Stihl MS 660 No Spark (Diagnostic Guide)
A no-spark MS 660 often has a simple cause: a fouled plug, damaged wire, or stop-switch lead shorted to ground. Work outdoors, keep fuel vapors away, and test with an approved inline spark tester. If spark is absent, isolate the stop circuit and check wiring and module air gap before replacing parts.
Could you find the fault without buying parts at random or paying for a full dealer diagnosis? Work through the checks in order. The MS 660 is a large, 91.6 cm³, air-cooled two-stroke saw, and its ignition parts can vary by version. Check the saw’s markings and use the service manual that matches it before setting or replacing components.
Equipment setup and safety preparation
Before testing, make the saw safe and confirm the parts and procedures match your MS 660. A spark test creates an ignition source, so fuel fumes, spilled mix, and fuel-soaked parts must be kept well away. Do not test beside an open fuel tank, or anywhere you smell fuel.
- Wear eye protection, gloves, hearing protection, leg protection, and sturdy footwear.
- Move the saw outdoors, away from flames, sparks, and fuel vapors. Let it cool.
- Engage the chain brake and set the ignition switch to off before handling the plug or wires.
- Wipe up spilled fuel and allow any fuel-soaked parts to dry fully before testing.
- Keep the chain clear of people and objects. Follow the saw’s manual for safe handling and starting.
Use a clean, dry work area. Keep track of any small parts you remove, and do not pull the starter with loose tools near the flywheel or chain.
Confirm the no-spark symptom
“No spark” should mean that a suitable tester shows no spark while you crank the engine. A saw that sparks but will not start has a different problem to diagnose. A plug that looks wet or sooty can give a clue, but it does not prove the ignition system is faulty.
First check the ignition switch is set to run, the plug cap is fully seated, and the switch wire and visible connectors are connected. Remove the plug and inspect it for a cracked insulator, damage, heavy fouling, or the wrong type. Replace a damaged or badly fouled plug with the correct type for the saw’s version.
Common plugs include NGK BPMR7A and Bosch WSR 6 F. The listed electrode gap is 0.5 mm (0.020 in), but confirm the plug specification and gap for your saw. Do not use a screwdriver or loose plug held near the cylinder as a spark test. Use an approved inline spark tester and follow its instructions.
| Observation | What it tells you | Next check |
|---|---|---|
| Tester shows regular spark while cranking | Spark is present during this test | Check fuel, starting procedure, and other causes of a no-start |
| Tester shows no spark | Ignition is not producing a visible test spark | Inspect the switch circuit, wires, cap, and module gap |
| Spark returns after disconnecting the stop-switch lead | The stop circuit may be grounding the ignition | Find and repair the switch or wire fault |
| Spark is still absent with the stop lead disconnected | The stop circuit is less likely to be the cause | Check wiring, module gap, flywheel, and module |
A tester result is not proof that every ignition part is healthy under all conditions. It is a useful first split in the diagnosis. Next, isolate the circuit that can deliberately stop the spark.
Isolate the stop switch and wiring
The stop switch turns the engine off by grounding the ignition circuit. A damaged or pinched stop wire can ground it all the time, so the saw may act as if the ignition module has failed. Disconnecting the stop-switch lead is a test only; never run or use the saw with that circuit disconnected.
With the ignition off, inspect the plug lead, cap, switch wiring, and connectors. Look for crushed insulation, bare wire touching metal, loose terminals, or damage near the flywheel cover and switch. Correct visible damage before further testing.
If there is no spark, follow the applicable service manual to disconnect the stop-switch lead at the ignition module. Keep the area dry and clear of fuel vapors, then retest with the inline tester. Do not leave the lead disconnected after the test.
- Spark returns: The switch circuit is grounding the ignition. Inspect the stop switch and wire for a short, pinch, or damaged insulation. Repair the fault, reconnect the circuit, and confirm the switch reliably stops the engine.
- Spark remains absent: The stop circuit is less likely to be the cause. Reconnect it after testing, then inspect the ignition module, its mounting, and flywheel area.
Never operate the saw with the stop wire disconnected. The switch is a key safety control, and the saw must be able to shut down reliably before it goes back to work.
Check the module gap and flywheel
The ignition-module air gap is the space between the module and flywheel. A gap that is out of range can interfere with ignition. On many MS 660 versions, the typical gap is 0.2–0.3 mm (0.008–0.012 in). Confirm the correct value and setting method in the service manual for your saw before adjusting it.
With the saw off and cool, inspect for loose module fasteners, debris, damaged parts, or signs of contact between the flywheel and module. If the service procedure calls for adjustment, use the specified gauge and method. Do not guess the gap by eye or force the flywheel.
