Graham FisherBy Graham Fisher

Updated November 2026

If your power smart lawn mower 170cc won’t start, the problem is almost always one of a handful of fixable issues: stale or missing fuel, a fouled or disconnected spark plug, a clogged air filter, a flooded carburetor, or a safety interlock that isn’t engaged. This guide walks you through the checks in the order that saves you the most time, so you can identify the cause and get the engine running without guesswork.

Before You Start: Safety Checks

Work on the mower with the engine off and cool. Disconnect the spark plug wire before touching the blade area or anything near the cutting deck, and never reach under the deck while the engine is running. Keep the mower on a flat, level surface and make sure the fuel cap is closed when you are not actively pouring or inspecting. If you smell strong gasoline, leaking fuel, or see damaged wiring, stop and have a qualified small-engine technician inspect the unit before continuing.

Tools You Will Need

  • Spark plug socket (typically 5/8-inch or 13/16-inch, depending on the plug)
  • Feeler gauge or spark plug gap tool
  • Screwdriver set and a socket wrench
  • Clean rags and a small container for fuel
  • Fresh gasoline and fuel stabilizer if the old fuel is questionable
  • Replacement air filter and spark plug (keep them on hand before you need them)
  • Carburetor cleaner spray (use only on the carburetor body and jets)

Step 1: Check the Fuel

Fuel is the most common reason a small engine refuses to start. Gasoline that sits for 30 days or more begins to break down, leaving varnish that clogs the carburetor jets. Ethanol-blended fuel is especially prone to this, and it can absorb moisture over time.

  1. Look at the fuel in the tank. If it is dark, smells sour, or has been sitting since last season, drain it into an approved container and dispose of it properly.
  2. Refill with fresh regular unleaded gasoline. Many manufacturers recommend fuel with 87 octane or higher and caution against using ethanol blends above E10 unless the owner’s manual explicitly allows it.
  3. Check the fuel shutoff valve, if your model has one, and confirm it is in the open position.
  4. Inspect the fuel line for cracks, stiffness, or leaks. Replace it if it looks hardened.

Step 2: Inspect the Spark Plug

A spark plug that is fouled, cracked, or wrongly gapped will prevent ignition even when fuel and air are present. Remove the plug and look at the tip.

  • Black, sooty tip: The engine is running rich. Often this points to a clogged air filter or carburetor issues.
  • White or blistered insulator: The engine may be running lean or overheating.
  • Oily film: Oil may be entering the combustion chamber, which points to a worn engine and warrants a professional diagnosis.
  • Cracked porcelain or heavy electrode wear: Replace the plug.

Clean the plug with a wire brush if it is only lightly fouled, then check the gap against the specification in your owner’s manual. Most small engines call for a gap somewhere around 0.030 inches, but confirm the exact figure for your model. Reinstall the plug snugly, then reconnect the wire firmly until it clicks.

Step 3: Check the Air Filter

A restricted air filter starves the engine of air, causing a rich mixture that can flood the cylinder and make starting difficult. Remove the air filter cover and inspect the element.

  1. Tap a paper filter gently to dislodge loose debris. If it is heavily clogged, black, or oily, replace it.
  2. For foam filters, wash in warm soapy water, rinse, squeeze dry, and lightly coat with the oil recommended by the manufacturer. Squeeze out any excess oil.
  3. Make sure the filter seats correctly and the cover is latched down with no gaps.

Step 4: Address a Flooded Engine

If you have pulled the starter cord repeatedly and the engine smells strongly of fuel, it may be flooded. Flooding happens when too much fuel reaches the cylinder, and the spark plug becomes wet.

  1. Set the choke to the off or run position and, if applicable, open the throttle fully.
  2. Remove the spark plug and wipe it dry.
  3. Pull the starter cord several times with the plug out to clear excess fuel from the cylinder.
  4. Dry the plug, reinstall it, and try again with the choke in the appropriate position for a cold or warm start.

Step 5: Confirm the Choke and Throttle Settings

Starting a cold engine usually requires the choke to be closed (or set to “start”), and a warm engine often needs the choke open. Setting these incorrectly is a frequent source of frustration. Follow the starting sequence printed on your mower’s decal or in the manual. As a general rule, use the choke for a cold start, move it to run once the engine has fired, and set the throttle to a moderate position.

Step 6: Check the Safety Interlocks

Many walk-behind mowers will not start unless the operator presence control bar is held against the handle, and some models also require the blade engagement control to be disengaged. Test the bar while attempting to start. Inspect the cable for kinks or slack, and confirm that the bail or control bar moves freely. A stuck or misadjusted cable can prevent the engine from firing even when everything else is correct.

