Stihl MS 661 Chainsaw (Troubleshooting Guide)
If your MS 661 will not start, stop pulling and check it in order: controls and starting procedure, fresh correctly mixed fuel, air and exhaust flow, spark, then compression. Work with the engine cool and chain clear. This avoids buying parts at random. If you find an air leak, damaged safety part, or low compression, stop and use a qualified service shop.
The MS 661 is a large professional saw, not a small homeowner model. Its 91.1 cm³ engine is rated at 5.4 kW (7.3 bhp), so a simple starting fault can have costly consequences if you guess at a repair. A clean diagnostic sequence is safer than repeated pulling or arbitrary carburetor adjustments.
Current versions may have STIHL M-Tronic electronic engine management; equipment and procedures can vary by version and market. Check the saw’s model label and its matching operator’s manual before testing or servicing. The aim here is to isolate the fault without bypassing emissions controls or replacing good parts.
Equipment Setup & Safety Preparation
This section establishes a safe starting point before diagnosis. A saw that is being tested must be stable, undamaged, and clear of people and material. Check the manual for the exact controls and starting steps; do not test a running saw if its chain, brake, throttle control, or other safety part is faulty.
- Wear chainsaw-rated leg protection, helmet, eye and face protection, hearing protection, gloves, and suitable boots. Keep bystanders out of the work area.
- Inspect the chain brake, throttle interlock, stop switch, handles, chain catcher, bar, chain, and fasteners. Do not use the saw with broken or modified safety parts.
- Check chain tension with the engine off and brake released, as directed in the manual. Confirm the chain is not contacting anything before starting.
- Use the correct bar and chain combination for the saw. Match bar mount, chain pitch, gauge, sprocket, and drive-link count. A common MS 661 setup uses 3/8-inch pitch and .063-inch (1.6 mm) gauge, but check the actual bar stamp and chain.
- Start on stable ground, following the manual’s cold- or warm-start procedure. Never drop-start the saw or start it with the chain touching wood.
If the saw starts, confirm the chain stops at idle. Do not rev the engine with the chain brake engaged. Stop if the chain moves at idle, the saw behaves abnormally, or you cannot control it safely.
Diagnose an MS 661 That Will Not Start
This workflow separates common causes instead of treating every no-start as a carburetor problem. “No-start” means the engine will not run after the correct starting procedure. Record what happens: whether it fires briefly, stays completely silent, or runs only with the choke on. Those clues guide the next test.
Start with the controls, fuel, and airflow
Check that the stop switch is set to run and follow the correct cold- or warm-start procedure. A flooded engine may smell strongly of fuel or have a wet spark plug. Follow the manual’s flooded-engine instructions rather than continuing to pull with the choke on.
Use fresh fuel mixed at 50:1 with STIHL two-stroke oil, as specified for the saw. Do not use straight gasoline or an incorrect mix. If fuel has been stored for a long time, replace it with fresh, correctly mixed fuel. Check that the fuel filter and lines are intact; a damaged line or leak needs repair, not carburetor adjustment.
Inspect the air filter and clean it by the method in the manual. Check the muffler outlet and spark-arresting screen for blockage, if fitted. Let the engine cool before working near the exhaust. A clogged filter or exhaust can restrict airflow and cause poor running, but cleaning it will not fix an ignition or compression fault.
Check spark safely
A spark plug that is wet, damaged, or heavily fouled can point to a problem, but appearance alone does not prove the cause. Remove and inspect it only with the engine off and cool. Use the plug type and gap listed for the exact saw version.
For a spark test, use a proper spark tester and follow its instructions. Keep the test away from spilled fuel, fuel vapor, and the open spark-plug hole. Do not hold the plug by hand against the cylinder while pulling the starter. If there is no spark, inspect the stop-switch wiring and plug connection as the service manual allows; ignition module testing may need a technician.
Assess compression and fuel delivery
Compression is the pressure created as the piston rises in the cylinder. If spark and fresh fuel are present but the engine will not run, compression or an air leak may be involved. Pulling feel is only a rough clue, not a measurement. Use the model-specific service procedure and a suitable gauge; do not rely on a universal pressure cutoff.
If the saw fires briefly and dies, check fuel flow, filter, fuel lines, and tank vent according to the manual. A restricted fuel supply can mimic a carburetor fault. If the cylinder is scored, compression is low, or an intake leak is suspected, stop. A leak can make a two-stroke engine run too lean and overheat; turning the carburetor screws to hide it can cause serious damage.
For M-Tronic versions, do not assume conventional carburetor adjustment applies. Follow the version-specific manual and service procedure. Maximum engine speed must be checked with a service tachometer using the specified method; the approximate maximum is 13,500 rpm. Idle is approximately 2,800 rpm. These are reference specifications, not permission to tune by ear.
