Jonsered is a Swedish-made chain saw produced by the same company that makes the Husqvarna brand. Troubleshooting them depends on the nature of the problem: the saw won't start, it runs roughly or dies frequently, the chain does not cut efficiently or cuts at an angle. Some problems are obvious, such as a broken starter rope. Many problems can be cleared up by an owner, following simple steps and a chain saw owner's manual.
Begin with obvious visual inspection: Note that the starter rope is satisfactory, that the chain brake is not engaged, that the switch is on and that the throttle/trigger is properly set. Check the fuel tank for proper mixture; it should be at least half full. Be sure the chain oiler has oil and the chain moves freely and does not bind in the bar groove. Then follow the directions to try to start.
Move to the next level if the saw does not start or will not run properly. Cbeck the air filter by removing the top cover and unscrewing the filter; wash it with soap and water or replace it if it is very dirty. Then remove the spark plug and see if it is fouled; also check for the proper spark gap. Replace a seriously fouled plug. Remove the muffler and clear it and any screen of carbon build-up with a wire brush. Clean the carburettor and airbox and the cooling fins on the cylinder.
Change the fuel filter (and clean the tank if it is dirty--dump old fuel, flush with gasoline and replace with proper gas/oil mixture). Check the clutch, drum and spring for wear; you can replace if needed by following directions in the owner's manual. Adjust the carburettor with the screws labelled L, H and T. Turn L (low) clockwise to the stop, then turn back until engine idles well. Turn H (high) counterclockwise to the stop, then back until engine idles well. Then turn T (throttle) clockwise until the chain starts to move, then turn it back until the chain stops. A properly adjusted saw will idle smoothly without moving the chain.
Sharpen or replace the chain. If the saw runs but does not cut efficiently or cuts at an angle, there is a sharpening problem. Cutting at an angle indicates the chain teeth are filed incorrectly. Check the owner's manual for the proper sharpening; you can sharpen a chain with a round file and a guide bar which insures the proper filing angle. If chain teeth are seriously misaligned or badly worn, you may have to replace the chain.
Be careful adjusting a chain saw with the chain moving.