Small adjustments made to a riding lawnmower carburettor can make a huge difference in operating performance. Most carburettors will benefit from an annual adjustment, since the 2 adjustment screws are only held in place with the support of a small spring. Vibration and normal wear and tear of the lawn tractor engine will cause them to move enough each season to make adjustment a good idea. Adjusting a lawn tractor carburettor is a simple process, and only takes about 15 minutes to complete.
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Things you need
- Clean rags
- Small screwdriver set
- Gasoline (if necessary)
Start the engine and allow it to warm up to several minutes, so that the engine is at it's normal operating temperature. Shut off the engine.
Set the idle adjustment by turning the throttle stop screw (located at the top of the carburettor) in the clockwise direction until it makes contact with the throttle lever. Turn it 3 more full turns to completely open the throttle plate (component within the carburettor). A screwdriver may or may not be required, depending on the spring tension on the adjustment screw.
Turn the pilot screw (located below the throttle stop screw) clockwise until it begins to become slightly tight, then back it out counterclockwise 1 and 1/2 full turns. Do not over tighten the pilot screw because carburettor damage will result. A screwdriver may or may not be required, depending on the spring tension on the adjustment screw.
Start the engine again, and let it warm up for 2 minutes. Once the engine is at normal operating temperature again, slowly twist the throttle stop screw counterclockwise. Stop turning the screw when the engine idles at it's normal rate of idle. This may be verified (if desired) with an rpm gauge. The normal rate of idle will be specified in the user manual for each lawnmower.
Turn the pilot screw in both directions (clockwise and counter clockwise) to locate the best screw position to obtain the highest engine rpm (revolutions per minute). Repeat the adjustment of the throttle stop screw (step 4 above) and the pilot screw (step 5) until the pilot screw setting is found that produces the correct "normal" idle speed. It may take several adjustments of each screw until this setting is found.
Set the maximum speed adjustment on the carburettor by adjusting the set screw on the carburettor control lever (this lever is where the throttle control cable meets the carburettor linkage). Turn the set screw clockwise to decrease the maximum rpm attained, and counterclockwise to increase the maximum rpm attained. Close the throttle to idle speed. Slowly open the throttle to attain maximum engine speed. The maximum engine speed should not exceed manufacturers specifications. This may be verified (if desired) with an rpm gauge. The maximum rpm's allowed will be specified in the user manual for each lawnmower.
Turn off the engine. The carburettor is now set. Be sure to check the operation of the engine while cutting grass (and the engine is under full load). Make any adjustments as necessary.
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