Check the flywheel and its key if the saw suffered an impact, was recently taken apart, or showed signs of abnormal timing. A damaged key can affect timing, but it is not the first explanation for every no-spark complaint. Follow the manual’s inspection steps; do not pry against the flywheel or strike it.
| Check | Finding | Action |
|---|---|---|
| Module mounting | Loose or visibly damaged | Correct or replace only as the service procedure directs |
| Air gap | Outside the version-specific specification | Set it using the manual’s method |
| Flywheel area | Debris, damage, or signs of contact | Clean or inspect further as directed |
| Stop circuit and gap verified, still no spark | Fault may be inside the module or related parts | Test or replace parts using the correct service procedure |
Do not replace the ignition module until the plug, cap, wiring, stop circuit, and module gap have been checked. Parts can differ across production runs, so verify the correct module against the saw’s markings or serial number.
Diagnostic examples: follow the evidence
A useful diagnosis follows test results rather than guesses. These examples show how the checks narrow the fault. They are diagnostic patterns, not claims about a particular saw; the same symptom can have different causes, so confirm each finding on your own machine.
In one common pattern, a saw has no tester spark, but spark appears when the stop lead is disconnected. That points toward a grounded stop circuit, not automatically a failed module. Inspect the switch and wire, repair the damage, reconnect the circuit, and retest both spark and shutdown.
In another pattern, there is still no spark with the stop lead disconnected. If the plug and lead look sound, check the module mount and air gap against the correct manual. If those checks pass, use the service procedure to test the remaining ignition parts before fitting a replacement module.
If the tester shows spark, stop treating the problem as “no spark.” Check that the saw has fresh, correctly mixed fuel and that you are following the right cold- or warm-start steps. STIHL specifies a 50:1 gasoline-to-two-stroke-oil mix when using STIHL 2-cycle oil. A wet plug may suggest flooding, but does not by itself identify the cause.
Return the saw to service safely
A spark test is only one step. Before starting, restore the stop-switch connection, secure the wiring, and reinstall the correct plug. Confirm spark again with the inline tester, then start outdoors using the correct procedure in the manual. Keep people clear of the chain.
Once running, check for a steady idle, normal throttle response, and a chain that stays still at idle. The manual’s approximate idle reference is 2,500 rpm, and its approximate maximum engine-speed reference is 13,500 rpm. These are operating references, not spark-test targets; use a tachometer and the version-specific procedure for tuning.
Before cutting, inspect chain tension, lubrication, controls, and the chain brake. MS 660 bar and chain setups vary. Match chain pitch, gauge, and drive links to the bar and sprocket; do not assume every saw uses the same combination. Common setups use full-size 3/8-inch pitch and .063-inch gauge chain, but verify the parts on your saw.
If the engine will not idle safely, the chain moves at idle, or the stop switch cannot shut the engine down, do not cut with the saw. Shut it off and resolve the fault first. After use, let it cool, clean it, and inspect for fuel or oil leaks.
Storage and seasonal care
Storage problems can make a later no-spark check harder, especially if dirt or moisture reaches the switch, plug cap, or wiring. A quick inspection before storage and before the next start can reveal damage early. Follow the MS 660 manual for fuel handling and storage steps for your version.
Keep the saw clean and dry, and inspect the ignition wire, plug cap, and stop-switch wiring for wear or pinching. Use fresh, correctly mixed fuel when preparing to run it. If the saw has been stored, do not assume an old plug or a new ignition module is the answer; repeat the spark test and circuit checks.
Frequently asked questions
Why does my MS 660 have no spark?
Common causes include a fouled or incorrect plug, a damaged plug lead or cap, a stop-switch wire shorted to ground, an incorrect module gap, or a failed ignition part. Test in that order so you do not replace parts before checking the simpler causes.
How can I test spark safely?
Use an approved inline spark tester while cranking, following the tester instructions. Work outdoors, away from open fuel, spills, and fuel vapors. Do not hold a loose plug near the cylinder or use an improvised tool to create a spark.
What spark plug gap does the MS 660 use?
The commonly listed gap is 0.5 mm (0.020 in), with common plug types including NGK BPMR7A and Bosch WSR 6 F. Confirm the plug and gap for your saw’s version before installation.
What is the ignition-module air gap?
Many MS 660 versions use a typical module gap of 0.2–0.3 mm (0.008–0.012 in). Check the matching service manual before adjustment because specifications and parts can vary.
Can a bad stop switch cause no spark?
Yes. A damaged or pinched stop wire can ground the ignition and prevent spark. Disconnecting the stop lead can help isolate the fault, but it is a diagnostic test only. Repair the circuit and verify shutdown before running the saw.
Should I replace the ignition module if there is no spark?
Not before checking the plug, cap, wiring, stop circuit, and module gap. Verify part fit against the saw’s markings or serial number, then follow the version-specific test procedure before replacement.
What if the tester shows spark but the saw will not start?
The ignition has produced spark during that test, so check the starting procedure and fuel next. Confirm fresh fuel mixed at the correct ratio, and inspect for a fouled plug. Spark alone does not prove the engine will start.
Can I run the saw with the stop wire disconnected?
No. The stop switch may not shut the engine down. Reconnect and secure the wire, then verify that the switch reliably stops the engine before returning the saw to service.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
