Internal combustion requires fuel to mix with air. The carburetor manages this for Mcculloch chain saws, just as for any small gas-powered lawn tools. Needles regulate the amount of fuel and air that are mixed. The needles are controlled by three adjustment screws: an lazy, a low-speed and a high-speed screw. Carburetors are pre-set by the producer, but can fall out of adjustment after several hours of use. Fixing the needles that control the consumption and mix isn’t a task for the inexperienced.
Unscrew and remove the main cover, air filter cover and air filter. Underneath these, you will notice a steel plate covering the carburetor. Unscrew the plate and pull it away to reveal the carburetor and its own three adjustment screws. If you are working with a tachometer, attach it in accordance with manufacturer directions.
Turn the high- and low-speed adjustment screws clockwise until they stop, but do not force them. Turn each screw one complete turn and start the engine. Permit the engine to warm up for approximately five minutes.
Gauge the idle speed if using a tachometer. Ideally, it should examine about 2,700 RPM. If you choose not to use one, use the screwdriver to turn the idle screw as high as possible without engaging the clutch. If the chains begin spinning, then turn the screw back till it stops.
Adjust the low-speed screw with the screwdriver. Turn it slightly back and forth and listen for the engine sounds to change. On one end, it will surge heavily. On the other end, it will almost die. Find the midway point between these two extremes. You might need to return to the idle adjustment after finishing the low-speed adjustment. In that case, repeat Step 3 and then continue on to Step 5.
Open the trigger fully and listen to the engine. It should shout when it is totally open, then flutter when lazy. A tachometer reading should be normally between 10,000 and 12,000 RPM when completely open. Only attempt to correct the high-speed screw if it is totally necessary to machine operation. Use extreme caution with this alteration and flip just 1/8 turn at one time. Making hasty alterations can ignore the motor and cause irreparable damage.