A big part of ensuring a smooth-running engine is keeping your carburetor and linkages clean and well adjusted. First, make sure that the air coming into the carburetor is clean and free of debris by inspecting the air filter. A clogged air filter is a common cause for black smoke emitting from the exhaust. Next, the linkages attached to the carburetor's throttle and choke plates can bind or stick when dirty. Constant vibration and wear can affect the setting of the carburetor's mixture screws (if equipped).
And with all of the grass, twigs and other debris that a small engine encounters, it's not surprising that even passages inside the carburetor eventually pay a price. Deposits inside the carburetor can clog fuel and air passages and reduce performance or stop the engine altogether.
Luckily, you can take care of many of these problems quickly and easily-often without even removing the carburetor from the engine. Commercially available carburetor cleaner comes in convenient spray cans for periodic cleaning of both inside and outside the carburetor.
Adjusting The Idle Speed And Mixture
On some float-type carburetors, you can adjust the air-fuel mixture and engine speed at idle. 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. If your carburetor contains these screws, proceed below.
With the engine off, remove the air filter and air cartridge.
Locate the idle mixture screw and turn it clockwise until the needle lightly touches the seat. Then, turn the screw counterclockwise 1-1/2 turns.
If your carburetor 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 to 1-1/2 turns. Replace the air cleaner assembly and start the engine for final carburetor adjustments.
Run the engine for five minutes at half throttle to bring it to its operating temperature. Then, turn the idle mixture screw slowly clockwise until the engine begins to slow. Turn the screw in the opposite direction until the engine again begins to slow (image A)
Image A. Finally, turn the screw back to the midpoint.
Using a tachometer to gauge engine speed (image B)
Image B, 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.
With the engine running at idle (image D)
Image D, hold the throttle lever against the idle speed screw to bring the engine speed to "true idle." Then, repeat the idle mixture screw adjustments from Step 4 to fine-tune the mixture (image C)
Adjusting The High Speed Mixture
Some older carburetors contain a high speed mixture screw, near the throttle plate and opposite the idle speed screw. Under load, the high speed circuit increases air flow through the throat. Setting the high speed mixture involves running the engine until it is warm, stopping it to adjust the high speed mixture and then restarting for final adjustments.
Run the engine for five minutes at half throttle to bring it to its operating temperature. Then, stop the engine.
Locate the high speed mixture screw and turn it clockwise until the needle just touches the seat. Then turn the screw counterclockwise 1-1/4 to 1-1/2 turns.
Restart the engine and set the throttle position to HIGH or FAST (image E)
Image E. Turn the high speed or main jet screw clockwise until the engine begins to slow. Then, turn the screw the other way until the engine begins to slow. Turn the screw back to the midpoint (image F)
Once adjusted, check engine acceleration by moving the throttle from idle to fast. The engine should accelerate smoothly. If necessary, readjust mixture screws.