A chain saw's starter rope connects to a pulley, which, through a recoil spring and two pawls, connects to the flywheel. The flywheel connects to the crankshaft through a thin metal piece called the key. Any problems along this line can cause the flywheel to stick. A bent or sheared flywheel key will also cause the flywheel to stop spinning. But the flywheel is also attached to the crankshaft, which can stop spinning if there are problems in the cylinder and piston area. It's generally cheaper to replace the saw than to fix these problems.
Unscrew the cylinder cover, which sits atop the engine, protecting the cylinder and the air filter; remove the cover. Unscrew the starter cover, which sits on the side, protecting the starter area; remove the cover.
Hold the starter cover in your hand and pull on the starter rope. Does the starter rope pull out and retract automatically on the pulley? If not, replace the starter recoil assembly. If it spins, your problem lies deeper in the engine.
Turn the flywheel by hand. Does it spin, but only if you push really hard? Your key is sheared and will need to be replaced. If it doesn't spin, your problem is further into the engine; you will need to remove the flywheel.
Use the plier to disconnect the lead ignition wire, which runs to the "stop" switch, from its post on the ignition module, which is to the left of the flywheel. Disconnect the HT lead wire from the spark plug by removing the rubber boot. Lift both wires from their cable guide above the flywheel, and slowly lift them out of the engine.
Unscrew the spark plug with the socket wrench, and lift it out. Push the piston stop tool into the open plug neck. Unscrew the two bolts holding the ignition module in place on the engine. Lift the module out of the engine, slowly and carefully. Unhook the two starter pawls from the flywheel, and remove them.
Loosen the flywheel's centre bolt until the bottom edge comes in line with the flywheel. Set the flywheel puller over the flywheel. Tighten the two outer bolts on the puller to grasp the flywheel. Tighten the centre bolt of the flywheel puller until the flywheel comes loose from the crankshaft.
Remove the nut, washer, flywheel and the puller. Separate them, and wash the flywheel in a white spirit. Check the flywheel for signs of bent or damaged spokes. Look for any cracks on the underbelly of the flywheel. Pop the key out of its slot on the crankshaft. Replace if it's bent or damaged.
Turn the crankshaft by hand; it should spin a few rotations. If it doesn't spin, your problem lies in the crankshaft, piston or cylinder. Most of which are very expensive to fix. Install the flywheel key, flywheel, ignition module, wires and the rest of the starter assembly in reverse order.