Adjusting the carburetor on your Stihl weed trimmer is necessary if the engine is running rough, idling poorly, or not reaching full RPMs. While Stihl trimmers are generally reliable, over time you may need to fine-tune the carburetor for optimal performance. I’ve done this a few times, and with the right steps, you can easily get your trimmer back to running smoothly.
Here’s a detailed guide to help you adjust the carburetor on your Stihl weed trimmer.
Tools You’ll Need:
- Small Flathead Screwdriver: This is used to adjust the carburetor screws.
- Tachometer (optional): For precise RPM adjustments.
- Safety Gear: Gloves and safety glasses.
Understanding the Carburetor Adjustment Screws
On a Stihl weed trimmer, the carburetor typically has three adjustment screws:
- L Screw: Controls the low-speed fuel mixture.
- H Screw: Controls the high-speed fuel mixture.
- LA Screw: Adjusts the idle speed.
Note: Some newer models may have limiter caps or tamper-resistant screws, so refer to your user manual for specific details if these are present.
Step-by-Step Guide to Adjusting the Carburetor
1. Warm Up the Engine
Before making any adjustments, it’s important to warm up the engine so that it’s operating at normal temperatures.
- Start the Weed Trimmer: Let it run for about 5 minutes to get the engine warmed up.
- Check the Current Performance: If it’s idling too high, low, or cutting out, you’ll need to adjust the corresponding screws.
2. Adjust the Low-Speed (L) Screw
The L screw adjusts the air-fuel mixture at low speeds. If the trimmer stalls at idle or hesitates when you pull the throttle, this adjustment may fix it.
- Locate the L Screw: It is usually on the carburetor body, often marked with an “L.”
- Turn the L Screw Clockwise Until Seated: Turn the L screw clockwise until it gently seats (don’t over-tighten).
- Back It Off 1 to 1.5 Turns: Turn the screw counterclockwise 1 to 1.5 turns. This is your starting point.
- Fine-Tune:
- Start the trimmer and rev it.
- If the engine bogs down or hesitates, turn the L screw counterclockwise slightly to richen the mixture (more fuel).
- If the engine smokes or sputters, turn the L screw clockwise to lean the mixture (less fuel).
Make small adjustments (1/8 of a turn) until the engine runs smoothly at low speeds and transitions smoothly when you increase the throttle.
3. Adjust the High-Speed (H) Screw
The H screw controls the air-fuel mixture at full throttle. If your trimmer isn’t reaching full RPM or feels underpowered, you’ll need to adjust this screw.
- Locate the H Screw: It is usually next to the L screw and marked with an “H.”
- Set the H Screw: Start by turning it clockwise until it seats (again, don’t overtighten), then back it off about 1 turn counterclockwise.
- Fine-Tune the H Screw:
- Hold the throttle wide open (full throttle).
- Turn the H screw clockwise to lean the mixture or counterclockwise to richen it.
- Listen for RPM changes: The engine should sound smooth but not too high-pitched. If it’s screaming (too lean), back off a little by turning the H screw counterclockwise. If it sputters or smokes (too rich), turn it clockwise.
Tip: If you have a tachometer, adjust the H screw to the manufacturer’s recommended max RPM, usually around 12,500 to 13,500 RPM for most Stihl models.
4. Adjust the Idle Speed (LA) Screw
The LA screw controls the idle speed by adjusting the throttle position at idle. If the trimmer stalls at idle or the head spins while idling, this adjustment is necessary.
- Locate the LA Screw: It’s usually positioned separately from the L and H screws.
- Adjust the Idle:
- Turn the LA screw clockwise to increase the idle speed.
- Turn the LA screw counterclockwise to decrease the idle speed.
The ideal setting is when the engine idles smoothly without stalling, and the trimmer head does not rotate while at idle.
Testing the Trimmer
Once you’ve made the adjustments:
- Test the Throttle Response: Pull the throttle quickly. The engine should rev smoothly without hesitation.
- Test at Full Throttle: Ensure the trimmer runs at full power without bogging down or running too lean.
- Check Idle: Make sure the engine idles smoothly without the head spinning.
Final Tips:
- Small Adjustments: Always make small adjustments (1/8 turns) and test the performance before making more changes.
- Use Fresh Fuel: Ensure you’re using fresh, properly mixed fuel to avoid carburetor problems.
- Tachometer for Precision: If you want precise tuning, especially for high-speed adjustments, using a tachometer is recommended.
- Clean Air Filter: A dirty air filter can cause poor performance. Check and clean or replace the filter before adjusting the carburetor.
Conclusion
Adjusting the carburetor on your Stihl weed trimmer can restore smooth operation and full power. By tuning the L screw, H screw, and LA screw, you can improve low-speed performance, ensure proper throttle response, and set the correct idle speed. Always make adjustments slowly, and your trimmer will be running smoothly in no time!