Old Gas and Lawn Mower Problems
In This Guide
The spark plug is the cheapest part on your mower and the most informative: a few dollars, ten minutes to change, and the old plug comes out carrying a written report on how the engine has been running. Our technicians replace plugs on nearly every service visit and read hundreds a season. This guide covers the signs a plug has gone bad, the change itself done properly (gapping and torque are where DIY goes wrong), and the color chart that turns your old plug into a diagnosis.
Symptoms
Old fuel and fuel-system deposits can contribute to hard starting, rough running, or a mower that will not fire.
- Harder starting, more pulls or longer cranking than the engine used to need.
- Misfiring and rough running, stumbles and skips, especially under load in thick grass.
- Noticeably higher fuel use, because weak incomplete combustion wastes gas.
- Engine cutting out momentarily, or refusing to start at all when the plug finally fails, the symptom tree covered in our cranks but will not start guide.
- Visible damage on inspection: a rounded, eroded center electrode, cracked porcelain, or heavy deposits bridging the gap.
Because the plug costs so little, the threshold for replacement is low: when in doubt, swap it, and keep the old one in the garage as a get home spare.
Inspection
Review fuel condition as part of diagnosis rather than assuming it is the only cause of the symptom.
Fuel system context
Cool engine, wire off
A complete no-start inspection can include fuel, ignition, and airflow because several systems can produce similar symptoms.
Handle fuel safely
Use approved containers and follow local rules for unwanted fuel.
Avoid unsafe disposal
Do not pour fuel onto the ground or into household drains.
Do not assume
Fuel age alone does not guarantee the diagnosis.
Handle fuel safely
Use approved containers and follow local rules for unwanted fuel.
Avoid unsafe disposal
Do not pour fuel onto the ground or into household drains.
Do not assume
Fuel age alone does not guarantee the diagnosis.
Prevention
Follow the equipment manufacturer's storage guidance and use safe, locally approved fuel-handling practices.
| Plug Appearance | What It Means | What to Do |
|---|---|---|
| Light tan or gray, dry | Healthy engine, correct mixture | Nothing, this is the goal |
| Black, dry and sooty | Running rich: too much fuel or too little air | Service the air filter, check the choke, suspect the carburetor |
| Black, wet and oily | Oil entering the cylinder: overfill, tipping, or wear | Check oil level, review how the mower was tilted, persistent oil fouling needs a technician |
| Wet with gasoline | Flooded engine or no spark reaching the plug | Dry it and follow our flooded engine guide |
| White, blistered or glazed | Running lean and hot: restricted fuel or an air leak | Suspect a varnished carburetor or intake leak, see the surging guide |
| Eroded electrode, wide gap | Simple old age | Replace and note the interval |
A black sooty plug is the one owners ask about most, and the message is rich running: the engine is getting more fuel than its air can burn. New plugs cure the symptom for a while, but the fix is on the air and fuel side, a clean filter and often a carburetor that needs cleaning.
Next Steps
If fresh, correctly handled fuel does not resolve the symptom, request an inspection before replacing parts.
Need an in-person inspection?
Share the equipment type and symptoms so the service scope and availability can be confirmed.