Diagnostic clues at a glance
| What you observe | Check next | Avoid |
|---|---|---|
| No firing at all | Controls, fuel condition, spark test | Replacing the carburetor first |
| Fires, then stalls | Fuel filter and lines, air filter, exhaust screen | Leaning the carburetor blindly |
| Wet plug after repeated pulls | Flooding and correct starting procedure | More choke and repeated pulling |
| Runs poorly under load | Filter, exhaust, chain sharpness, fuel supply | Forcing a dull or binding chain |
| Chain moves at idle | Stop work; inspect idle control and clutch system | Using the saw until repaired |
In a typical bench diagnosis, a saw that fires once and dies deserves a fuel-supply check before a carburetor swap. A completely silent saw calls for a spark and control check. These are diagnostic leads, not proof; confirm each fault with the matching test before buying parts.
Field Cutting & Landscaping Operation
Safe operation matters during both diagnosis and normal work. The MS 661 is intended for demanding cutting tasks, but its power does not remove the need for a planned cut and stable footing. Use only an approved bar-and-chain setup and stop if the saw, wood, or work area becomes unsafe.
Keep a firm two-handed grip and stay aware of the bar nose, pinch points, and changing wood forces. Rotational kickback can happen if the upper quadrant of the bar nose hits wood or another object; the bar may rise sharply toward you. Keep the nose clear, use a working chain brake, and do not cut with the tip unless trained and the task requires it.
For felling, assess tree lean, defects, wind, overhead hazards, targets, and escape routes. The notch, back cut, and hinge depend on the tree and method; there is no safe universal hinge measurement. Keep people outside the applicable exclusion zone. Do not fell near utilities without required utility coordination and qualified personnel.
Stop the engine before clearing a jam or adjusting chain tension. Stop work if the chain binds, footing or visibility worsens, or anyone enters the work zone. Apply ANSI Z133 and relevant OSHA requirements, including 29 CFR 1910.266 where its logging provisions apply.
Maintenance, Sharpening & Tuning
Routine cleaning and inspection can prevent faults that look like engine trouble. Service the saw only with the engine off and cool, following the manual. Keep safety parts intact, and use chain, bar, and fuel specifications that match the exact MS 661 setup.
After use, clean the air filter, cooling fins, starter intake, and sprocket area. Check the guide-bar groove and oil holes, chain condition, sprocket, and anti-vibration mounts. Use purpose-made bar-and-chain oil. Oiler delivery is adjustable and varies with conditions, so check lubrication at the bar as described in the manual rather than assuming a fixed flow rate.
Sharpen the chain with the file size, angles, and depth-gauge clearance specified by its manufacturer. Depth-gauge settings are chain-specific; do not use one universal number. Keep cutter lengths consistent and replace a cracked or excessively worn chain, bar, or sprocket.
If performance drops, check fresh fuel, filter condition, spark plug, exhaust screen, and signs of air leaks before considering tuning. Do not compensate for an air leak or fuel-system fault by leaning the carburetor. If a repair requires carburetor adjustment, maximum-speed checks, or internal engine work, use the exact service procedure and proper test equipment.
Storage & Seasonal Care
Correct storage helps reduce stale-fuel and corrosion problems between jobs. Let the saw cool, clean it, inspect for damage, and store it securely with the chain covered. Follow STIHL’s instructions for fuel storage or draining and long-term preservation; use fresh, correctly mixed fuel when returning the saw to service.
Before the next start, check the chain, brake, bar, controls, fuel, and oil. Do not operate the saw if a safety part is damaged or the chain fails to stop at idle. A simple pre-start check can prevent a repairable fault from becoming an injury or engine failure.
Frequently Asked Questions
Why will my MS 661 not start after many pulls?
Check the starting procedure, stop switch, fresh 50:1 fuel mix, air filter, and spark. A wet plug may indicate flooding. Do not keep pulling with the choke on; follow the manual’s flooded-engine procedure.
What fuel mix does the MS 661 use?
Use gasoline mixed 50:1 with STIHL two-stroke engine oil, as specified for the saw. Do not run straight gasoline or guess at a different ratio.
What should I check if the saw starts and then dies?
Check fresh fuel, the fuel filter and lines, air filter, and exhaust screen. A restricted fuel supply can cause this symptom. Confirm the fault before replacing or adjusting the carburetor.
What are the MS 661’s idle and maximum engine speeds?
Approximate reference values are 2,800 rpm at idle and 13,500 rpm maximum. Verify and adjust only with the model-specific service procedure and a suitable tachometer.
Can I adjust the carburetor by ear?
No. Use the manual’s procedure and appropriate equipment. M-Tronic versions require their specific service process, and carburetor adjustment will not repair an air leak or blocked fuel supply.
Why does the chain move while the saw idles?
The saw may have an idle-control or clutch-related fault. Stop using it until repaired. The chain should stop at idle; do not try to compensate by holding the brake while revving.
What chain pitch and gauge does the MS 661 use?
A common setup is 3/8-inch pitch and .063-inch (1.6 mm) gauge, but setups vary. Match the chain to the bar stamp, sprocket, drive-link count, and approved saw configuration.
How do I know if the bar is receiving oil?
Follow the manual’s lubrication check and inspect the bar oil holes and groove. Use bar-and-chain oil and do not assume one fixed oiler flow applies to every condition.
When should I stop troubleshooting at home?
Stop if you find low compression, cylinder damage, an air leak, fuel leakage, a faulty safety part, or a need for internal engine or ignition testing. Those faults need the correct service procedure and equipment.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
