McCulloch gas chainsaws use a carburettor with needles that regulate the inflow of gas during different engine speeds. These needles can be opened and closed slightly through adjusting screws on the outside of the carburettor. Adjusting the needles will help solve many engine problems and improve the overall performance of the engine. However, if done improperly, adjustment can damage the carburettor and the engine. For this reason, McCulloch recommends a professional makes these adjustments. Before making adjustments, make sure this won't void your warranty.
Loosen and remove the top cover's screws with the screwdriver. Lift the cover off the cylinder and air filter. Take out the air filter box beside the cylinder. Take the filter out of the box.
Clean the filter, box and area around it, including the cylinder fins, with the brush. Brush off all dust and dirt. Leave the filter and box out to access the carburettor.
Open up the fuel tank and dispose of any old or bad fuel left in the tank for longer than one week according to environmental regulations. Fill the chainsaw with the freshly mixed McCulloch chainsaw gas. Start the engine and allow it to warm up for several minutes. Locate the carburettor adjusting screws on the side of the carburettor.
Locate the idle speed adjusting screw on your McCulloch chainsaw's carburettor. Turn the idle adjusting screw with the small screwdriver slowly clockwise until the clutch engages and the chain starts spinning; make sure the moving bar will not hit anyone or anything when it starts to spin.
Stop turning the screw immediately once the chain starts rotating. Turn the idle screw counterclockwise until the chain stops spinning. Turn the screw in slowly clockwise to find the highest idle speed before the chain starts moving.
Locate the low speed adjusting screw on your McCulloch chainsaw's carburettor. Turn the low speed adjusting screw with the small screwdriver slowly clockwise until your hear the engine producing surging noises. Stop turning and rotate it counterclockwise until your hear the engine producing bubbling noises.
Rotate the low speed screw in between these two lean and rich settings to find the optimal low speed. Press the trigger and make sure the acceleration is sharp and clean. Adjust the screw slowly as needed. Don't adjust past the surging or bubbling noises. Readjust the idle speed screw as before once the low speed is set.
Let a professional adjust the high speed screw only if necessary.