Nothing is more frustrating than a chainsaw that starts but won’t idle, or one that bogs down the moment you squeeze the throttle on a log. If your Stihl chainsaw is struggling to run smoothly, the culprit is often the fuel-air mixture.
While modern Stihl saws are robust, changes in altitude, temperature, humidity, and fuel quality can throw off the factory tuning. Adjusting the carburetor isn’t just about performance—it’s about engine longevity. A saw running too “lean” (not enough fuel) can overheat and seize the piston, ruining the engine in seconds.
In this guide, we will walk through the exact, safe procedure to tune a Stihl carburetor, covering the specific functions of the H, L, and LA screws and the baseline settings for popular models like the MS 170, MS 180, and MS 250.
Warning: Carburetor adjustment affects engine lubrication. If you are unsure, always consult a certified Stihl technician. Running a saw too lean voids warranties and destroys engines.
Safety Precautions Before Adjusting
Before you touch a screwdriver, you must ensure the environment and the saw are safe.
- PPE is Mandatory: Wear eye protection and ear defenders. You will be running the saw at high RPMs during tuning.
- Install Bar and Chain: Never tune a chainsaw without the bar and chain attached. The lack of resistance can cause the engine to over-rev and shatter internal components.
- Clear the Area: Ensure the chain path is clear of debris. The chain will spin during this process.
- Stable Surface: Place the saw on flat ground or a solid workbench where it won’t vibrate off.
Credit: homeguides.sfgate.com
Preparation Checklist: Do This First!
90% of “carburetor problems” are actually dirty filters. Adjusting the carb on a saw with a clogged filter will lead to engine damage once the filter is cleaned.
- Clean the Air Filter: Tap it out or wash it with soapy water (if it’s a fleece/nylon filter) and let it dry.
- Check the Fuel Filter: Ensure the pickup body in the tank isn’t clogged.
- Fresh Fuel Mix: Use fresh, high-octane fuel mixed with high-quality 2-cycle oil (Stihl HP Ultra is recommended) at a 50:1 ratio. Old gas separates and causes tuning issues.
- Spark Arrestor: Check the muffler screen. If it’s clogged with carbon, the saw can’t exhale and won’t tune properly.
- Warm Up the Engine: Start the saw and let it idle or make a few light cuts for 1–2 minutes. You cannot tune a cold engine.
Locate and Understand Stihl Carb Screws (H, L, LA)
Stihl carburetors generally have three adjustment screws, usually accessed through holes in the side cover near the pull handle. They are often marked on the plastic housing:
1. L (Low Speed Jet)
This controls the fuel mixture at idle and during the initial acceleration.
- If too Lean: The saw will hesitate or “bog” when you hit the gas.
- If too Rich: The saw will smoke at idle and struggle to build RPMs.
2. H (High Speed Jet)
This controls the fuel mixture at full throttle (maximum RPM). This is the most critical screw for engine safety.
- If too Lean: The saw screams at a very high pitch and lacks power in the cut. Danger: This causes overheating and piston seizure.
- If too Rich: The saw sounds “burbley” (four-stroking) and won’t reach full power.
3. LA (Idle Speed Stop)
Sometimes labeled “T” on other brands. This mechanically pushes the throttle open slightly to keep the engine running when you let go of the trigger. It does not change the mixture, only the engine speed.
Baseline Carb Settings for Popular Stihl Models
If you have completely lost your settings, use these baselines to get the saw running again so you can fine-tune it.
Note: “1 Turn Out” means turn the screw gently clockwise until it stops (seats), then turn it counter-clockwise 1 full rotation.
| Stihl Model | L Screw (Low) | H Screw (High) | Notes |
| MS 170 / MS 180 | 1 turn out | Fixed (or 3/4 turn) | H screw is often non-adjustable on newer EPA models. |
| MS 250 / MS 251 | 1 turn out | 1 turn out | Standard baseline. |
| MS 261 / MS 362 | Standard Setting | Standard Setting | Many Pro models now use M-Tronic (auto-tune) and have no screws. |
| MS 290 / MS 291 | 1 turn out | 1 turn out | Older “Farm Boss” models usually allow 1 turn. |
| Old Stihl (026, 028) | 1 turn out | 1 turn out | Classic setting for nearly all older Stihl saws. |
Pro Tip: Many modern Stihl saws feature Limiter Caps (plastic tabs on the screws) that prevent you from turning them more than 1/4 or 1/2 turn. Do not force them past the stops unless you are a trained technician, as you can break the screw heads.
