![]() Some older carburettors contain a high speed mixture screw, near the throttle plate and opposite the idle speed screw. Then, repeat the idle mixture screw adjustments from Step 4 to fine-tune the mixture. With the engine running at idle, hold the throttle lever against the idle speed screw to bring the engine speed to "true idle".Using a tachometer to gauge engine speed, set the idle speed screw to bring the engine to 1750 RPM for aluminum-cylinder engine or 1200 RPM for engine with a cast-iron cylinder sleeve.Finally, turn the screw back to the midpoint. Turn the screw in the opposite direction until the engine again begins to slow. Then, turn the idle mixture screw slowly clockwise until the engine begins to slow. Run the engine for five minutes at half throttle to bring it to its operating temperature.Replace the air cleaner assembly and start the engine for final carburettor adjustments. Then, turn the screw counterclockwise 1 to 1-1/2 turns. If your carburettor has a main jet adjustment screw at the base of the float bowl, turn the screw clockwise until you feel it just touch the seat inside the emulsion tube.Then, turn the screw counterclockwise 1-1/2 turns. Locate the idle mixture screw and turn it clockwise until the needle lightly touches the seat.With the engine off, remove the air filter and air cartridge.If your carburettor contains these screws, proceed below. Check for an idle speed screw designed to keep the throttle plate from closing completely, and an idle mixture screw that limits the flow of fuel at idle. On some float-type carburettors, you can adjust the air-fuel mixture and engine speed at idle. ![]() If it fires only a few times and then quits, assume a dry plug condition and consider the causes of a dry plug, listed in Step 5. Screw the spark plug back in and start the engine. Pour a teaspoon of fuel into the spark plug hole.A dry plug may indicate a plugged fuel filter, leaking mounting gaskets on either end of the carburettor, or a stuck or clogged carburettor inlet needle. A wet plug may indicate over-choking, water in the fuel, or an excessively rich fuel mixture. If the engine is equipped with a fuel pump, make sure it operates properly.Remove the line and check for blockage.Fuel will not reach the carburettor if the fuel valve is closed. Open the fuel valve (if equipped), located at the base of the fuel tank where the fuel line is attached.Debris in the carburettor often causes performance problems. Spray a small amount of carburettor cleaner on the shaft of a sluggish choke and into the venturi to loosen grit.A choke that does not move freely or close properly can cause difficulties in starting. Check that the choke plate closes easily and completely. Remove the air cleaner and inspect the choke plate mounted on a shaft at the opening of the carburettor's throat.Commercially available carburettor cleaner comes in convenient spray cans for periodic cleaning of both inside and outside the carburettor. ![]() ![]() Luckily, you can take care of many of these problems quickly and easily-often without even removing the carburettor from the engine. Deposits inside the carburettor can clog fuel and air passages and reduce performance or stop the engine altogether. Constant vibration and wear can affect the setting of the carburettor's mixture screws (if equipped).Īnd with all of the grass, twigs and other debris that a small engine encounters, it's not surprising that even passages inside the carburettor eventually pay a price. Next, the linkages attached to the carburettor's throttle and choke plates can bind or stick when dirty. A clogged air filter is a common cause for black smoke emitting from the exhaust. First, make sure that the air coming into the carburettor is clean and free of debris by inspecting the air filter. A big part of ensuring a smooth-running engine is keeping your carburettor and linkages clean and well adjusted.
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