What Is Stihl M-Tronic on the MS 391 (Auto-Tune Fix)
The STIHL MS 391 is a 64.1 cm³ chainsaw with a conventional carburetor, not STIHL M-Tronic. It has no M-Tronic auto-tune reset or calibration procedure. If your saw runs poorly, check fuel, air, spark, exhaust, chain brake, and cutting gear first. If the problem remains, ask a STIHL dealer to test the carburetor and engine.
If a manual, video, or forum mentions an “M-Tronic reset,” it is easy to think you have found the fix for a rough-running saw. But applying a reset that does not match your model can waste time or lead you toward unsafe adjustments. I start with one question: does this saw actually have M-Tronic?
What M-Tronic means on a STIHL chainsaw
M-Tronic is STIHL’s electronic engine-management system on models designed to use it. It adjusts fuel metering as operating conditions change, so the operator does not make the same manual carburetor adjustments used on a conventional system. The name does not describe a universal STIHL feature or a reset that works on every saw.
The MS 391 does not use M-Tronic. Its engine uses a conventional carburetor, so an M-Tronic reset or calibration cannot correct its running problem. “Auto-Tune” is often used informally for electronic engine adjustment, but that phrase does not make an MS 391 an electronically managed saw.
This distinction matters because a carburetor fault and an electronic-control fault call for different checks. Do not search for or turn adjustment screws based only on a video about a different STIHL model. Confirm your model and follow its manual.
Is the STIHL MS 391 compatible with M-Tronic?
The MS 391 is a 64.1 cm³ conventional-carburetor saw, according to the model information in the supplied technical guidance. Exact parts and instructions can vary by production version and market, so check the serial-number-specific STIHL manual and the parts fitted to your saw before ordering or servicing anything.
There is no M-Tronic reset procedure to perform on this model. Nor should you assume that adding an electronic module is a simple upgrade. The MS 391’s ignition, carburetor, and related parts must match its approved design; ask a STIHL dealer about compatibility rather than attempting a conversion.
| Question | MS 391 answer | What to do |
|---|---|---|
| Does it have M-Tronic? | No; it uses a conventional carburetor. | Skip M-Tronic reset instructions. |
| Is there an auto-tune calibration? | No M-Tronic calibration applies. | Diagnose the carburetor system and basic causes. |
| Can I use a 20-inch bar? | It is a common application, not a universal requirement. | Check approved combinations for your saw. |
| Is every 3/8-inch chain suitable? | No. Pitch alone does not confirm fit. | Match pitch, gauge, and drive-link count to the bar. |
What to check before calling it an “auto-tune” problem
A saw that bogs, idles poorly, or loses power may have a basic fuel, air, exhaust, or cutting problem. I recommend checking those items first, because they do not require guessing at carburetor settings. Stop using the saw if it races at idle, surges, or loses power; a lean-running engine can overheat and seize.
Confirm the details for your serial number in the STIHL manual. The MS 391’s listed displacement is 64.1 cm³, and the specified fuel mix is 50:1 gasoline to STIHL two-stroke oil. Use fresh, correctly mixed fuel. Do not use straight gasoline.
| Check | What to look for | Safe next step |
|---|---|---|
| Fuel | Stale fuel, wrong mix, or an uncertain mix | Drain and replace it as the manual directs. |
| Air filter | Dirt or blockage | Clean or replace it using the manual. |
| Spark plug | Fouling, damage, or an incorrect part | Inspect and service to the model’s instructions. |
| Fuel filter and lines | Restriction, damage, or signs of a leak | Have a dealer inspect if the fault is unclear. |
| Muffler and spark-arresting screen | Debris or blockage | Clean only as the manual permits, with the saw cool. |
| Chain brake and cutting gear | Brake drag, a damaged chain, or a binding bar | Stop and correct the issue before cutting. |
Do not use carburetor screw turns, torque values, raker clearances, or oil-flow figures copied from another model. Parts and specifications can differ. If basic checks do not solve the problem, a dealer can test the carburetor, fuel delivery, intake sealing, and ignition with the correct model information and tools.
What to do when the saw runs poorly
A methodical check helps separate a simple maintenance issue from a fault that needs service. Work with the engine off and cool unless the manual specifically directs otherwise. Keep the chain brake engaged when handling the saw, and do not run the chain while inspecting the bar or chain.
- Stop and identify the symptom. Note whether the saw will not start, stalls at idle, bogs under load, or races when you release the throttle. Do not keep cutting to “see if it clears.”
- Check the fuel. Confirm it is fresh and mixed at the specified 50:1 ratio. If you are unsure what is in the tank, follow the manual’s fuel-draining instructions and refill correctly.
- Inspect service items. Check the air filter, spark plug, fuel filter and lines, and muffler or spark-arresting screen as directed by the manual.
- Check for drag or cutting problems. With the engine off, inspect chain tension and the bar and chain. Confirm the brake is not dragging. A blunt or damaged chain can make the saw work harder, but it does not call for an M-Tronic reset.
- Stop if the fault continues. A dealer should test the conventional carburetor, intake sealing, fuel supply, and ignition if basic checks do not resolve the symptom.
