Adjusting the Carburettor

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A big part of ensuring a smooth-running engine is keeping your carburettor and linkages clean and well adjusted. First, make sure that the air coming into the carburettor 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 carburettor's throttle and choke plates can bind or stick when dirty. Constant vibration and wear can affect the setting of the carburettor'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 carburettor eventually pay a price. Deposits inside the carburettor 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 carburettor from the engine. Commercially available carburettor cleaner comes in convenient spray cans for periodic cleaning of both inside and outside the carburettor.

Many engine performance problems can be linked to maintenance issues such as stale fuel, dirty air filter, fouled spark plug, and deteriorated oil.

A great way to help avoid these problems would be to perform an annual tune-up using a Briggs & Stratton engine Maintenance Kit.

Troubleshooting a Fuel Supply Problem

  1. Remove the air cleaner and inspect the choke plate mounted on a shaft at the opening of the carburettor's throat . Check that the choke plate closes easily and completely. A choke that does not move freely or close properly can cause difficulties in starting.
  2. Spray a small amount of carburettor cleaner on the shaft of a sluggish choke and into the venturi to loosen grit. Debris in the carburettor often causes performance problems.
  3. Open the fuel valve (if equipped), located at the base of the fuel tank where the fuel line is attached. Remove the line and check for blockage.Fuel will not reach the carburettor if the fuel valve is closed.
  4. If the engine is equipped with a fuel pump, make sure it operates properly.
  5. Remove and inspect the spark plug. A wet plug may indicate over-choking, water in the fuel, or an excessively rich fuel mixture. 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.
  6. Pour a teaspoon of fuel into the spark plug hole . Screw the spark plug back in and start the engine. 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.

Adjusting the Idle Speed and Mixture

On some float-type carburettors, 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 carburettor contains these screws, proceed below.

  1. With the engine off, remove the air filter and air cartridge.
  2. 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.
  3. 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 to 1-1/2 turns. Replace the air cleaner assembly and start the engine for final carburettor adjustments.
  4. 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. Finally, turn the screw back to the midpoint.
  5. 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.
  6. With the engine running at idle , 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.

Adjusting the High Speed Mixture

Some older carburettors 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.

  1. Run the engine for five minutes at half throttle to bring it to its operating temperature. Then, stop the engine.
  2. 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.
  3. Restart the engine and set the throttle position to HIGH or FAST. 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.
  4. Once adjusted, check engine acceleration by moving the throttle from idle to fast. The engine should accelerate smoothly. If necessary, readjust mixture screws.

Adjusting the Choke Linkage

  1. Remove the air cleaner and locate the choke lever on the engine or on the remote engine speed controls.
  2. Move the equipment controls to FAST or HIGH . Loosen the cable mounting bracket to allow movement of the cable casing.
  3. Move the cable casing so the choke is closed. Tighten the cable mounting screw  and check the motion of the control lever. Repeat the steps, as necessary, until the cable moves freely.


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Take advantage of the expertise your local dealer can provide for service and support on your Vanguard engine.

Please read and abide by any applicable Safety Information contained in your engine Operator's Manual. The material provided above is not intended to replace work performed by a Briggs & Stratton authorized dealerTerms and Conditions apply to all of the information presented on this website. Always be sure to completely read and understand your engine Operator's Manual.