The chainsaw does not start when cold and floods the spark plug: 7 reasons and solutions

A problem familiar to every chainsaw owner: a flooded spark plug and failure to start.

You pulled the starter, the engine made a couple of “sneezes” and died. When you try to start it again, the chainsaw refuses to respond, and when you unscrew the spark plug, a wet, fuel-filled part is discovered. Typical situation: the chainsaw does not start when cold and floods the spark plug. Moreover, the problem arises as with budget Chinese models Carver or Champion, as well as professional Stihl MS 260 or Husqvarna 550 XP.

In 80% of cases, it is not the engine itself that is to blame, but carburetor malfunction, ignition or fuel system. But before disassembling the saw, it is worth understanding the mechanism of the problem. When you pull the starter to cold, the fuel mixture enters the combustion chamber in excess - this is how the cold start algorithm works. If spark is weak or compression is insufficient, the fuel does not ignite, but simply floods the spark plug. Next, we will analyze all possible reasons - from banal “flooding” to hidden defects that are not written about in the instructions.

Important: if the saw starts “hot” after several attempts, but stalls again when it cools down, this is a direct sign of problems with enriching the fuel mixture on a cold engine. Ignoring the symptom will lead to more serious damage, for example, scuffing on the piston or ring wear.

1. Over-enriched fuel mixture: why the carburetor “spouts” gasoline

The most common reason for a candle to flood is excess fuel in the combustion chamber. The chainsaw carburetor is configured to supply a rich mixture (more gasoline, less air) during a cold start. If the adjustments are messed up or there are malfunctions, the fuel enters the cylinder in excess, does not have time to evaporate and “floods” the spark plug.

How does this manifest itself:

• 🔧 The saw does not start after 1-3 pulls, the spark plug is wet after 2-3 attempts.
• 💨 The muffler smells like pure gasoline (no exhaust smell).
• 🔥 When the spark plug is unscrewed, fuel is visible in the cylinder (if you turn the saw over).

Reasons for over-enrichment of the mixture:

1. Excessive fuel level in the float chamber (worn needle valve or float).
2. Air filter clogged - the engine is “choking”, the ECU (if equipped) or carburetor mechanics compensate for this by increasing the supply of gasoline.
3. Incorrect screw adjustment L (low speed) - the mixture is too “rich” on a cold start.
4. Air leaks through carburetor gaskets — sensors (if any) or mechanics mistakenly “think” that the mixture is lean and add fuel.

2. Ignition problems: weak spark or no spark

Even if the carburetor works perfectly, a weak spark will not be able to ignite an over-rich mixture. As a result, fuel sticks to the spark plug electrodes and the saw stalls. Checking the spark is easy:

1. Unscrew the spark plug and insert it into the cap of the high-voltage wire.
2. Press the spark plug skirt onto the cylinder (through a dielectric, such as a dry wooden stick).
3. Pull the starter - there should be a spark bright blue and stable.

If there is no spark or it is weak (orange, intermittent), the reasons may be as follows:

• ⚡ Faulty spark plug - cracks on the insulator, carbon deposits, incorrect clearance (must be 0.5–0.65 mm for most saws).
• 🔌 High voltage wire breakdown - Check for cracks or abrasions.
• 🛠️ Ignition coil malfunction - if after replacing the spark plug and wire there is no spark, the coil is to blame.
• 🔋 Weak ground contact - oxidation or corrosion at the connections of the reel to the saw frame.

3. Fuel system malfunctions: from filter to fuel pump

If the carburetor and ignition are in order, but the saw still floods the spark plug, the problem may lie in fuel supply. Here are the key points to check:

System element	Symptoms of a problem	How to fix
Fuel filter	Clogged with dirt → low pressure in the system → carburetor “spouts” gasoline	Replace the filter (costs 50–200 ₽). Flushing doesn't help!
Petrol hoses	Cracks, softening → air leaks or fuel leaks	Replace the hoses with silicone ones (original or Tygon).
Carburetor needle valve	Does not hold pressure → fuel overflows into the float chamber	Clean or replace the valve (included in the carburetor repair kit).
Gasoline pump (membrane)	Membrane torn → low performance → lean mixture when hot, rich when cold	Replace the membrane (repair kit ~300 ₽).

Pay special attention quality of the fuel mixture. If you use cheap gasoline with a lower octane rating A-92 or old oil (over 1 month), this leads to:

• 🛢️ Formation of resinous deposits on the carburetor needle.
• 🔥 Unstable operation of the candle (carbon deposits on the electrodes).
• 💀 Accelerated wear of the piston group.

How to properly store the fuel mixture?

A mixture of gasoline and chainsaw oil can be stored for no more than 14 days in a plastic canister and 30 days in metal. After this, the octane number of gasoline drops and the oil oxidizes. For long-term storage (in winter), drain the fuel from the tank and carburetor, and before storing, run the engine “dry” (without load) until the gasoline is completely exhausted.

