Mower Won’t Start Flowchart

No Fluff. Just the Fix.

Module A: Correct Starting Sequence & Pre-Checks

A surprisingly common reason a mower won’t start is simply not following the correct sequence. Don’t worry—it’s easy to miss a step, especially when switching between mower types.

Here’s a quick refresher to make sure everything’s in order:

✅ Step-by-Step Starting Checklist

🧰 Extra Tips

✅ If you’ve done all the above and the mower still won’t start…
Go back to the flowchart and continue to ➡️ Step 2: Is the fuel fresh?

Module B: Drain Fuel Bowl & Refill with Fresh Gas

Old, stale fuel is one of the most common reasons a mower won’t start. Even if your tank looks full, if the gas is over 30 days old—or contaminated—it can clog the carburetor and prevent starting.

Here’s how to safely drain the fuel and give your mower a fresh start:

✅ Step-by-Step Instructions

🛠️ Tools You’ll Need

• Small wrench or socket set
• Catch container for old gas
• Fuel siphon (optional, for tank draining)

If you need more info, check out this video – Drain gas bowl

⚡ Important Safety Tips

• Always drain fuel outdoors or in a well-ventilated area.
• Keep fuel away from sparks, flames, or ignition sources.
• Dispose of old gas properly—check local regulations.

📌 Pro Tip

“If you’re draining old gas, it’s a good idea to also replace the fuel line filter if your mower has one. Tiny bits of debris from old fuel often hide there and cause trouble later.”

✅ Fuel drained and replaced?
Go back to the flowchart and continue to ➡️ Step 3: Is the air filter clean and dry?

Module C: Air Filter Cleaning or Replacement

A clogged, soaked, or heavily soiled air filter can choke your engine by restricting airflow to the carburetor, causing hard starting or a no-start condition.
Luckily, it’s an easy fix you can check in minutes.

✅ Step-by-Step Instructions

🛠️ Tools You’ll Need

• Flathead screwdriver (sometimes)
• Mild dish soap (for foam filters)
• Replacement filter (if necessary)

Need more info on air filter replacement check out this video – Mower blowing black smoke

⚡ Important Safety Tips

• Avoid running a mower for long periods without an air filter—it risks engine damage.
• Make sure a foam filter is properly oiled (but not dripping) before reinstalling. Do not oil paper filters.

📌 Pro Tip

“If your mower floods with gas or blows black smoke and sputters, a dirty air filter is often to blame. Remove the air filter and try starting, if she now runs fine clean or replace the filter.”

✅ Air filter checked or replaced?
Go back to the flowchart and continue to ➡️ Step 4: Is the spark plug in good condition and securely connected?

Module D: Plug Inspection, Cleaning, Gapping & Refit

The spark plug is a critical link in starting your engine. A fouled, worn, incorrectly gapped, or loose spark plug is a very common cause of a mower failing to start.
Here’s how to inspect and fix it properly.

✅ Step-by-Step Instructions

🛠️ Tools You’ll Need

• Spark plug socket and ratchet
• Wire brush (for cleaning)
• Spark plug gap tool
• New spark plug (if replacement needed)

If you need more help on spark plug gapping or cleaning, check out the following videos:

Gapping spark plug

Cleaning spark plug

⚡ Important Safety Tips

• Always unplug the spark wire before working on the engine.
• If unsure about plug condition, it’s cheap insurance to replace it.

📌 Pro Tip

“Even a brand-new mower can suffer from a factory mis-gapped spark plug. Always check the gap—it’s a two-minute job that can save hours of frustration.”

✅ Spark plug cleaned, gapped, or replaced?
Go back to the flowchart and continue to ➡️ Step 5: Do you see a spark when tested with a spark tester?

Module E: Kill Switch, Ignition Coil & Spark Troubleshooting

If your mower still has no spark after inspecting, cleaning, or replacing the spark plug, the next step is to check the ignition coil and kill switch circuit.

The ignition system is simple—but small faults like a shorted kill wire or a failed coil will stop everything.

If you prefer to watch a video on testing the system, check it out here – Check mower spark

✅ Step-by-Step Instructions

If you still have no spark:
➔ The ignition coil itself is faulty and must be replaced.

I’ve covered the coil replacement process here in this video – Checking spark & replacing coil

🛠️ Tools You’ll Need

• Screwdriver or socket set (for engine cover)
• Spark tester
• Feeler gauge (for coil replacement)

⚡ Important Safety Tips

• Never pull the starter cord without the engine cover secured in place.
• Keep fingers and tools clear of the blade when testing spark.

📌 Pro Tip

“Coils can fail without warning—but kill wire insulation rubbing through is a possible no spark reason too. Always check for grounding faults first before replacing parts.”

✅ Spark restored?
Go back to the flowchart and continue to ➡️ Step 6: Does the engine feel like it has compression?

Module F: Compression Test & Flywheel Key Clues

If your mower shows no spark issues and fresh fuel, but still refuses to start, low or no compression could be the real problem.
Luckily, you don’t need fancy tools to get a quick idea.

✅ Step-by-Step Instructions

1. Quick Compression Test (The Thumb Test)

• Remove the spark plug.
• Place a gloved thumb firmly over the plug hole.
• Gently pull the starter cord.

If compression feels weak or missing:
➔ You may have a sticking open valve, especially common after winter lay-up.

2. Fixing a Sticking Valve

I’ve covered inspecting valves in this video – Sticking valve inspection

3. Sheared Flywheel Key Symptoms

If compression seems okay but the starter cord violently snaps back when you pull it, suspect a sheared flywheel key. This typically happens after the blade strikes a hard object (like a tree stump or curb).

