When your Stihl chainsaw starts, but only after battling the starter rope, you likely have one of two problems: a bad spark plug or poor gas flow. These common problems will drop any engine, which is why you should check for these during your scheduled maintenance checks and tune-ups. If you're consistently having starting problems, it may even be an old and dirty carburettor. Due to the difficulty required, leave carburettor servicing to the professionals.
Fill your chainsaw, if it isn't already, with freshly mixed fuel with a ratio of 50:1 regular unleaded gasoline to two-cycle engine oil. Put the fuel cap back on.
Push the flat tip of the screwdriver underneath the edge of the spark plug's rubber boot, on top of the engine, near the cylinder. Gently wiggle the boot free with the screwdriver and pull it up slowly with your fingers. Attach the socket wrench and remove the spark plug.
Check the spark plug. If it's dirty and covered in carbon, throw it away; it's easier than trying to clean and re-gap an old plug, which, if in a pinch, you can do with a brush, rag and a little gasoline. Place the new spark plug, with metal tip facing out, into the rubber boot. Press it down firmly into the boot.
Put on your leather gloves. Set the chainsaw on the ground so that the electrical circuit is grounded. Place the tip of the spark plug next to a grounded metal point on the engine block, where grounded means a metal spot that runs continuously to the ground.
Hold the rubber boot in hand, with the tip about ¾ inch away from the metal point. Pull the starter rope out three to four times and look for a spark. If there's a blue spark, move onto the next test to diagnose your starting problem. If the spark isn't present or is a weak yellow, you will need to service or replace your ignition module. Leave these repairs to a professional as the electricity can kill if not handled properly.
Open your fuel tank and drain out all of the fuel in the tank into a fuel container. Use a flashlight to inspect the tank's interior. Clean the tank with a brush and rag if the tank is dirty.
Pull the fuel line and fuel filter out the tank's opening. Check them with a flashlight to make sure they aren't clogged. A lot of starting problems begin with a plugged fuel filter or dirty fuel line. Replace the fuel filter and fuel line if you suspect they aren't allowing enough gas to reach the carburettor.
Open the cylinder cover and lift it off the engine. Take the air filter out. Set the choke lever, near the throttle, to the half-open position. Spray a one second squirt of starter fluid into the choke opening.
Set the saw on the ground and try starting it without the air filter and cover on. If it starts, your carburettor is dirty and needs to be cleaned. If it doesn't start, your carburettor is even worse and you may need to upgrade it with a carb kit and new filters. Leave these repairs to a professional.