignition Coil (ignition)
Functioning spark is an important and necessary part of a functioning saw. After verifying your spark plug health you can verify spark. This can be tested by removing the spark plug, reconnecting the ignition wire, and then holding the end of the plug so it’s touching a metallic part of the engine cylinder head. You can also test using an in-line spark tester which a built in light.
Turn the ignition switch on. As you pull the cord and turn over the engine check the tip of the spark plug for a spark. You should clearly see a blue spark between the tip of the electrode and the piece of curved metal. If you see the spark, great! Ignition is fine.
If your spark is not visible, or is absent and weak, then check the condition of the spark plug. Typical use gives spark plugs a brown or light grey residue, and not covered in oil or other black deposits, then replace the plug. Test the spark again. If there still is no spark, the only remaining culprits are the wire, and ignition coil. Replace both.
Follow this guide for more help testing an ignition coil.
Carburetor Screw Adjustment by Craig Kirk