Stihl FS 350 Carb Screws (Adjustment Guide)
The FS 350 is a 40.2 cm³ two-stroke brushcutter, and its carburetor screws are not a one-setting-fits-all adjustment. Confirm the carburetor and matching STIHL instructions first. L controls low-speed mixture, H controls high-speed mixture, and LA sets the idle stop. Adjust only where permitted, use the specified limits, and stop if the cutting attachment turns at idle.
A dependable seasonal start saves more than time: it can spare you from replacing parts that were never the problem. If your FS 350 idles poorly or bogs on acceleration, a screw adjustment may help, but a dirty filter, stale fuel, or air leak can cause similar symptoms. I check those basics before touching the carburetor.
Identify the carburetor and screws first
The carburetor meters fuel and air for the engine. The FS 350 model name alone does not confirm which carburetor or limiter arrangement is installed, so identify the unit and consult the matching STIHL operator or service instructions before adjusting anything.
Look for the model identification on the machine and the screw markings on the carburetor. The adjustment points may be restricted, and carburetor versions can differ. If markings are unclear, or the screws have limiter caps, do not force, remove, or bypass them. Ask a STIHL dealer to identify the carburetor and the permitted procedure.
| Marking | What it controls | What it may affect |
|---|---|---|
| L | Low-speed fuel mixture | Idle and the transition to acceleration |
| H | High-speed fuel mixture | Fuel mixture at full throttle |
| LA | Idle-speed stop | How far the throttle remains open at idle |
The letters describe different functions; they are not interchangeable settings. Use the manual for the exact adjustment limits and specified engine speeds. Do not use a generic screw-turn count or RPM figure as a substitute.
Prepare the FS 350 safely
Preparation means making the machine safe and ruling out basic faults before tuning. Use fresh, correctly mixed fuel, inspect the machine, and secure the work area. Carburetor adjustments should never be used to hide a leak, damaged part, or dirty filter.
The FS 350 has a two-stroke engine with a stated displacement of 40.2 cm³. When using suitable STIHL two-stroke oil, the specified fuel mixture is 50:1 gasoline to oil. Follow the applicable machine manual and oil label; do not run straight gasoline.
Before starting:
- Check for fuel leaks, damaged fuel hoses, loose fasteners, and a dirty air filter.
- Inspect the cutting attachment and guard for damage or trapped debris.
- Wear eye and face, hearing, hand, leg, and foot protection.
- Clear people and obstacles from the work area.
- Keep the cutting attachment off the ground and clear of debris during starting and adjustment.
If a hose is cracked, a fitting is loose, or fuel is leaking, stop and repair the fault before running the engine. A leak can affect how the engine runs and create a fire risk. Do not try to correct it by turning L or H.
Understand L, H, and LA
L and H change the fuel mixture at different engine speeds. LA is an idle-speed stop, not a mixture screw. Knowing the distinction helps you avoid using an idle adjustment to mask a mixture or fuel-system problem.
L screw: The low-speed mixture setting affects idle and the engine’s response as you open the throttle. A poor transition can have causes beyond L, including restricted fuel flow or an intake leak.
H screw: The high-speed mixture setting affects full-throttle operation. An incorrect, overly lean setting can cause excessive engine heat and serious damage. Do not lean H to chase higher speed, and do not set it by sound alone.
LA screw: This stop sets the throttle position at idle. The engine should run steadily while the cutting attachment remains stationary. If the attachment rotates at the lowest stable idle, stop and address the cause instead of continuing to operate the brushcutter.
Adjust only by the matching procedure
A safe adjustment follows the instructions for the carburetor actually fitted to the machine. If the required service procedure, adjustment limits, or tools are unavailable, leave the settings alone and have a STIHL dealer perform the work.
- Confirm the cause. Check fuel freshness and mixture, air-filter condition, fuel lines, fuel filter, and visible signs of damage or leakage. Correct faults before tuning.
- Warm the engine as the operator’s manual directs. Keep the machine securely controlled, with the cutting attachment clear of the ground and other objects.
- Follow the specified sequence and limits. Make only permitted adjustments, in small steps, and one screw at a time. Use a tachometer if the model-specific procedure calls for one.
- Set LA after mixture settings. Adjust the idle stop only as directed. Confirm that the engine idles reliably and the cutting attachment does not rotate.
- Recheck operation and safety. Stop the engine if it surges, stalls repeatedly, overheats, or accelerates poorly. Recheck for leaks and refit covers before use.
Do not hold the engine at full throttle for a prolonged period while trying to tune H. If the manual requires a high-speed adjustment and you do not have its procedure or the specified measuring equipment, use a qualified service shop.
