Stihl MS 660 Idle Speed Adjustment (RPM Specs)
For an STIHL MS 660, target an idle speed of 2,500 rpm, measured with a tachometer, and confirm the chain stays still. Adjust only the LA idle-stop screw: clockwise raises speed; counterclockwise lowers it. If the saw stalls or the chain creeps at specification, stop and diagnose the fault instead of masking it with a higher idle.
A saw that will not idle can turn a short job into a lost weekend. Before buying parts or changing carburetor settings, check the idle with a tachometer and follow a simple sequence. Correct adjustment can reduce wasted fuel and avoid needless repairs, but it cannot fix a fuel, air, ignition, or clutch problem.
MS 660 idle-speed specification
The normal idle target listed for the MS 660 is 2,500 rpm. Treat that as the setting to check, not a speed to guess by sound. Verify the manual for your saw’s exact model and market, since carburetor versions and service conditions can differ.
At the correct setting, the engine should idle without stalling and the chain should remain stationary. The clutch should not engage at idle. A chain that moves is a safety fault, even if the saw seems to cut well.
Use a tachometer that can read a small two-stroke engine. Follow its instructions for placing the sensor or pickup. Take the reading with the saw warmed as directed in the owner’s manual; a cold engine may not idle like a warm one.
| Check | Target or condition | What it means |
|---|---|---|
| Idle speed | 2,500 rpm | Use the manual for your saw’s specification |
| Chain at idle | Stationary | If it creeps, stop and diagnose |
| Adjustment screw | LA idle-stop | Clockwise raises speed; counterclockwise lowers it |
| Fuel mix | 50:1, fresh fuel | Use unleaded gasoline and quality air-cooled two-cycle oil |
| Chain fit | Exact bar and chain match | Confirm pitch, gauge, drive-link count, and sprocket |
The common MS 660 chain pitch is 3/8 inch, but gauge and bar options vary. Match the chain to the bar stamp and the saw’s approved setup. A specification from another MS 660 configuration may not fit your saw.
Prepare the saw and work area
Safe adjustment means the saw is stable, the cutting attachment cannot contact anyone or anything, and you can stop the engine at once. Check the controls and chain before starting. Do not try to adjust a saw with a damaged throttle, faulty chain brake, or moving chain.
Work outdoors on level ground. Wear eye and face protection, hearing protection, gloves, suitable leg protection, and boots. Keep other people away, clear loose items from the work area, and point the bar away from yourself and others.
Before starting, inspect the stop switch, throttle trigger and lockout, chain catcher, handles, chain brake, bar, and chain tension. Confirm the chain has suitable lubrication and that the bar-and-chain combination matches the saw. The brake is for starting and handling as directed by the manual, not a way to hold the chain while testing idle.
Use fresh fuel mixed at 50:1 with quality air-cooled two-cycle oil. Do not use straight gasoline or stale, incorrectly mixed fuel. If the saw has been sitting with old fuel, address that before trying to tune around a poor idle.
Adjust idle with the LA screw
The LA screw is the idle-stop screw. It sets how far the throttle can close at idle. It does not repair a clogged carburetor, air leak, or worn clutch, and it is not the high-speed mixture control.
- Start the saw according to its manual, with the chain brake engaged for starting. Move to a clear, level spot.
- Warm the engine as the manual directs. Release the brake only when you are ready to check idle, with the bar and chain clear of people and objects.
- Measure engine speed with a tachometer. Do not estimate rpm by sound.
- If adjustment is needed, turn the LA screw a small amount at a time. Clockwise raises idle speed; counterclockwise lowers it.
- Recheck rpm after each small change. Aim for 2,500 rpm, and confirm the chain remains still.
- Stop the engine if the chain moves, the throttle sticks, or the speed will not settle. Do not keep raising idle to prevent stalling.
Use the carburetor’s marked screw locations and the owner’s manual to identify LA. Do not confuse it with the L or H mixture screws. Do not adjust the H screw by ear; incorrect high-speed tuning can cause excessive engine speed or a lean mixture that damages the engine.
