The carburettor on a Husqvarna trimmer comes with three adjustable jets that regulate the speed of the engine by controlling the amount fuel and air mixing inside the carburettor. The high speed and low speed jet regulate the fuel and air mixing when the engine is throttled. The idle screw regulates the speed of the engine when it is idling. While adjusting the carburettor is a necessity, improper adjustment of the high and low speed can cause serious engine damage. You should leave it to a qualified technician if you are worried about damaging the engine or voiding the warranty.
Discharge any fuel remaining in the tank by running the engine out or by pouring it into an approved fuel container. Fill the tank up with properly mixed fresh trimmer gas. Follow the mixing directions for your model as described in the operator's manual.
Unscrew the cover to the air filter box with the screwdriver and remove the foam filter. Wash the foam filter in warm, soapy water and let it dry thoroughly before reusing it. Replace the air filter if it's beyond cleaning. Reattach air filter and its cover before adjusting the carburettor.
Start the trimmer's engine and let it run at a high idle for about 10 minutes to warm up. Locate the three carburettor screw holes, which on most Husqvarna's are above the fuel tank and next to the air filter.
Insert the screwdriver into the hole marked "L." Turn the screw in clockwise to try and find the highest idle speed. Turn the "L" screw back ¼ turn counterclockwise once you have found the highest idle speed. If the cutting head starts to rotate while idling you will need to adjust the idle screw.
Adjust the idle speed by inserting the screwdriver into the hole marked "T." Turn the screw in clockwise until the trimmer head starts to rotate. Turn the screw back in ¼ increments until the cutting head stops spinning. Readjust the screw marked "L" to fine tune for the optimum engine performance.
Let a qualified service mechanic adjust the high speed, as it requires a tachometer and careful, fine adjustments.
Do not attempt to adjust the H speed screw as it can cause serious engine damage if it is run at too high of a speed for longer than 10 seconds.