Stihl MS 390 Power Upgrades (Tuning Guide)
The safest way to improve an MS 390’s cutting performance is to restore its factory condition, fit a compatible bar and chain, sharpen the chain correctly, and verify carburetor speed with a tachometer. The 64.1 cm³ saw is rated at about 3.4 kW (4.6 bhp), depending on version. Avoid unverified engine modifications and never exceed the specified maximum speed.
Treat a power upgrade as an investment in reliable cutting, not a shortcut to more engine output. A dull chain, restricted air filter, poor lubrication, or mismatched bar can make a healthy saw feel weak. I start with those checks before considering any parts change; they often reveal a problem without risking engine damage.
The MS 390’s specifications can vary by market and fitted equipment. Use the model’s manual and the markings on the bar and chain to confirm the exact setup before buying parts or adjusting the carburetor.
Equipment Setup and Safety Preparation
A compatible setup means the bar and chain match the saw and each other. Check pitch, gauge, and drive-link count rather than relying on bar length alone. A 16–20 inch bar is a practical general-purpose range, but the manual and approved equipment list decide what fits your saw.
Before a tune-up or cutting session, inspect the chain brake, throttle interlock, chain catcher, handles, controls, fasteners, bar, and chain. Replace damaged parts before starting. Wear a helmet, eye and face protection, hearing protection, chainsaw chaps, gloves, and suitable boots.
Confirm bar and chain fit
The MS 390 commonly uses a 3/8-inch pitch chain, but the gauge may be .050 or .063 inch, depending on the bar. The pitch is the spacing of the chain’s drive links; gauge is the thickness of the drive links that sit in the bar groove. A mismatch can damage parts or cause poor chain movement.
Read the bar stamp and chain markings. Then confirm drive-link count, which depends on the specific bar. Do not order by model name or bar length alone.
| Check | What to confirm | Why it matters |
|---|---|---|
| Pitch | Commonly 3/8 inch; verify markings | Chain must match sprocket and bar |
| Gauge | .050 or .063 inch, depending on bar | Chain must fit the bar groove |
| Drive links | Count required for the fitted bar | Too many or too few links will not fit correctly |
| Bar length | Manual-approved fit; 16–20 inches is a practical range | Longer bars increase cutting load |
A longer bar is not automatically an upgrade. It adds load, especially in sustained hardwood cutting. Use an approved combination and choose the shortest bar that suits the work.
Field Cutting and Landscaping Operation
A saw that is ready to cut should start, idle, lubricate, and accelerate correctly before it enters the wood. A sharp chain and suitable bar reduce needless strain, but neither makes an unsafe cutting position safe. Check the site and the saw before each job.
Clear your footing, escape route, bystanders, overhead hazards, and intended drop zone. For felling, assess tree lean, defects, and surroundings, then plan the notch and hinge for that tree. No generic hinge thickness guarantees a safe fall; follow suitable training and applicable safety rules.
Set tension and verify lubrication
With the engine off and cool, loosen the bar nuts as directed by the manual. Adjust chain tension so the drive links remain seated in the bar groove while the chain can still move by hand with a gloved hand. Check the manual’s procedure, then retighten the nuts and recheck tension.
Fill the oil tank with dedicated bar-and-chain oil. There is no reliable universal oil-flow rate or viscosity figure for every setup. Confirm the oil passage and pump work, and use the manual’s lubrication test. Do not run the chain dry.
Start the saw on stable ground using the manual’s cold- or warm-start procedure. Confirm it idles without chain movement and that lubrication is present before cutting. Use a firm two-hand grip, secure footing, and avoid the bar tip. Contact between wood and the upper quadrant near the tip can cause rapid rotational kickback. Keep the tip clear, do not cut above shoulder height, and maintain a working chain brake; the brake does not replace control.
Maintenance, Sharpening, and Factory-Spec Tuning
Factory-spec tuning means restoring the saw to the settings listed for its exact version, not adjusting it by ear for extra speed. The MS 390 reference figures are about 2,800 rpm at idle and 12,500 rpm maximum no-load speed. Verify them in the correct manual and use a tachometer.
Start with basic service: clean the air filter as the manual allows, inspect the spark plug and fuel system, and clear debris from the bar groove, oil port, sprocket area, and cooling fins. A clogged filter or blocked oil passage can undermine performance even when the engine starts.
Sharpen the chain to its own specification
A file size and filing angle depend on the chain, not just the saw. A 7/32-inch (5.5 mm) round file is common for many standard 3/8-inch chains, but it is not universal. Read the chain maker’s markings or instructions before filing. The same rule applies to top-plate angle: use the specified angle, rather than assuming one angle fits every chain.
File each cutter evenly, keep the file level as directed, and use a guide if you are unsure of the angle. A chain that cuts crooked may have uneven cutter lengths or damage that needs more than routine filing. Set depth gauges (often called rakers) to the chain manufacturer’s specification. A 0.025-inch (0.65 mm) setting is a common reference, not a universal target.