Match symptoms to likely checks
A symptom is a clue, not proof that a screw needs turning. Check the simple causes first, then use the service instructions to decide whether adjustment is allowed.
| Symptom | First checks | Safe next step |
|---|---|---|
| Engine stalls at idle | Fresh fuel, air filter, fuel lines, and visible leaks | Follow the model-specific L and LA procedure only if permitted |
| Hesitates as throttle opens | Fuel condition, filter, and fuel-system condition | Do not turn L repeatedly; investigate restrictions or leaks |
| Runs poorly at full throttle | Air filter, fuel supply, and carburetor condition | Do not guess at H; use specified service data or a dealer |
| Cutting attachment turns at idle | Idle-stop setting and signs of a throttle or linkage fault | Stop using it until the attachment remains stationary at idle |
| Surging, abnormal heat, or loss of power | Possible air leak, fuel restriction, or incorrect mixture | Stop the engine; do not continue cutting or lean H further |
A carburetor setting cannot repair a deteriorated fuel hose or an air leak. In particular, an air leak can make the engine behave as if its mixture setting is wrong. Repeatedly turning screws to compensate may increase the risk of engine damage.
Field examples: when adjustment is and is not the answer
These examples show how I sort common FS 350 complaints. They are diagnostic scenarios, not factory settings: the correct screw limits and engine speeds still come from the instructions for the installed carburetor.
Scenario: it idles, then dies when the throttle opens. I first check that the fuel is fresh and correctly mixed, then inspect the air filter and fuel lines. If those checks pass, I follow the carburetor-specific procedure for the low-speed circuit. I do not assume that opening L by a guessed amount will solve the problem.
Scenario: the attachment spins while the engine idles. I stop using the machine and check the idle setting and throttle linkage. LA may need adjustment under the specified procedure, but a fault may also be responsible. The attachment must remain stationary at idle before the machine returns to work.
Scenario: it loses power and runs hot at full throttle. I stop the engine rather than turning H toward a leaner setting. I check fuel supply, filter condition, and possible air leaks. If the cause is unclear, a dealer can test and set the carburetor using the required service data.
Maintain the carburetor between seasons
Routine care reduces the chance that a real fuel or air-system fault will be mistaken for a screw-setting problem. Let the engine cool before cleaning or servicing, and follow the FS 350 manual for filter care and storage.
Clean or replace the air filter as specified. Inspect the spark plug, fuel filter, fuel lines, and carburetor for contamination, wear, or leaks. After storage, check the fuel system and machine condition before restarting, and use fresh, correctly mixed fuel.
For longer storage, follow STIHL’s instructions for the expected storage period rather than relying on a general rule. Keep the machine secure, with the cutting attachment protected. After any permitted adjustment, confirm dependable idle, clean acceleration, and a stationary attachment at idle; check for leaks and secure the covers.
Frequently asked questions
Answers below cover common adjustment questions. The installed carburetor and the matching STIHL instructions decide which adjustments are allowed and what limits apply.
What do L, H, and LA mean on an FS 350 carburetor?
L adjusts the low-speed mixture, H adjusts the high-speed mixture, and LA sets the idle-speed stop. Their functions differ, so do not use one screw to compensate for a fault associated with another.
Which direction should I turn the L or H screw?
There is no safe universal direction or turn count for every FS 350 carburetor. Follow the procedure and limits for the carburetor fitted to your machine, and do not force restricted screws.
How many turns out should the FS 350 carb screws be?
Do not assume a generic number of turns. Carburetor versions and limiter arrangements can differ. Use matching STIHL service information or ask a dealer to identify the correct setting.
What RPM should the FS 350 idle at?
Use the idle-speed specification in the matching instructions. Do not substitute a generic RPM value. At idle, the engine should run reliably and the cutting attachment must remain stationary.
Can I adjust the H screw by listening to the engine?
No. Sound alone does not confirm a safe high-speed mixture. An overly lean H setting can overheat and damage the engine. Use the specified service procedure and measuring equipment, or have a dealer do the adjustment.
Why does the cutting attachment spin at idle?
The idle stop may need attention, or a throttle or linkage fault may be present. Stop operating the machine until the attachment stays still at idle, and follow the model-specific procedure.
Should I adjust the carburetor if the engine surges?
Not as a first step. Stop the engine and check fuel condition, filter condition, fuel lines, and possible air leaks. Surging can signal a fault that screw adjustment will not fix.
What fuel mixture does the FS 350 use?
The engine is a two-stroke. With suitable STIHL two-stroke oil, the specified ratio is 50:1 gasoline to oil. Follow the applicable manual and oil label, and do not use straight gasoline.
When should a STIHL dealer adjust the carburetor?
Use a dealer if the carburetor is restricted, its type is unclear, the service limits are unavailable, or high-speed adjustment requires equipment you do not have. Stop immediately if the engine overheats, surges, or loses power.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
