Chainsaw Won’t Start? The Checks to Make Before the Shop

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When a chainsaw won’t start, Husqvarna points to four causes: the switch isn’t on run, the engine is flooded, the fuel mix is old or wrong, or the spark plug is worn. You can check all of them on the spot with a spark plug wrench. Allow 10 to 15 minutes of waiting if a hot engine is flooded.

Common symptoms and causes

• Chainsaw won’t start cold: check the choke, the primer bulb (the small clear pump that pushes fuel to the carburetor), the fuel level and the spark plug.
• Chainsaw won’t start hot: the procedure differs from a cold engine, and a full choke can flood it.
• Spark plug wet with gasoline: the engine is flooded, for instance because the choke stayed closed after the first cough.
• Spark plug dry after several pulls: check the fuel level and the primer bulb.
• Spark plug black and fouled: too much oil in the mix or a dirty air filter, according to Stihl.
• Starts, then stalls: choke, air filter or carburetor adjustment, common leads that the manufacturers don’t detail.

Before you start

• Set the chainsaw on the ground, on a stable surface.
• Engage the chain brake: push the front hand guard forward, which locks the chain.
• Wear gloves, safety glasses and hearing protection.
• Keep everyone out of the machine’s working area.
• Refuel with a cold engine, outdoors, away from any flame or source of sparks.
• Keep the manual within reach: the position of the choke and the switch changes with the brand and model.
• Have a spark plug wrench and a spare spark plug of the type listed in the manual.

Warning: before any work, stop the engine and wait until the chain has stopped completely. Disconnect the spark plug boot (the cap attached to the ignition wire) before you touch the plug.

Warning: when you reassemble, the spark plug connector nut must be fixed and tight. If it is loose, it can produce sparks and start a fire near the gasoline.

Warning: don’t take the carburetor apart or touch its adjustment screws without the manual. Opening the machine can void the warranty: check its end date first, and leave this work to an authorized shop.

Solution 1: check the switch and follow the starting procedure

These steps follow the Husqvarna cold-start procedure for models 135, 435, 440, 445 and 450; other models and brands differ, so follow your manual.

1. Set the on/off switch to the run position.
2. Check that the chain brake is engaged.
3. Pull the choke out to the full choke position.
4. Press the primer bulb 6 times, until you can see fuel inside it.
5. Hold the machine firmly on the ground.
6. Pull the starter cord until the engine starts or gives a short cough.
7. Push the choke back to the run position.
8. Pull the cord again until the engine starts.
9. Let the engine warm up, then squeeze the throttle gently to bring it back to idle.

The engine should idle smoothly.

Warning: don’t rev it with the chain brake engaged. Stihl states that this damages the clutch and the brake.

Hot engine, on a Husqvarna: pull the choke and push it straight back to its original position, pull the cord and let the engine run before you accelerate gently.

Differences on the Stihl MS 271 and MS 291:

• Press the primer bulb at least 5 times, even if it already looks full.
• The universal control lever goes to the closed choke position when cold, and to the start position when hot (an engine that has run for about a minute).
• To move to the closed choke position, press the throttle trigger lockout and the trigger at the same time.
• After the first cough, move the lever to the start position and pull again; once the engine runs, a short squeeze of the trigger returns it to normal run F.

The Husqvarna 450X has no separate choke: the switch goes to its start position, and the procedure is the same cold or hot.

Solution 2: flooded engine, wet spark plug

A flooded engine has received too much gasoline. On a Stihl, this happens when the lever wasn’t moved from the closed choke to the start position in time after the first cough. If the engine is hot, Husqvarna advises waiting 10 to 15 minutes before restarting, or starting with the throttle wide open and no choke.

Otherwise, clear the cylinder as Stihl describes.

1. Set the switch or lever to stop 0.
2. Disconnect the spark plug boot.
3. Clean around the spark plug.
4. Unscrew the spark plug with the spark plug wrench, pushed fully into the socket.
5. Dry the spark plug.
6. Pull the starter cord several times with the plug removed, to ventilate the combustion chamber.
7. Screw the spark plug back in.
8. Reconnect the spark plug boot, pressed on firmly.
9. Set the lever to the start position, even if the engine is cold.
10. Restart the engine.

The engine should start without going back to the closed choke. If it floods again, the spark plug may be fouled: go to solution 5.

Good to know: on a Husqvarna, the other option for a flooded hot engine is to pull the cord with the throttle wide open and no choke. The excess gasoline clears faster.

Solution 3: mix fresh, correctly measured 2-stroke fuel

A 2-stroke engine is lubricated by oil mixed into the gasoline. Husqvarna lists an old or incorrect mix among the causes of a no-start, and Stihl notes that too much oil fouls the spark plug.

Stihl’s reference amounts for a 50:1 mix with its 2-stroke oil:

In US units, 50:1 comes to about 2.6 fl oz of oil per gallon of gasoline. For another brand, follow the ratio in the manual: the Husqvarna page consulted gives none. Stihl accepts a high-performance 2-stroke oil of class JASO FB, FC or FD, or ISO-L-EGB, EGC or EGD.

1. Don’t keep a mix for more than 30 days; make a fresh one.
2. Pour the 2-stroke oil into an approved fuel can first, clean and dry.
3. Add the gasoline, with an octane rating of at least 90 RON (about 86 AKI in the US).
4. Close the can and shake it to blend.
5. Fill the chainsaw’s tank with this fresh mix.
6. Press the primer bulb several times.
7. Start with the choke closed, as after running out of fuel, on the Stihl MS 271 and MS 291; for other models, follow the manual.

