How to Set Jets in a Lawn Mower Carburetor

The jets on a lawnmower carburetor control the amount of fuel mixing with air inside the carburetor. When these jets are opened or closed the fuel either increases or decrease, and this sets the engine's operating speeds. These adjustments will help an old lawnmower carburetor perform better. However, improperly adjusting the jets on a lawnmower can permanently damage the carburetor and the piston. Allow a professional to make these adjustments if you don’t feel confident making them at home.

Screwdriver
Tachometer

Instructions

- 1
  Turn off the lawnmower engine. Unscrew and remove the air filter box from the top of the carburetor. Find the idle speed adjusting screw, on the side of the carburetor.
- 2
  Turn the idle mixture screw clockwise until the needle lightly touches the seat. Turn the screw counterclockwise 1½ turns.
- 3
  Find the main jet adjustment screw at the base of the carburetor (some carburetors may not have this). Turn the screw clockwise slowly until it touches the seat inside the emulsion tube. Turn the screw counterclockwise 1½ turns.
- 4
  Refit the air filter box to the engine and tighten the screw. Start the engine and wait 10 minutes for it to warm up fully.
- 5
  Turn the idle speed mixture screw clockwise until the engine begins to slow. Turn the idle speed mixture screw counterclockwise until the engine slows again. Turn the screw back to its midpoint.
- 6
  Connect a tachometer to the engine. Adjust the idle speed screw to bring the idle speed to the lawnmower’s idle speed specifications (See your lawnmower's product manual for exact specifications).

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