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How to adjust a small engine carburetor?

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Quick Answer

Adjusting a small engine carburetor is a simple job that involves adjustments to its idle speed and mixture settings. In addition, troubleshooting fuel supply problems before making adjustments to the carburetor rules out poor performance because of issues with the fuel supply.

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Full Answer

Small engines depend heavily on a clean and well-adjusted carburetor for trouble-free performance. Ensuring smooth running involves troubleshooting fuel supply problems and then making adjustments to the carburetor settings.

Step 1: Troubleshoot fuel supply problems

Small engines have to deal with debris and grass, which can easily clog fuel passages and negatively affect performance. Inspecting the lines and cleaning any blockages ensures an uninterrupted supply to the carburetor.

  • If the choke plate does not move freely, use carburetor cleaner to remove grit.
  • Open the fuel valve, remove the line and check for free flow.
  • Inspect the spark plug. If it is wet, it may indicate water in the fuel. If it is dry, it could mean a blocked fuel filter or leaking gaskets.

Step 2: Adjust carburetor idle speed

Some carburetors allow adjustments to the air-fuel mixture and idle speed, indicated by the presence of an idle speed screw and an idle mixture screw.

  • Turn the engine off and remove the air filter, to access the carburetor freely.
  • Turn the idle mixture screw clockwise until the needle touches the throttle plate. Turn counterclockwise by 1/2 to 1 turn of the screw.
  • Repeat the process with the main jet adjustment screw, but turn it counterclockwise by 1 to 1-1/2 turns.
  • Replace the air filter, turn on the engine and bring it up to operating temperature.Turn the idle mixture screw clockwise until the engine slows. Repeat in the opposite direction. Bring it back to midpoint.
  • Using a tachometer, bring the engine idle speed down to the settings recommended by the manufacturer.

Step 3: High speed mixture adjustments

A few early carburetors may be equipped with a high speed mixture screw that controls the flow of air under load. Follow the same steps as those for the idle settings. However, the engine should be at operating temperature and the screw should be turned counterclockwise by 1-1/4 to 1-1/2 turns.

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