Stihl 020 AV Carb Tuning (Carb Adjustments)
Reliable tuning starts with identifying the carburetor fitted to your 020 AV and finding the matching STIHL service data. There is no safe, universal screw setting or maximum-RPM figure for every version. Prepare the saw, correct faults first, then use a tachometer to set engine speed and verify idle, acceleration, and safe operation.
Carburetor adjustment can seem like a small repair, but a wrong setting can turn into an expensive engine failure. A saw that idles poorly may need cleaning or repair, not a different screw position. I approach the job as a sequence: identify the saw and carburetor, check the engine and fuel system, then tune only to the correct specification.
The 020 AV is a legacy model. Parts may have been replaced over time, so the saw’s name alone does not identify the carburetor or its adjustment limits. If you cannot match the installed parts to reliable service data, stop before adjusting the high-speed circuit.
Equipment setup, carburetor identification, and safety
The goal here is to confirm which saw and carburetor you have, prepare it for a fair test, and prevent avoidable injury. A model label or old manual may not describe replacement parts now fitted. Identify the actual hardware before using any adjustment values.
Start by recording the saw’s model and serial information. Use those details to ask a STIHL dealer or qualified service shop for the matching service information. Do not assume that specifications for an 020T, another 020 version, or a different STIHL saw apply.
Next, look for the carburetor’s maker and identification marks. These may be stamped or printed on the body, which can require removing a cover. Do not force parts or remove the carburetor just to read a mark if you are not equipped to do so. Some versions or replacement carburetors may not have the same adjustable screws.
If your carburetor has adjustment labels, they commonly mean:
- L: low-speed mixture, which affects idle and the transition to acceleration.
- H: high-speed mixture, which affects operation at high throttle.
- LA, T, or an equivalent mark: idle-speed adjustment, which sets how fast the engine idles.
Labels and layouts vary. These terms explain common functions; they do not prove that a given screw is adjustable or provide a setting.
Before starting, work outdoors, away from flames and sparks. Wear eye and hearing protection, gloves, chainsaw-rated leg protection, and suitable boots. Check the chain brake, throttle lockout, stop control, chain catcher, handles, bar, chain, sprocket, fuel cap, and oiling system. Do not run the saw if a safety part is damaged, fuel is leaking, or the bar or chain is unsafe.
Use fresh two-stroke fuel at the ratio specified for your exact saw and oil. STIHL two-stroke oil is commonly used at 50:1, but follow the manual for your version and oil. Never use straight gasoline or fuel with an unknown mix. Clean the air filter and inspect the spark plug and fuel filter before tuning; repair known faults first.
How to tune the 020 AV carburetor
A safe adjustment depends on the matching service specification, not a guessed baseline. Warm the engine as directed, change only the adjustments the service data allows, and check engine speed with an inductive tachometer. Do not set maximum speed by ear.
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Confirm the service data. Match it to the saw version and the carburetor actually installed. Find the specified idle and maximum engine speeds, the permitted adjustment method, and any limits. If the correct data is unavailable, do not guess at H or L settings.
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Record the starting point. Photograph the carburetor and write down any screw positions or measured settings before adjustment. Do not force a screw against its seat. If a screw is capped, missing, damaged, or appears fixed, use the correct service procedure rather than modifying it.
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Start and warm the saw safely. Follow the saw’s starting instructions. Keep the bar clear of people and objects, and use a stable test area. Let the engine reach the temperature required by the service procedure. A cold engine can give misleading idle and response.
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Set the low-speed circuit only as specified. The L circuit affects low-speed running and acceleration. Make small changes using the matching procedure, then check whether the saw idles smoothly and responds cleanly to throttle. Do not use a mixture screw to hide a blocked passage, air leak, or other fault.
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Set idle speed. Adjust the idle-speed screw to the specified speed. The chain must remain stationary at idle. If the chain keeps moving after a correct adjustment, stop and diagnose the clutch, idle setting, or another fault; do not operate the saw in that condition.
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Set high speed with a tachometer. Use the exact maximum-RPM limit for the identified version and carburetor. An inductive tachometer reads engine speed without contacting moving parts. Follow the service procedure for where and how to take the reading. Do not hold the throttle open longer than the procedure requires.
If the engine will not meet specification, stop instead of leaning the mixture or raising the speed. Check for a clogged fuel passage, restricted exhaust, damaged fuel line, intake or crankcase air leak, or low compression. A suspected leak needs proper pressure and vacuum testing.
Common symptoms and what to check
A symptom points to a test, not always to a carburetor screw. Fuel, air, ignition, exhaust, and engine condition all affect how a two-stroke runs. Use the table to decide what to inspect before changing a setting.
| Symptom | First checks | Tuning response |
|---|---|---|
| Poor idle or stalling | Air filter, fuel freshness, plug, fuel filter, carburetor passages | Adjust L or idle speed only if service data supports it |
| Slow or uneven acceleration | Fuel supply, filter, intake condition, exhaust restriction | Verify L setting and repair faults before further tuning |
| Chain moves at idle | Idle speed, clutch, clutch spring, throttle return | Stop using the saw until the chain stays still at idle |
| Hanging or unusually high idle | Throttle linkage, intake and crankcase leaks | Shut down; do not try to correct a suspected leak with screw changes |
| Surging, power loss, or abnormal heat | Fuel delivery, air leaks, exhaust, engine condition | Stop; avoid further high-speed testing until diagnosed |
| Maximum speed is unknown | Carburetor identity and applicable service data | Do not tune H by ear or use another model’s RPM limit |
A lean seizure is a key risk. An air leak or an over-lean high-speed setting can raise engine speed while reducing the fuel mixture that helps cool and lubricate the engine. The result can be piston scoring or seizure. If the saw’s idle hangs, speed surges, or the engine heats abnormally, shut it down and have it checked.