Tune carburetor speed carefully
Carburetor adjustment affects idle and engine mixture. Too much speed or an overly lean mixture can damage the engine. Do not use generic screw-turn counts from another saw as a substitute for the model’s procedure.
Use a tachometer to check idle and maximum no-load speed against the exact manual. The reference values above are not permission to exceed the listed limit. If the saw cannot reach specification, surges, stalls, or runs poorly after basic service, stop and diagnose the fuel, air, exhaust, and carburetor system rather than turning screws at random.
| Symptom | First checks | Avoid |
|---|---|---|
| Chain slows or burns wood | Sharpness, chain tension, bar and chain match | Forcing a dull chain through the cut |
| Poor power under load | Air filter, fuel condition, exhaust and chain condition | Leaning the mixture to chase speed |
| Chain moves at idle | Idle setting and throttle return; manual procedure | Cutting before idle is safe |
| Bar and chain seem dry | Oil level, oil port and pump function | Continuing to cut without lubrication |
Practical shop scenarios
In one common diagnostic pattern, an owner reports that the saw feels weak in hardwood. Inspection finds a dull chain and a bar-and-chain combination that needs verification. The useful first steps are to confirm the fit, sharpen to the chain maker’s specifications, set tension, and test lubrication. This restores the known baseline before any carburetor adjustment.
Another common pattern is a saw that idles unevenly after storage. Old fuel or a restricted air filter may be part of the problem, but symptoms alone do not identify a cause. I check fuel freshness, filter condition, controls, and the manual’s tuning procedure, then use a tachometer. If it still falls outside specification, a qualified repair is safer than a speculative adjustment.
These are diagnostic examples, not proof that one fix suits every saw. The key is to change one known condition at a time and recheck the result.
Storage and Seasonal Care
Good storage protects the saw’s baseline performance between jobs. Let the engine cool, stop it, and clean away sawdust and oil residue. Inspect the chain and bar for damage, check tension, and follow the manual’s fuel-storage guidance for the fuel type you use.
Before the next use, inspect the chain brake and controls again, check the oil tank, and confirm the chain moves freely with the engine off. Clean the air filter and cooling areas as needed. A few minutes of care can prevent a problem from being mistaken for a need for more power.
Which Power Upgrade Makes Sense?
For most owners, the most useful “upgrade” is a correctly maintained saw with a sharp, matched chain and an approved bar. Parts that claim to increase power should not be fitted unless compatibility and safety are confirmed by reliable manufacturer information. Avoid changes that defeat safety features, exceed engine speed limits, or rely on unverified performance claims.
Follow the saw’s manual and applicable requirements, including ANSI Z133 and OSHA 29 CFR 1910.266 where they apply. Neither a displacement-to-bar ratio nor a generic felling measurement can replace a site-specific assessment and proper training.
Bottom line: Check fit, service, sharpening, and factory settings before looking for more output. That sequence protects the engine and gives you a clear basis for judging whether a repair is needed.
Frequently Asked Questions
What is the displacement and power of the MS 390?
It is a 64.1 cm³ saw rated at about 3.4 kW (4.6 bhp), subject to market and version.
What fuel mixture does an MS 390 use?
The reference mixture is 50:1 gasoline to quality air-cooled two-cycle oil. Use fresh fuel and follow the manual’s fuel recommendations.
What bar length is best for an MS 390?
A 16–20 inch bar is practical for general use, but use only a bar approved for the saw. Longer bars increase load.
What chain pitch does it commonly use?
Three-eighths-inch pitch is common. Check the bar and chain markings to confirm your actual setup.
Does the MS 390 use .050- or .063-inch chain gauge?
Either may apply, depending on the fitted bar. Match the chain gauge to the bar stamp.
What file size and angle should I use?
Follow the chain maker’s specification. A 7/32-inch (5.5 mm) file is common for many standard 3/8-inch chains, but file size and filing angle are not universal.
What are the reference idle and maximum speeds?
About 2,800 rpm at idle and 12,500 rpm maximum no-load speed are reference figures. Verify the exact manual and use a tachometer; do not exceed its limit.
Can I gain power by adjusting the carburetor?
Do not tune beyond factory specifications to seek power. Incorrect or lean adjustment can damage the engine; diagnose service issues and use the manual’s procedure.
What depth-gauge setting should I use?
Use the chain maker’s specification. 0.025 inch (0.65 mm) is a common reference, not a universal setting.
Why does the saw burn wood instead of cutting cleanly?
A dull chain is a common cause. Check sharpness, tension, bar-and-chain compatibility, and lubrication before cutting again.
(This article was written by one of our staff writers, David Hensley. Visit our Meet the Team page.)