Never turn H or L carburetor screws by guesswork. An incorrect setting can make the engine run too lean and cause serious damage. The chain speed at maximum engine power is approximately 25.6 m/s (84 ft/s), but this is not a target for adjusting no-load engine speed.
Bar, chain, oiling, and operating checks
The chain and bar must be a matched set. MS 391 setups commonly use 3/8-inch pitch, but the bar stamp determines the correct chain pitch, gauge, and drive-link count. Gauge is commonly .050 or .063 inch, but it depends on the bar. Do not order a chain by pitch alone.
A 20-inch bar is a common application, not a requirement for every job or operator. Choose only a bar and chain combination approved for your saw, and do not exceed STIHL’s listed combinations. If a marking is hard to read, take the bar or its details to a dealer rather than guessing.
Check chain oil delivery using the procedure in the manual and use suitable chain-and-bar lubricant. Do not substitute a guessed oil viscosity or adjust oil flow based on a figure for another model. Before cutting, inspect the chain brake, throttle trigger lockout, chain catcher, bar, chain tension, and fasteners. With the engine off and the brake released, confirm the chain moves freely by hand.
| Item | Confirm before use |
|---|---|
| Bar and chain | Approved combination; pitch, gauge, and drive-link count match the bar. |
| Chain brake | Works as described in the manual; no signs of drag. |
| Chain movement | Moves freely by hand with engine off and brake released. |
| Oiling | Chain receives bar oil using the manual’s check procedure. |
| Protective equipment | Wear suitable chainsaw PPE and keep bystanders out of the work zone. |
For cutting, assess footing, wind, tree lean, overhead hazards, escape routes, and limbs under tension. Start the saw as the manual directs, with the brake engaged. Release the brake only when ready to cut. Use a firm two-handed grip, keep the saw below shoulder height, avoid the guide-bar nose, and let a sharp chain cut without forcing it. Tree felling, hung-up trees, or work with significant targets or lean calls for a qualified professional.
Common “auto-tune fix” scenarios
These examples are diagnostic situations, not reports of specific customer repairs. They show why the symptom alone cannot confirm a carburetor fault. A saw that bogs may need service, but it may also have stale fuel, restricted airflow, exhaust blockage, brake drag, or a problem with its cutting attachment.
It starts, then stalls or bogs
Check the fuel mix and the service items listed above. If fresh fuel and clean, correctly fitted parts do not help, stop troubleshooting by trial and error. A dealer can test fuel delivery and the conventional carburetor.
It races at idle or surges
Stop using the saw. A lean-running conventional carburetor, restricted fuel supply, stale fuel, or an intake leak can overheat the engine. An M-Tronic reset is not a remedy, and adjusting screws without the model-specific procedure can make the problem worse.
The chain seems slow or the saw struggles in wood
Inspect the chain, bar, tension, and brake. A blunt or binding cutting attachment can reduce cutting performance even when the engine starts. Do not raise engine speed or alter carburetor settings to compensate.
Storage and seasonal care
For storage, follow the fuel-system instructions in your STIHL manual. Use fresh fuel for operation and avoid leaving unsuitable fuel in the saw. Clean the air filter and cooling fins as directed, inspect the cutting attachment, and store the saw secured with its chain covered.
Before returning it to service, check the fuel, chain condition and tension, brake, and oil delivery. If it surges, races at idle, or loses power, leave it shut down until the cause is found. These symptoms can point to a lean-running condition, which risks overheating and engine seizure.
FAQ: STIHL MS 391 M-Tronic and troubleshooting
The direct answer is that the MS 391 is not an M-Tronic saw, so M-Tronic reset steps do not apply. These answers clarify common searches about auto-tune claims, fuel, chains, and when to seek service.
Does the STIHL MS 391 have M-Tronic?
No. The MS 391 uses a conventional carburetor, not M-Tronic.
How do I reset M-Tronic on an MS 391?
You do not. There is no M-Tronic reset procedure for this model.
Is “Auto-Tune” another name for M-Tronic?
It is often used as a general term for electronic engine adjustment, but it does not confirm that a particular saw has M-Tronic. Check the model and manual.
Why does my MS 391 bog down?
Possible causes include stale or incorrectly mixed fuel, a dirty air filter, restricted fuel delivery, exhaust blockage, brake drag, or a cutting-attachment problem. If basic checks do not help, have a dealer diagnose it.
Can I adjust the carburetor myself?
Do not turn the H or L screws by guesswork. Use only the exact model procedure and proper measurements, or have a STIHL dealer test and adjust the carburetor.
What fuel mix does the MS 391 use?
The supplied model guidance specifies a 50:1 gasoline-to-STIHL two-stroke oil mix. Confirm the instruction in your serial-number-specific manual and use fresh, correctly mixed fuel.
Does every MS 391 use a 3/8-inch chain?
No chain should be selected by pitch alone. Check the bar stamp and match pitch, gauge, and drive-link count.
Is a 20-inch bar required?
No. It is a common application, but the right bar depends on the job and must be an approved combination for the saw.
Can a dealer convert an MS 391 to M-Tronic?
Do not assume an M-Tronic conversion is available or appropriate. Ask a STIHL dealer about parts and approved configurations for your serial number.
When should I stop using the saw?
Stop if it races at idle, surges, loses power, or continues to run poorly after basic checks. Have a dealer inspect it before further use.
(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)
