Sometimes it seems like nothing you do will get your Stihl chainsaw to start and run properly. With so many parts and even more problems for those parts, it can be difficult even knowing where to start. But using the three engine principles of compression, spark and gas, you can quickly troubleshoot your problem into the affected engine area. Then, you can further isolate the parts that need repairing and get your Stihl chainsaw running again.
Switch the Stihl's ignition switch to the "Off" setting. Disconnect the lead wire's rubber boot from the spark plug. Hook up the socket wrench and unscrew the plug. Remove the spark plug and insert the compression gauge into the spark plug hole.
Crank the starter rope six or seven times and check for a reading of at least 90 psi. If pressure is below this or rapidly falls below, you have air leakage either in the oil seals, gaskets or cracks in the housing. Take the Stihl to a service dealer to vacuum and pressure test the crankcase.
Check the spark plug if the pressure reading is fine. Replace oxidised or dirty plugs -- don't try to clean them. Insert a new spark plug into the rubber boot. Switch the ignition switch back to the "On" position. Hold the boot and wire next to a grounded metal point on the engine block. Pull the rope briskly three times.
Check for a spark across the plug. If there's no spark, check the ground and ignition wires to make sure they aren't contacting a metal point, prematurely grounding the circuit. Make sure wires aren't corroded or broken. Check the ignition switch to make sure it is contacting the wires properly. Unscrew the starter cover and check the flywheel to make sure it isn't damaged. Check to make sure lead wires aren't loose at the ignition module.
Repeat the spark plug test again. If there's still no spark, a service mechanic will need to replace your ignition module. If there's a spark now or there was one at the original test, refit the spark plug and plug wire. Move the choke lever into the closed position; hold the throttle trigger down and pull the starter rope three times. Remove the spark plug and check the tip to see if it is wet or dry with fuel.
Unscrew and remove the cover to the muffler if the spark plug was wet. Remove the spark arrester screen from the muffler. Clean the screen with the brush and rag. Clean and inspect the muffler's exhaust port. Replace bent, cracked or broken mufflers. Replace dirty mufflers that can't be cleaned.
Check the fuel supply if the spark plug was dry during the test. Make sure you are using 50:1 regular unleaded 87-octane gas with two-cycle mix oil. Get rid of any old or bad gas. Pull out the fuel filter and replace with a new filter. Check the fuel line to make sure there are no holes or cracks in it. Pull out and replace the fuel lines if fuel isn't reaching the carburettor.
Remove and have a service mechanic pressure test the carburettor if you have compression, spark and proper fuel in the carburettor. Your problem is likely an old carburettor. Try cleaning the carburettor and installing a carb kit to the front of the carburettor. If the carb kit doesn't solve problem, you will need to buy a new carburettor.