Small engines have a float-type carburetor and most follow the same basic design. Tuning a small engine carburetor is easier with a little mechanical knowledge, but this is not necessary. A carburetor is a delicate and sensitive piece of equipment, so a light touch is needed to perform proper tuning and maintenance. The carburetor idle is tuned and the float bowl is cleaned during a comprehensive engine tuneup.
Things You'll Need
Flat blade screwdriver
Air compressor with air duster attachment
Remove the air cleaner. Unscrew the air cleaner cover screw with the flat head screwdriver. Pull out the air cleaner filter. Clean the air cleaner filter with an air duster. Point the air duster from inside the air cleaner to blow the dirt off the air cleaner element. Replace the air cleaner element and tighten up the air cleaner cover.
Look for the fuel valve leading to the carburetor and set it to the "off" position. Locate the fuel bowl under the carburetor. Unscrew the single screw on the base of the fuel bowl with the flat head screwdriver. Take care not to bend or misalign the float inside. The float is set to a certain level so the carburetor does not flood.
Pour out the gasoline and wipe off the sediment in the fuel bowl with a rag. Also wipe the rim of the fuel bowl so that any accumulated dirt will not drop into the bowl when you put it back under the carburetor. Replace the fuel bowl on the bottom of the carburetor. Screw the fuel bowl back into place. Do not overtighten. It should be just tight enough so that no leaks occur when the engine is started.
Turn the fuel valve lever to the "on" position. Start up the engine and allow it to warm up to operating temperature. Rev the engine once in a while so carbon does not build up with extended idle. If the engine dies because the rpm are too low, turn the idle adjustment screw about ¼ turn to increase the idle speed.
Locate the air mixture screw above the carburetor--it is the only screw facing outward above the carburetor where the air cleaner filter hose connects to the it. Using a flat blade screwdriver, turn the air mixture screw clockwise until the engine almost dies. Just before the engine dies, turn the air mixture screw counterclockwise approximately 1 ¼ turns.
Turn the air mixture screw up to one whole turn clockwise or counterclockwise. What you are looking for is the smoothest idle possible. Make the turns gradually. You will have to experiment both ways to see which screw position produces the smoothest idle.
Adjust the idle adjustment screw so that the engine does not die at idle speed. Then turn the idle adjustment screw about 1/8 turn clockwise. You should not set a very high idle speed because this uses more fuel and decreases engine life.
It helps if you fill the fuel bowl about 1/3 full when you put it back in the carburetor. The engine will start faster.
All adjustments to the carburetor should be made with the engine warm, using fresh fuel and the air cleaner clean and in place.
Never run an engine in an enclosed space because of the ever-present danger of carbon monoxide poisoning. Never leave a running engine unattended.