You should see fuel in the bulb. The engine should then start.

Good to know: Stihl says gasoline containing more than 10% alcohol can disturb carburetor engines that are adjusted by hand. Stihl France advises against keeping a mix made with SP95-E10 for more than a month. Store the can in a cool place, out of the sun.

To avoid any dosing mistake, a ready-to-use 2-stroke fuel at 50:1 exists, made with Stihl oil; check that this ratio matches your manual.

Stihl MotoMix ready-mixed 2-stroke fuel

Alkylate fuel already mixed with 2-stroke oil at 1:50. It avoids mixing mistakes and keeps better than ordinary petrol.

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Solution 4: clean or replace the air filter

According to Stihl, a clogged air filter reduces power and fouls the spark plug. How you clean it depends on the material. A nonwoven filter isn’t oiled; an HD2 filter is tapped out, then rinsed with water from the outside.

1. Remove the air filter cover.
2. Take out the filter.
3. Clean the filter according to its material.
4. Let it dry out of direct sun.
5. Replace the filter if it is still clogged after cleaning.
6. Refit the filter and the cover.

If the spark plug was blackened, restore it (solution 5), then restart with the procedure from solution 1.

If the filter stays clogged, here is a genuine Stihl HD2 filter, compatible with the MS 231 to MS 391; check the part number for your model.

Original Stihl HD2 air filter (MS 231 to MS 391)

Original Stihl air filter in washable HD2 material. It replaces a filter that stays clogged after cleaning.

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Solution 5: check, clean or replace the spark plug

Stihl recommends checking the spark plug first when starting is difficult. A new plug must also be the right type and correctly gapped.

1. Disconnect the spark plug boot.
2. Clean around the spark plug.
3. Unscrew the spark plug with the spark plug wrench.
4. Examine the electrodes, the two metal tips the spark jumps between.
5. Clean the spark plug if it is fouled.
6. Measure the electrode gap: 0.5 mm (0.020 in) on the Stihl MS 271 and MS 291.
7. Replace the spark plug if it is worn or has about 100 hours of running time.
8. Screw the spark plug back in.
9. Reconnect the spark plug boot, pressed on firmly.

Then restart with the procedure from solution 1. If the spark plug stays dry after several pulls, check the fuel level and the primer bulb.

Tip: only use a resistor spark plug of the type listed in the manual. For the MS 271 and MS 291, Stihl lists the Bosch WSR 6 F and NGK BPMR 7 A.

Good to know: according to user reports, a primer bulb that stays soft points to a punctured diaphragm. The official sources consulted don’t confirm this.

If the spark plug needs replacing, here is the NGK BPMR7A, equivalent to the one Stihl lists for the MS 271 and MS 291; check the one in your manual.

NGK BPMR7A spark plug

Resistor spark plug for 2-stroke chainsaw and garden tool engines. It replaces a worn or fouled plug, provided the reference matches your manual.

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If nothing works

If the chainsaw still won’t start after the right procedure, a clean or new spark plug, a fresh mix and a clean air filter, the problem lies in the carburetor, its adjustment or the ignition. Take the machine to a dealer or an authorized shop for the brand. Stihl provides L, H and LA adjustment screws, but asks you not to lean out the setting and sends you to the dealer if the chain turns at idle.

With Husqvarna, the product assistant and the contact page on the official site cover special cases. Have the model, the fuel you used and a list of what you’ve already checked ready.

The same fuel, filter and spark plug checks apply to a gas lawn mower that won’t start. A robot mower that won’t return to its dock calls for different checks.

Take better care of your chainsaw: our toolbox

These products help you mix fuel properly, work on the spark plug and stay safe.

Frequently asked questions

Why won’t my chainsaw start when the spark plug is new?

The spark plug is only one of the four causes Husqvarna lists. Check that the switch is on run, that the engine isn’t flooded and that the mix is fresh and correctly measured. Also make sure the new plug is a resistor type, the part number in the manual, with the right electrode gap.

What should I do if the chainsaw’s spark plug is wet?

The engine is flooded: dry the spark plug and ventilate the cylinder. Remove the plug, dry it, pull the cord several times, then refit it. On the Stihl MS 271 and MS 291, restart with the lever in the start position, even with a cold engine; on other models, follow the manual.

Why does my chainsaw start, then stall?

The usual leads are the choke, the air filter and the carburetor adjustment. On the Stihl MS 271 and MS 291, the manual calls for the choke closed when the engine stalls on acceleration. A clogged air filter also cuts power; if the engine still stalls, the carburetor adjustment is a job for the shop.

Why won’t my chainsaw start when it’s hot?

Most often it is flooded by a choke used as if the engine were cold. On a Husqvarna, pull the choke and push it straight back to its original position before pulling the cord; if the engine is flooded, wait 10 to 15 minutes. On the Stihl MS 271 and MS 291, with a hot engine, the lever goes to the start position.

What gas-to-oil mix does a chainsaw need?

Stihl prescribes a 50:1 mix with its 2-stroke oil, which is 20 ml of oil per liter of gasoline. Pour the oil in first, then the gasoline, in an approved can. For another brand, follow the ratio in the manual, and don’t keep the mix for more than 30 days.