Verification, cutting setup, and illustrative cases
Verification means checking that the repair works under safe conditions, not just that the engine runs in the shop. Recheck hot idle, throttle return, acceleration, chain-brake function, and oil delivery. Use only a bar and chain approved for the specific saw, and match pitch and gauge across the chain, bar, and sprocket.
The following are diagnostic examples, not reports of named customer repairs. They show why an adjustment should follow identification and testing rather than guesswork.
| Situation | What the checks show | Appropriate next step |
|---|---|---|
| An 020 AV stalls after starting | The filter is dirty and the fuel is old | Replace stale fuel, service the filter, then retest before adjusting |
| The engine races after warming | The idle hangs and the intake may be leaking | Stop running; arrange leak testing and repair |
| The saw runs but its high-speed limit is unknown | Carburetor identity does not match the available manual | Ask a STIHL dealer or qualified shop to identify the carburetor and source its data |
| The chain creeps at idle | The chain remains in motion after idle adjustment | Stop; inspect the clutch and confirm the idle setting |
During a cutting test, use a safe, open area and keep bystanders clear. Felling and professional tree work require trained operators and compliance with applicable ANSI Z133 and OSHA requirements. A carburetor adjustment does not make an unsafe saw or worksite safe.
Maintenance, shutdown, and carburetor verification
Good maintenance makes tuning more stable and helps prevent repeat adjustments. Use the manual for cleaning and service intervals, and check the saw after work. A tune cannot compensate for a worn part, poor fuel, a damaged chain, or a leak.
Use the correct bar-and-chain oil and confirm oil delivery by the method in the saw manual. Check chain tension when the saw is cool, and keep the chain sharp. Set depth gauges to the chain maker’s specification; one clearance value does not suit every chain type. Confirm pitch, gauge, and drive-link count from the bar or chain markings, and make sure the sprocket pitch matches.
After a test, shut the saw down and let it cool. Clean debris from the saw, check for leaks or damage, and recheck chain tension. For seasonal storage, follow the manual’s fuel and storage instructions. When the saw returns to service, use fresh correctly mixed fuel and verify its safety parts before starting.
If a carburetor adjustment does not hold, document the symptom, fuel mix, carburetor identification, and any service data used. That record helps a repair shop distinguish a tuning issue from a fuel, ignition, air-leak, or compression problem.
Frequently asked questions
What are the factory H and L settings for an 020 AV?
There is no single safe H and L baseline for every 020 AV. Carburetor versions and replacement parts differ. Identify the installed carburetor, then use the matching STIHL service information. Do not copy a setting from an 020T or another model.
What RPM should an 020 AV idle at?
Use the idle-speed limit in the service data for your saw and carburetor. The chain must remain stationary at idle. Do not substitute a generic STIHL figure; incorrect idle speed can cause chain movement or poor running.
Can I tune the high-speed screw by ear?
No. Engine sound cannot reliably confirm the safe maximum speed. Use the matching service limit and an inductive tachometer. If you do not have the correct data or test equipment, have a qualified shop set it.
What does the L screw adjust?
The L circuit generally affects low-speed mixture, idle behavior, and the transition to acceleration. Its exact adjustment method depends on the carburetor. A dirty passage, weak fuel supply, or air leak can cause similar symptoms.
What does the H screw adjust?
The H circuit generally affects mixture at high throttle. An incorrect setting can cause poor power or dangerous overspeed and overheating. Adjust it only by the procedure and RPM limit for the identified carburetor.
Why does my chain move when the saw idles?
The idle may be too fast, or the clutch may have a fault. Set idle only to the correct service specification. If the chain still moves, stop using the saw and have the clutch and controls checked.
Why does the engine race or hang at idle?
A hanging idle can point to an air leak, throttle problem, or incorrect setting. Shut the saw down. Do not keep adjusting mixture screws to mask a possible leak; request proper pressure and vacuum testing.
Can I use an 020T carburetor setting on an 020 AV?
Do not assume the settings match. The saw versions and carburetors may differ, and replacement parts can complicate identification. Confirm the carburetor and obtain its corresponding service specification before tuning.
What fuel mix should I use?
Use fresh two-stroke fuel mixed to the ratio specified for your exact saw and oil. STIHL oil is commonly used at 50:1, but verify the manual for your version. Never run straight gasoline or unknown-ratio fuel.
When should I stop trying to tune the saw?
Stop if the saw has a fuel leak, damaged safety parts, a moving chain at idle, a hanging idle, surging, abnormal heat, or suspected air leak. Do not continue high-speed testing when the correct service data is missing.
The practical rule is simple: identify first, repair faults, then adjust to the correct data and verify with a tachometer. If the carburetor or service specification cannot be confirmed, a qualified STIHL dealer or chainsaw repair shop is a safer investment than a guessed setting.
(This article was written by one of our staff writers, Elliott Fowler. Visit our Meet the Team page.)
