CH18-25 Carburetor Troubleshooting and Adjustments

NOTE: Carburetor adjustments should be made only after the engine has warmed up.

The carburetor is designed to deliver the correct fuel-to-air mixture to the engine under all operating conditions. The high idle is set at the factory and cannot be adjusted. The low idle fuel adjusting needle is also set at the factory and normally does not need adjustment.

If the engine is hard to start, runs roughly, or stalls at low idle speed, it may be necessary to adjust or service the carburetor.

 

Low Idle

 

Fuel Adj.

Low Idle

Needle &

Spring

Speed Adj.

 

Screw

Fixed Main Jet

 

 

(under optional

 

solenoid)

 

 

Figure 13. Carburetor (CH18-25).

 

Troubleshooting

If engine troubles are experienced that appear to be fuel system related, check the following areas before adjusting the carburetor.

•Make sure the fuel tank is filled with clean, fresh gasoline.

•Make sure the fuel tank cap vent is not blocked and that it is operating properly.

•If the fuel tank is equipped with a shut-off valve, make sure it is open.

•If the engine is equipped with an in-line fuel filter, make sure it is clean and unobstructed. Replace the filter if necessary.

•Make sure fuel is reaching the carburetor. This includes checking the fuel lines and fuel pump for restrictions or faulty components, replace as necessary.

•Make sure the air cleaner element is clean and all air cleaner element components are fastened securely.

If, after checking the items listed above, the engine is hard to start, runs roughly, or stalls at low idle speed, it may be necessary to adjust or service the carburetor.

Adjust Carburetor

NOTE: Certified engines may have a fixed idle or limiter cap on the idle fuel adjusting needle. Do not attempt steps 1 and 2 below. Proceed directly to step 3. Step 4 can only be performed within the limits allowed by the cap.

1.With the engine stopped, turn the low idle fuel adjusting needle in (clockwise) until it bottoms lightly.

NOTE: The tip of the low idle fuel adjusting needle is tapered to critical dimensions. Damage to the needle and the seat in carburetor body will result if the needle is forced.

2.Preliminary Low Idle Fuel Needle Setting: Turn the adjusting needle out (counterclockwise) from lightly bottomed position 1-5/8 turns.

3.Start the engine and run at half throttle for 5 to 10 minutes to warm up. The engine must be warm before making final settings (steps 4 and 5).

4.Low Idle Fuel Needle Setting: Place the throttle into the “idle” or “slow” position. Turn the low idle fuel adjusting needle in (slowly) from the preliminary setting until engine speed decreases and then back out approximately 3/4 to 1 turn to obtain the best low-speed performance.

5.Low Idle Speed Setting: Place the throttle control into the “idle” or “slow” position. Set the low idle speed to 1200 RPM* (± 75 RPM) by turning the low idle speed adjusting screw in or out. Check the speed using a tachometer.

*NOTE: The actual low idle speed depends on the application – refer to equipment manufacturer’s recommendations. The recommended low idle speed for basic engines is 1200 RPM. To ensure best results when setting the low idle fuel needle, the low idle speed must not exceed 1200 RPM (± 75 RPM).

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