Step 7: Look at the Carburetor

If fuel, spark, and air are all in order, the carburetor is the next suspect. Small jets and passages can clog with varnish from old gasoline. Remove the air filter housing and inspect the carburetor for visible debris. Spray carburetor cleaner into the throat and the jets as directed on the can, and let the engine rest before attempting a restart. Avoid poking deep into the jets with wire, which can enlarge the openings and throw off the fuel mixture. A full carburetor rebuild is often the best option for an engine that has sat unused for a long time, and this is best left to a technician if you are not comfortable with small disassembly.

Step 8: Check the Engine Compression and Fuel Supply

If none of the above solves the problem, the engine may have low compression, a stuck valve, or a fuel pump or primer issue. Pull the starter cord slowly and listen: a weak, lifeless pull often indicates compression loss. Check the primer bulb if your model has one, and make sure it returns to shape after squeezing. At this stage, consult a qualified small-engine repair shop before disassembling the engine further.

Quick Reference: Symptoms and Likely Causes

Symptom Most likely cause First thing to check
Cranks but never fires No spark or no fuel Spark plug and fuel freshness
Smells of gas, won’t fire Flooded engine Dry the spark plug and crank with plug out
Starts then dies Clogged carburetor or air filter Air filter, then carburetor jets
Won’t crank at all Safety interlock or stuck blade engagement Operator presence bar and cable
Weak pull, no kick Low compression Consult a technician

Common Mistakes

  • Leaving old fuel in the tank over winter. Drain it or add stabilizer before storage.
  • Over-priming. Too many primer pumps can flood the engine quickly.
  • Ignoring a dirty air filter. It is one of the cheapest and most overlooked fixes.
  • Skipping the spark plug gap check. A slightly wrong gap can prevent ignition.
  • Forcing the starter cord. A jammed cord can break; if it resists, stop and investigate.

Tips for Preventing Future Starting Problems

  • Add fuel stabilizer to every tank you do not plan to use within a month.
  • Run the engine dry at the end of the season, or drain the carburetor bowl.
  • Change the spark plug and air filter at the intervals listed in your manual.
  • Keep the deck clean of grass buildup, which can restrict airflow around the engine.
  • Store the mower in a dry area out of direct weather.

A basic spark plug socket and a fresh set of replacement parts make the most difference. A fuel stabilizer keeps gasoline usable through storage, and a carburetor cleaner handles varnish buildup on the jets.

  • A 5/8-inch spark plug socket with a rubber insert helps you remove and reinstall the plug without damaging the porcelain. View on Amazon
  • Fuel stabilizer added to gasoline before storage prevents varnish formation in the carburetor. View on Amazon
  • Aerosol carburetor cleaner removes varnish from jets and passages without disassembly. View on Amazon

Further Reading

If the fix above does not resolve your issue, the broader Troubleshooting a Lawn Mower That Won't Start: 8 Steps to Fix It walkthrough covers additional diagnostic paths. For models with similar starting complaints, see Sun Joe Lawn Mower Won't Start: 8 Causes and Fixes and Worx 40V Lawn Mower Won't Start: 8 Fixes to Try First. If you suspect a worn component, 4 Best Power Smart Lawn Mower Won't Start Parts and Replacements (2026) compares common replacement options.

FAQ

How long can gasoline sit in a mower before it causes starting problems?

Gasoline can begin to degrade within about 30 days, especially ethanol blends. Fuel that has been stored for a season or more should be drained and replaced.

Why does my mower start and then stall after a few seconds?

This usually points to a restricted air filter, a dirty carburetor jet, or an incorrect choke setting. Check the air filter first, then the carburetor.

Can I use starting fluid on a 170cc engine?

Starting fluid can help diagnose a fuel delivery problem, but it should be used sparingly. Overuse can damage the engine or cause flooding, and it is not a permanent fix.

How often should I replace the spark plug?

Follow the interval in your owner’s manual, which is often once per season or every 50 hours of operation. Replace it sooner if the electrode is worn or the tip is heavily fouled.

Does the blade need to be disengaged before starting?

On many models, yes. Check your manual, since some designs use a blade engagement control that must be off before the engine will crank.

What should I do if the starter cord feels stuck?

Stop pulling. A stuck cord may indicate a seized engine, a jammed recoil starter, or a damaged cord. Consult a qualified repair shop rather than forcing it.

When should I take the mower to a professional?

If you have checked fuel, spark, air, and safety interlocks without success, or if you notice oil leaks, smoke, unusual noises, or weak compression, a qualified small-engine technician can diagnose the problem safely.

Conclusion

Most power smart lawn mower 170cc won’t start problems come down to fuel, spark, air, or a safety interlock. Work through the steps in order, starting with the cheapest and easiest checks, and you will usually find the culprit within an hour. If the engine still refuses to run, do not keep cranking; a qualified technician can diagnose internal issues safely.

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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.