Gas Lawn Mower Won't Start

Common Symptoms

• Pull cord cranks the engine but it won't catch and run
• Engine fires briefly then stalls immediately
• Black smoke or strong fuel smell when cranking
• Primer bulb feels hard, cracked, or doesn't spring back
• Spark plug is wet with fuel (flooded) or covered in black deposits
• Mower sat unused for 2+ months over winter

Most Likely Causes

1. 1

   Stale / Varnished Fuel (Most Common)

   Ethanol-blend gasoline begins to degrade in as little as 30 days in a sealed tank. The light fractions evaporate, leaving a sticky varnish residue that coats the carburetor float bowl, needle, and jets. A mower stored over winter without fuel stabilizer will almost always have this problem. Draining and replacing the fuel — plus cleaning the carb — resolves it.

2. 2

   Clogged Carburetor Main Jet

   The main jet is a small brass orifice (roughly 0.5–1mm diameter) that meters fuel into the intake. Varnish from old fuel plugs this opening entirely. Even one season's worth of degraded fuel can block it completely. Removal and cleaning with carb cleaner spray through the jet, or replacement with a $6 jet kit, usually fixes it.

3. 3

   Fouled or Gapped Spark Plug

   Small engine spark plugs (typically NGK BPR6ES or Champion RJ19LM) should be replaced every season. A plug with a gap wider than 0.030" or coated in black carbon won't produce a reliable spark. Testing is simple: remove the plug, reconnect the wire, ground the plug threads against the engine block, and pull the cord — you should see a strong blue spark.

4. 4

   Cracked or Hardened Primer Bulb

   The primer bulb pumps a small squirt of fuel directly into the carb throat to aid cold starts. Over time, the rubber hardens, cracks, or the check valve inside fails. A bad primer bulb means the engine never gets that initial fuel charge. Press the bulb — it should feel soft, compress fully, and spring back. If it stays compressed or you feel air, it needs replacement.

5. 5

   Choke Not Engaging or Stuck Open

   Cold engines need full choke (butterfly valve closed) to create a rich mixture for startup. If the choke plate is stuck open due to a seized pivot or broken choke spring, the engine won't start cold. Check that the choke plate moves freely and closes fully when the primer/choke lever is in the CHOKE position.

6. 6

   Low Oil Shutoff Triggered

   All modern Briggs & Stratton and Honda engines have a low-oil shutoff sensor (SAE-compliant). If the oil level is even slightly below the MIN mark on the dipstick, the sensor prevents starting to protect the engine. Check the oil level every time before starting — this is commonly overlooked after an oil change that was slightly underfilled.

Quick DIY Checks

1. 1Check the oil level first: locate the dipstick (usually yellow or red handle), wipe it clean, reinsert fully, then remove and read. The oil should be at or slightly above the FULL mark. Top up with SAE 30 or 10W-30 as needed. The low-oil sensor will prevent starting if underfilled.
2. 2Drain the old fuel completely. Tip the mower back (air filter side up to avoid flooding the carb), or disconnect the fuel line at the carb and let it drain into a container. Refill with fresh 87-octane or higher gasoline — use non-ethanol fuel if available near you for small engines.
3. 3Remove the spark plug with a spark plug socket (typically 5/8" or 3/4"). Inspect the electrode — if it's black and sooty, gap is wide, or the ceramic is cracked, replace it. Most Briggs engines use NGK BPR6ES or Champion RC12YC. Gap to 0.030" with a feeler gauge before reinstalling.

1. 4With the spark plug removed, spray a 2-second burst of carburetor cleaner directly into the intake throat. Reinstall the plug, set the choke to CHOKE (cold start position), and pull the cord 3–5 times. If it fires and runs briefly then stalls, the carb's fuel delivery is the confirmed problem.
2. 5Remove the air filter cover and air filter. Locate the primer bulb — it's a small dome (usually red or black) pressed through the carb body. Remove it by prying with a flathead and check the two rubber check valve discs inside. Replace the bulb if it's cracked, hard, or the valves are deformed. Part cost is under $5.
3. 6Remove the carburetor bowl (one bolt underneath). Inside you'll find the float and main jet. The jet is a small brass plug with a tiny hole — spray carb cleaner through it and confirm you can see light through the orifice. If the hole is blocked, use a thin wire strand (from a wire brush) to gently clear it, or replace the $6 jet. Reinstall bowl and test.
4. 7If the engine still won't start, test ignition: remove the spark plug, reconnect its wire, hold the plug threads firmly against a metal part of the engine block (or use an ignition tester), and pull the cord. A strong blue spark every pull = ignition is fine. Weak/orange spark or no spark = ignition coil failure. Set the air gap between coil and flywheel magnet to 0.010" (business card thickness) before replacing the coil.

Repair vs Replace

Recommended Tools & Parts

• Carburetor Rebuild Kit (Briggs & Stratton)

  Includes float, needle, jets, gaskets, and primer bulb. Fits most Briggs 450E–550EX series engines found on Craftsman, Husqvarna, Toro, and Murray mowers.

  $8–$18

  Buy on Amazon →

• Spark Plug NGK BPR6ES

  OEM-equivalent plug for most Briggs & Stratton and Honda GCV engines. Pre-gapped to 0.028–0.030". Replace every season for reliable starting.

  $3–$6

  Buy on Amazon →

• Primer Bulb Replacement Kit

  Rubber primer bulb and check valve discs for Briggs & Stratton and Tecumseh carburetors. Resolves no-prime and flooded engine symptoms.

  $4–$8

  Buy on Amazon →

• Carburetor Cleaner Spray (12 oz)

  Fast-acting solvent for dissolving varnish and gum deposits in carburetor passages. Essential first step before replacing any parts.

  $6–$10

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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