My Lawn Mower Won't Start? Basic Troubleshooting Tips for Small Engines
Lawn Mower Won't Start After Winter
For an engine to startup, it basically requires two things: fuel and a spark. If you aren't getting a spark at the plug or fuel isn't getting through to the engine (known as fuel starvation), the machine will never startup. Bad compression can also lead to difficult starting. After storage during winter, seals, pumps and valves in the carburettor can become gummed up with varnish deposits if you leave fuel in the tank. During the summer, dust and dirt can block the fuel system or clog filters.
While the information here applies to lawnmower engines, it is equally valid for other small engines used on yard equipment, such as portable generators, pumps, cultivators, snow blowers, compactors (wackers), cement mixers etc.
WARNING!!! Before You Start Any Mower Troubleshooting...
- When working on a mower, remember that gasoline/petrol is flammable. Remove all sources of ignition such as naked flames, stoves, sparks, cigarettes or other hot objects in the vicinity when working on the tank or fuel lines.
Disconnect the spark lead if you have to remove any clumps of grass jammed under the deck
- Keep the carburetor side of the engine upwards to avoid oil spills if the mower is placed on its side
- Drain the tank if you have to turn the mower on its side If there's only a small amount of gas in the tank, or you think it won't reach the tank cap, you don't need to do this
- Check oil level If oil is low, it can cause premature engine wear. If there's no oil left, an engine can quickly seize
Checking Lawn Mower Oil
Check the oil level is ok before starting. Move the mower to a level surface, remove the dip stick and wipe clean. Replace it and remove again. Check the oil is between the high and low marks before starting. Top up if necessary. Oil should be changed after every 25 to 50 hours of run time as advised in your engine manual. An oil change is reasonable easy to do. You can find more details in my guide How to Change Lawn Mower Oil
How Do You Start a Small Engine?
- Set the throttle to high revs or the choke position. If the engine has a manual choke turn it on
- Press the primer button (if fitted) about 8 times
- Pull the starter rope quickly, but try not to yank it too hard
- Allow the engine to reach top speed before using. If the engine has a manual choke, turn it off after about 5 seconds
- Don't turn a manual choke back on again if you need to start a hot engine
A Checklist to Get Your Mower Started
Use fresh gas. Don't use old gasoline which can cause difficult starting. Make sure there is enough fuel in the gas tank and check the vent in the tank cap is unblocked
Check the spark lead is firmly attached to the plug, and the plug is tightly screwed in. Try replacing spark plug with a new one
Make sure the "dead man's handle" control on the mower is held fully against the mower handle while starting.
Turn on choke if a manual choke fitted. Don't turn on the choke if the engine has been running in the last 5 minutes or so
Make sure primer bulb is pressed about 5 times (if fitted). If the mower runs out of gas during cutting, it will need to be primed again
Check the air filter isn't dirty. Wash and dry foam type air filter (see below for details), or replace dirty paper filter
Check the cable connecting the "dead man's handle" on the frame to the engine is not damaged or snapped
Check the carburettor is tightly screwed or bolted to the engine
Make sure there is no water lodged at the bottom of the gas tank
If the starter cord is hard to pull, check there are no clumps of grass clippings clogging the underside of the deck. These can jam the blade. Disconnect the spark lead before attempting to move the blade to remove clippings!
Some newer push mowers have starter motors and batteries. The battery voltage should be over 12 volts for the engine to start.
More Advanced Troubleshooting
If you still can't get your mower started, checkout my advanced troubleshooting guide which covers more in-depth problem solving of issues with fuel flow and starvation, the carburetor, ignition (electrical) system, choke, valves and compression.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
How do I check for a spark on a lawn mower engine?
Follow the procedure mentioned in my article:
You may need an assistant if the plug can't be seen while pulling the cord.
1. Find a shaded location.
2. Remove the plug.
3. Attach the lead back onto the plug.
4. Hold the plug threads against the engine block. (An assistant could hold it with a plastic clothes peg)
5. Don't touch the lead or the plug!!
6. Pull the starter cord.Helpful 25
If I put starting fluid in the carburetor and it still does not start, can you pass over the carburetor in troubleshooting as a none starting problem?
You don't have to troubleshoot in sequence. If you want you can check the spark plug, connections, air filter, etc. first. The important thing is to check or change one thing at a time and see the results.Helpful 20
I replaced the magnetic pick up in my mower, but there is still no spark. What can I check next?
Replace the spark plug, check the ground of the pickup or that the ignition coil is making contact with the engine block. Also, check the spark lead and make sure the kill switch isn't shorting out the primary when the throttle is set to the run position.Helpful 15
© 2015 Eugene Brennan