Step-by-Step Stihl Carburetor Adjustment Guide
Follow this sequence exactly: L first, then H, then LA.
Step 1: Adjust the Low Speed (L) Screw
Goal: Crisp acceleration without bogging.
- Start the warmed-up saw.
- Slowly turn the L screw clockwise (leaner) until the idle speed surges upward.
- Then, turn it counter-clockwise (richer) until the engine starts to load up and die.
- Set the screw right in the middle of these two points—where the idle is highest.
- The Test: Pull the trigger quickly.
- Symptom: If the engine bogs or gasps → Action: Turn L slightly counter-clockwise (richer) approx 1/8 turn.
- Symptom: If the engine smokes and is sluggish → Action: Turn L slightly clockwise (leaner).
Step 2: Adjust the High Speed (H) Screw
Goal: Maximum power without over-revving.
- Hold the throttle wide open (ensure chain brake is OFF and path is clear).
- Listen to the engine tone. It should sound slightly rough or “burbley” (like a motorboat) when not cutting wood. This is called “four-stroking.”
- The Tuning:
- Turn the H screw clockwise (leaner) just until the “burble” disappears and it screams smoothly.
- CRITICAL STEP: Once it sounds smooth, back the screw out (counter-clockwise) just a tiny bit until you hear that slight burble return.
- Why? That “burble” means the saw is getting enough fuel to keep the piston cool. Under the load of a log, the burble will clear up, and the saw will pull hard. If it screams smooth without a load, it will overheat in the cut.
Step 3: Adjust the Idle (LA) Screw
Goal: Engine stays on, chain stays stopped.
If the engine dies: Turn the LA screw clockwise until the engine runs smoothly without stalling.
Let the saw idle.
If the chain is spinning: Turn the LA screw counter-clockwise until the chain stops completely.
How To Tune A Chainsaw | Adjusting The Carburetor
Troubleshooting Common Carb Adjustment Problems
Problem: The Chain Turns at Idle
- Fix: Your idle is too high. Turn the LA screw counter-clockwise. If that doesn’t work, check your clutch springs; they may be worn out.
Problem: Saw Dies When I Squeeze the Trigger
- Fix: The saw is starving for fuel during the transition. Enrich the L screw by turning it counter-clockwise 1/8 to 1/4 turn.
Problem: Saw “Screams” and Gets Very Hot
- Fix: Your High (H) setting is too lean. This is dangerous. Stop immediately and turn the H screw counter-clockwise (richer) to add more fuel.
Problem: I Can’t Find the Screws / They Won’t Turn
- Fix: Newer Stihl saws (like the MS 180 C-BE) often have fixed jets or “Limiter Caps” to meet EPA emissions. If your saw has M-Tronic (M-badge models), it tunes itself electronically—there are no screws to adjust.
When to Stop and Call a Technician
Sometimes, tuning won’t fix the issue. If you adjust the L and H screws and the saw still runs erratically, you likely have an air leak or a mechanical failure.
- Air Leak: If the idle RPM floats up and down on its own, you may have a cracked fuel line or a bad crank seal.
- Bad Diaphragm: If the carburetor’s internal rubber diaphragms are stiff (common in saws that sit for years), no amount of screw turning will fix it. You need a carb rebuild kit.
FAQ: Common Stihl Tuning Questions
Can I adjust a Stihl carburetor without a tachometer? Yes, experienced users tune “by ear” listening for the “four-stroke burble” at high RPM. However, using a tachometer is the safest way to ensure you stay within the max RPM limits listed in your manual.
Which way do I turn the screw for more gas (richer)? Turn the screw Counter-Clockwise (left) to open the needle and add more fuel (Richer). Turn Clockwise (right) to close the needle and reduce fuel (Leaner).
What tools do I need for Stihl carb adjustment? Most Stihl saws use a simple, small flathead screwdriver. However, some newer models require a specific spline driver or a hex key, available at Stihl dealers.
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