I’ve covered inspecting and replacing the shear key in detail right here – Checking Shear-key

🛠️ Tools You’ll Need

• Socket set (for spark plug and valve cover)
• Gloves (for thumb compression test)
• Soft mallet (optional for valve cover removal)

⚡ Important Safety Tips

• Disconnect the spark plug wire before handling any moving parts.
• Be cautious when removing covers—gaskets may be fragile.

📌 Pro Tip

“If the starter rope snaps out of your hand hard, always check the flywheel key before replacing expensive parts. It’s a cheap and simple fix once caught early.”

✅ Compression checked? Valve freed? Timing issue ruled out?
Go back to the flowchart and continue to ➡️ Step 7: Perform the Gas Shot Test.

Gas Shot Test

If your mower has good spark, good compression, and fresh fuel but still won’t fire, it’s time for a gas shot test.
This simple check helps confirm if the carburetor is the issue—or if you need to dig deeper into spark or compression.

✅ Step-by-Step Instructions

🔍 Interpret the Result

• If the engine fires briefly or tries to run:
  ➔ Carburetor is dirty or blocked.
  ➔ Fuel wasn’t reaching the combustion chamber properly — time to clean or rebuild the carb.
• If there’s no fire at all:
  ➔ Suspect deeper spark, compression, or timing issues.
  ➔ Recheck previous steps, especially flywheel key and ignition coil.

🛠️ Tools You’ll Need

• Carburetor cleaner spray
• Fresh gasoline
• Small container or syringe
• Screwdriver (to remove air filter housing if needed)

I covered the process in this video – Gas Shot Test

⚡ Important Safety Tips

• Only use small amounts of carb cleaner or gas—never flood the engine.
• Perform the test outdoors or in a well-ventilated area.
• Keep fuel sources clear of sparks or flames.

📌 Pro Tip

“The gas shot test is one of the fastest ways to tell if your problem is fuel-related. If the mower fires up—even briefly—you’ve just saved yourself hours of guessing.”

✅ Completed the gas shot test?

If it doesn’t: Recheck ignition and compression modules.

If it fires: Go to Module G: Carburetor Cleaning or Replacement

Did the Engine Fire Briefly?

When we say the engine “fired briefly,” we mean the engine tried to start — even if it only ran for a second or coughed once before dying.

Signs the Engine Fired Briefly:

• A loud “pop” or sharp bang from the exhaust
• The engine sputters, coughs, or runs for a second before cutting out
• You feel a strong kick through the starter cord when pulling

If you hear or feel any of these, it means the engine was able to burn the small amount of fuel you gave it — confirming that spark and compression are likely OK.

Go back to the flowchart.

Module G: Dirty Carb – Time to Clean or Replace

If your gas shot test caused the engine to fire briefly, it means spark and compression are good — and the problem is fuel delivery.
In almost every case, that points to a dirty or blocked carburetor.

Cleaning it properly can get your mower running like new again.

✅ Step-by-Step Instructions

I’ve covered the carburetor removal and cleaning process by engine model, and you can check these out below:

B & S Classic Carb Cleaning

B & S Intek and Quantum Carb Cleaning

B & S E Series Carb Cleaning

Kohler Carb Cleaning

Honda Carb Cleaning

🛠️ Tools You’ll Need

• Screwdriver set
• Socket set
• Carburetor cleaner spray
• Fine wire (for cleaning jets)
• New gaskets (optional)

⚡ Important Safety Tips

• Work in a well-ventilated area.
• Keep fuel sources away from sparks and open flames.
• Always turn off the fuel valve (if fitted) before removing the carburetor.

📌 Pro Tip

“Most ‘dead mowers’ after winter storage are suffering from clogged carb jets. A good clean fixes 90% of no-start issues without buying a new carb.”

✅ Carb cleaned and reinstalled?
Try starting the mower again following the correct start procedure from Module A.
If it starts and runs well, you’re good to go!
If it still won’t start, double-check your work.
If everything checks out, revisit Module H: Spark, Compression, and Timing.

Module H: Recheck Spark & Compression / Timing

If you’ve followed every step — fuel, spark, compression, timing — and your mower still won’t start, here are a few extra checks that are often overlooked:

✅ 1. Check for Flooding

• After repeated starting attempts, the engine can flood with fuel.
• Signs: Strong gas smell at the exhaust, wet spark plug.
• Fix: Remove the plug, dry it off, let the engine air out for 15–30 minutes, and try again with no choke.

✅ 2. Confirm Fresh Fuel is Really Reaching the Carb

• Double-check that fuel is actually flowing from the tank to the carb.
• Kinked, blocked, or cracked fuel lines can prevent delivery even if the tank is full.

✅ 3. Kill Switch Cable or Bail Lever Issue

• A sticking bail lever switch can sometimes ground the coil even after you think you’ve tested it.
• Make sure the bail lever fully releases and the kill cable isn’t damaged or sticking internally.

✅ 4. Low-Quality Aftermarket Parts

• Cheap aftermarket spark plugs, coils, or carbs sometimes fail right out of the box.
• If you recently replaced parts and it’s still dead, consider trying OEM parts if you can.

✅ 5. Look for Air Leaks

• Cracked intake manifolds or loose carb gaskets can cause massive air leaks and prevent starting.
• If your mower pops but won’t keep running, inspect carb mounting bolts and gaskets.

⚡ Quick Reminder

• Always double-check that the spark plug wire is fully seated.
• Make sure the fuel tap is turned on (easy to forget after draining!).
• Rethink the starting sequence—one missed primer shot or choke position can make the difference.

✅ If you’ve rechecked everything and still no joy:
At this point, it may be time to consider a deeper mechanical issue (worn valves, serious internal damage) or consult a professional.