Most chainsaw repairs are actually just maintenance. Major problems, such as bad cylinders or defective carburettors, either cannot be fixed or require service of an authorised repairman. (Environmental Protection Agency rules require carburettor adjustments be made by authorised servicemen.) Chainsaws are fairly simple devices -- an engine drives a sprocket that turns a chain with sharp teeth that cut wood. There are ways to troubleshoot a saw that won't start, won't keep running, won't cut effectively or won't cut straight.
Check all the basics first. Make sure the saw has the proper mixture of unleaded gasoline and two-cycle oil and that the bar-oil tank is full. Remove the cover and inspect the air filter; if it is dirty or clogged, clean or replace it. Remove the spark plug with the scrench; if it is oily or fouled, clean or replace it. Check that the sparkplug wire is clean and properly fastened and fits securely on the plug. Remove the exhaust cover at the front of the saw and check that the spark-arrester screen behind it is free of carbon; replace the screen if it is fouled or broken.
Remove the bar and chain by removing the retaining nuts on the side of the saw. Check that the chain groove is clean -- use the end of a small screwdriver or a piece of wire to clean out any debris. Replace the bar if it has nicks in it or is bent or damaged so the chain will not slide easily and smoothly. Examine the chain for sharpness and to make sure the underneath teeth, which catch on the drive sprocket, are not damaged. If the chain is damaged, replace it.
Replace the drive sprocket if it is grooved or severely worn. Remove the screw holding it in place by turning it to the right. Take out the spark plug, then insert a section of nylon cord into the sparkplug hole to secure the cylinder while you remove the sprocket, which should just pull off once the screw is removed. Put on a new sprocket and screw it in place, then remove the rope and put the spark plug back in. Test with the chain to make sure the sprocket holds the chain and will turn.
Repair a broken starter cord by removing its cover and taking off the circular spring assembly holding it. Remove the old cord, watching to see how it goes through a hole and is fastened on the inside with a knot. Put a new cord through that hole and tie a knot in it. Then wind the cord onto the spring, holding it with your hand as you turn to keep it from uncoiling. When the cord is wound in, reattach the spring assembly, pass the loose end of the cord through the hole in the saw and affix the cord handle by putting the cord through it and knotting it.
Most manufacturers suggest that a service centre perform sprocket replacement. You will have to get replacement sprockets from dealers or a parts house.