Diagnose stalling or chain creep
A correct idle setting depends on more than the screw position. If the saw stalls at 2,500 rpm or the chain turns there, stop using it until the cause is found. Raising idle to hide a fault can make the saw unsafe or cause engine damage.
| What you observe | First response | Likely areas to inspect |
|---|---|---|
| Chain turns at idle | Stop engine and do not cut | Idle too high, sticking throttle, carburetor issue, or clutch fault |
| Saw stalls at 2,500 rpm | Do not keep increasing LA | Low-speed circuit, fuel supply, intake leak, or filter condition |
| Idle surges or will not settle | Stop and inspect before use | Fuel quality, air leak, fuel line, or carburetor condition |
| RPM seems too high at full throttle | Do not tune by ear | Have high-speed carburetor setting checked to manual specification |
A practical diagnostic example: if the chain creeps after an LA adjustment, lower the idle slightly and measure again. If the tachometer reads 2,500 rpm but the chain still moves, the problem is not solved by changing the target. Stop the saw and inspect the throttle return and clutch system, or have a qualified technician do so.
If the engine dies at the specified idle, check the basics before buying a carburetor. Confirm the fuel is fresh and correctly mixed. Inspect the air filter, fuel filter, fuel lines, and spark arrestor for blockage or damage. A dirty carburetor or air leak may need proper repair; repeated screw changes can make diagnosis harder.
Use the manual for any carburetor limits or service steps. If the idle cannot be stabilized at specification, or you suspect an intake leak, clutch defect, or internal engine problem, stop and have the saw tested. Do not run it with a chain that moves at idle.
Confirm the repair and maintain the setting
A successful adjustment holds close to the specified idle and leaves the chain stationary. Once the setting is stable, check normal chain lubrication and confirm the chain brake works as intended before cutting. If the setting changes again, look for a fault rather than repeatedly turning LA.
Keep the chain sharp and correctly tensioned. Inspect the drive links, bar rails, sprocket, and oil delivery. Use the chain maker’s file size and depth-gauge setting; those details vary by chain model. There is no single depth-gauge value that applies to every MS 660 chain.
Bar length depends on the cutting task, approved setup, and operator skill. Longer bars can increase cutting load and kickback exposure. Match pitch, gauge, drive-link count, and sprocket, and do not assume one bar length or chain gauge fits every saw.
After use, let the saw cool before cleaning. Clear debris from the air-filter area, cooling fins, sprocket cover, and bar groove. For storage, follow STIHL’s fuel-system instructions, and use fresh, correctly mixed fuel when returning the saw to service.
Frequently asked questions
These answers cover the common idle-speed checks for the MS 660. Use the saw’s own manual when its carburetor or market specification differs. The key rule remains simple: measure idle with a tachometer, keep the chain stationary, and stop if adjustment cannot produce both conditions.
What is the idle speed for an STIHL MS 660?
The listed target is 2,500 rpm. Check your saw’s manual for its exact specification.
Which screw adjusts idle speed?
Use the LA idle-stop screw. Do not confuse it with the L or H mixture screws.
Which way do I turn the LA screw to raise idle?
Turn it clockwise to raise idle speed. Turn it counterclockwise to lower speed.
Can I set idle by ear?
No. Use a tachometer to check rpm. Sound alone cannot confirm the specified speed.
Should the chain move at idle?
No. The chain must remain stationary. Stop the saw if it creeps.
Can I raise idle if the saw stalls?
Only adjust LA to the specified idle. If it still stalls at 2,500 rpm, diagnose the fuel, low-speed circuit, intake, or other fault.
Why does the chain turn when the tachometer reads 2,500 rpm?
A throttle or clutch fault may be present. Stop the saw and inspect it before use; do not treat a moving chain as normal.
Should I adjust the H screw to fix idle?
No. The H screw controls the high-speed mixture. Incorrect adjustment can damage the engine; have it checked against the manual.
What fuel mix should I use?
Use fresh unleaded gasoline mixed 50:1 with quality air-cooled two-cycle oil, unless the manual for your saw specifies otherwise.
When should I stop troubleshooting and get service?
Stop if the chain moves at idle, the throttle sticks, the engine will not hold 2,500 rpm, or you suspect an air leak, clutch fault, or internal damage.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
