Graham FisherBy Graham Fisher

Updated November 2026

When an ex550 lawn mower won’t start, the cause is usually something simple: stale fuel, a fouled spark plug, a dead battery, or a safety switch that isn’t engaged. This guide walks through the checks in the order that solves most no-start problems, so you can get back to cutting without guessing. Work through each step before moving on to anything more involved.

Start With the Safety Checks

Before you touch the engine, confirm the mower is set up to run. Many no-start complaints come down to a control that hasn’t been engaged the way the machine expects.

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  • Make sure the engine stop switch or kill switch is in the run position.
  • Check that the throttle lever is in a position that allows starting, usually choke open or fast for a cold start.
  • Confirm the operator presence bar or blade control is pulled toward the handle if your model requires it.
  • Remove the spark plug wire and reconnect it firmly so it seats completely.

Check the Fuel System

Fuel is the most common cause of a mower that cranks but won’t fire. Gasoline that has sat for more than about 30 days can lose its volatility and leave varnish in the carburetor jets. If the mower has been stored since last season, assume the fuel is the problem until you prove otherwise.

Drain the old fuel into an approved container and refill with fresh unleaded gasoline. If the mower has been sitting for months, the carburetor may need cleaning or a rebuild kit. Look at the fuel line for cracks or softness, and make sure the fuel shutoff valve (if equipped) is open.

Quick fuel checklist

  1. Drain and replace any fuel older than about 30 days.
  2. Check the fuel filter and lines for cracks or blockages.
  3. Confirm the fuel shutoff valve is open.
  4. Prime the engine according to the manual, usually a few presses of the primer bulb.

Inspect the Spark Plug

A spark plug that is wet with fuel, covered in carbon, or has a worn gap will prevent ignition. Remove the plug and inspect the tip. A dry, light tan or gray tip usually means the plug is healthy. Black, oily deposits point to a rich fuel mixture or an air filter problem, while white, blistered insulators suggest overheating.

Clean the plug or replace it with the type listed in your owner’s manual. Set the gap to the specification in the manual, and reinstall it snug without overtightening. Clean the area around the plug before removing it so debris doesn’t fall into the cylinder.

Test the Battery and Starting System

If your ex550 has an electric start, a weak battery will cause the starter to click, turn slowly, or do nothing at all. Check that the battery terminals are clean and tight. A fully charged 12-volt battery should read around 12.6 volts or higher with a multimeter. If it reads well below that, charge it fully or replace it.

If the mower uses a recoil starter instead, a worn pull cord or a broken return spring can prevent the engine from turning over. Pull the cord slowly to see whether the engine turns freely.

Air Filter and Choke Issues

A clogged air filter starves the engine of air and can flood it with fuel, leaving it unable to start. Remove the filter and tap out loose dirt. If it’s oily or badly clogged, replace it. Check that the choke is set correctly for a cold start and that it moves freely without sticking.

Common Mistakes

  • Repeatedly cranking a flooded engine. Let it sit for a few minutes or remove and dry the spark plug first.
  • Using fuel that contains more than 10 percent ethanol in a small engine without checking the manual.
  • Skipping the spark plug wire check, which is a quick fix that often goes unnoticed.
  • Forgetting to disconnect the spark plug wire before doing any work on the blade or underside of the deck.

A basic multimeter helps you confirm battery voltage and check for continuity in switches and wiring. Search for a View on Amazon that includes a battery voltage range.

A spark plug socket with a rubber insert holds the plug securely during removal. Look for a View on Amazon that matches your plug size.

A feeler gauge lets you set the spark plug gap accurately. A View on Amazon is a small, inexpensive way to get that setting right.

FAQ

Why does my ex550 crank but not start?

Cranking without ignition usually points to fuel delivery or spark. Check for fresh fuel, a clean spark plug, and a properly seated spark plug wire before moving on to the carburetor.

How often should I change the spark plug?

Many owners replace the plug once a season or after about 25 to 50 hours of operation, but always follow the interval in your owner’s manual.

When should I take the mower to a repair shop?

If you have checked fuel, spark, and the safety controls and the engine still won’t run, or if you smell fuel leaking or see damaged wiring, have a qualified small engine technician inspect it.

Conclusion

Most ex550 no-start problems come down to fuel, spark, or a control that isn’t engaged. Work through the safety checks first, then fuel, plug, and battery, and you will usually find the cause within an hour. If you need model-specific details, your owner’s manual is the best source for torque values and part numbers.

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Graham Fisher

About Graham Fisher

Graham Fisher is part of our editorial team and reviews and edits our lawn mowers guides, checking product details against manufacturer specifications and owner feedback.