4. Compression and condition of the cylinder-piston group

Low compression is one of the most insidious causes of spark plug flooding. If there is insufficient pressure in the cylinder, the fuel mixture is not compressed properly and the spark cannot ignite it. Normal compression for a chainsaw:

• 🔧 Budget models (Carver, Champion): 8–10 atm.
• 🔧 Professional saws (Stihl, Husqvarna): 10–12 atm.

How to check compression without a compression gauge:

1. Unscrew the spark plug.
2. Close the spark plug hole with your finger.
3. Pull the starter: if the finger “shoots” with force, compression is normal; If you can barely feel the pressure, there's a problem.

Reasons for low compression:

• 🔩 Piston ring wear — check the piston play in the cylinder (should be minimal).
• 💥 Seizure on the cylinder or piston - often occur when working on a lean mixture or without oil.
• 🛠️ Cylinder head gasket failure - Check for oil leaks around the bolts.

5. Diagnostic algorithm: where to start if a spark plug is flooded

To avoid wasting time disassembling the carburetor or replacing a spark plug “at random,” follow this check procedure:

Unscrew the spark plug and check its condition (dry/wet/carbon deposits)

Check for spark (brightness, stability)

Inspect the air filter for clogging

Try to start the saw without a filter (if it starts, the filter is to blame)

Check the fuel hose and filter for blockages

Adjust screw L carburetor (turn 1/4 turn counterclockwise)

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If after these steps the saw does not start, proceed to in-depth diagnostics:

1. Compression check (as described above).
2. Carburetor diagnostics:
   • Blow out the channels with compressed air (needle, jets).
   • Check the fuel level in the float chamber (should be at the level of the cap connector).
   • Replace gaskets if there are gasoline leaks.

Checking the ignition coil multimeter (primary winding resistance: 0.5–2 Ohm, secondary: 2–5 kOhm).

6. Carburetor adjustment: step-by-step instructions

In 60% of cases, pouring a cold candle is eliminated correct carburetor adjustment. To do this, there are three screws on it:

• L — low speed adjustment (responsible for the mixture on a cold start).
• H — high speed adjustment.
• T - idle speed adjustment (sometimes referred to as LA or S).

Adjustment procedure (for most carburetors Walbro, Zama, Tillotson):

1. Start the saw and warm it up for 2-3 minutes.
2. Turn the screw L clockwise until it stops, then back to 1/4–1/2 turn.
3. Turn the screw T so that the chain does not rotate at idle, but the engine runs stably.
4. Check at maximum speed (screw H): There should be no dips when you press the gas.

Warnings:

⚠️ Attention: Do not adjust the screw H no load (no cutting wood)! This will lead to engine overheating and failure of the piston group.

⚠️ Attention: On some saws (eg Stihl MS 180) screw L can be closed with a plug. To access it you will need to remove the cover and drill out the plastic.

7. Prevention: how to avoid flooding a candle in the future

To prevent the problem from recurring, follow these rules:

• 🛠️ Use quality fuel: gasoline A-92 or A-95 from proven gas stations, oil for two-stroke engines (Stihl HP, Husqvarna HP).
• 📅 Do not store fuel mixture for longer than 2 weeks — the octane number drops, the oil oxidizes.
• 🔧 Clean the air filter after every 5–10 hours of operation (blow with compressed air or wash in gasoline).
• ❄️ In winter, use a “winter” mixture: Add 10% more oil (eg. 1:40 instead of 1:50).
• 🔥 Before winter storage drain the fuel, run the engine dry and add 5 ml of oil to the cylinder by turning the crankshaft by hand.

If the saw is used rarely (for example, 1-2 times a year), before starting:

1. Drain the old fuel from the tank.
2. Pour in fresh mixture.
3. Unscrew the spark plug, check the gap and dry it.
4. Start the saw with fully open damper (do not use the choke before warming up).

FAQ: Frequently asked questions about pouring candles

Is it possible to start a chainsaw with the spark plug unscrewed to blow out the cylinder?

Yes, but only as a last resort. Remove the spark plug, turn the saw over with the spark plug hole facing down and pull the starter 2-3 times to remove excess fuel. However, frequent repetition of this procedure may lead to dirt getting into the cylinder.

The saw starts only with a choke, without it it stalls - what to do?

This is a sign lean mixture at idle. Adjust the screw L (turn counterclockwise 1/8 turn) and check carburetor tightness - Possibly sucking air through the gaskets.

After replacing the spark plug, the saw again fills in a new one. What's the matter?

The problem is not in the candle, but in carburetor or ignition. First check the spark on the new spark plug (it should be bright blue). If the spark is weak, the coil or high-voltage wire is to blame. If the spark is good, look for the reason in fuel overflow (see section on carburetor).

The saw won't start when it's cold, but when it's hot it works fine. Why?

This is a typical symptom incorrect screw adjustment L (the mixture is too lean for a cold start) or air leak into the carburetor. Also check fuel pump diaphragm condition - when cold, it may not create the required pressure.

Is it possible to use a car spark plug instead of a standard one?

No! Car spark plugs have something else heat number and gap between electrodes. For chainsaws, use spark plugs marked BPM7A (Champion), BJM7A (NGK) or Bosch WSR6F. Non-compliance will lead to overheating or ignition failure.