Below are some quick fixes for common problems with petrol lawnmowers that won’t start. If your mower still isn’t working after trying these tips, consider taking it in for service. In most cases, the issue can be traced to one of a few things:
1. Check Your Fuel
Check is whether or not there is gas in the fuel tank. We can all overlook this assuming there is enough gas. A quick check is good.
Also, make sure that there isn’t other debris inside the fuel line. For example, if a spark plug were fouled, oil from an air filter would not have reached it.
A quick way to check for this problem is to look for signs of dirt or debris in the carburetor (the item between the air filter and the fuel line). If there is dirt, oil, leaves, etc., stuck in this area, clean it out so that your mower can start up properly again.
2. The Spark Plug May Be Foul/Too Wet From Over Oil Use
Check to make sure that you have spark: if you don’t have a plug gap tool, just grab two metal rods and slide them together. If they create a spark by touching each other, you should be fine; otherwise, check your owner’s manual for specific instructions.
3. The Air Filter May Be Clogged
As stated before, without proper airflow to the carburetor, your mower won’t start. To check for a clogged air filter or missing/broken air filter housing on lawnmowers under 30cc (2.9 cubic centimeters), put your hand over the vent holes where the air comes in and see if there is airflow. See the guide for changing and cleaning lawnmower air filters here.
4. The Spark Plug May Not Be Securely in Place/Missing or Damaged Wire Connections
If you have checked for spark and there is still no sign of life from your lawnmower (or if it won’t start after trying the above-listed fixes), the next thing you should check is whether or not the bolt connects wire to the plug has loosened.
This can happen easily when moving over rough terrain, so it may need tightening with pliers after every use. If that’s not the problem, take a look at your wiring: make sure that all connections are secure and there aren’t any loose wires.
If you find a wire that has come disconnected, make sure to reconnect it correctly (sometimes the red/positive wire connects to the red/positive terminal and vice versa).
For lawnmowers over 30cc with removable carburetors, check for cracks in the larger air intake hole(s). These are usually located near where the side of your mower meets the back; if you can see the light through these holes, they are most likely cracked.
5. The Petrol Mixture May Be Too Rich or Too Lean
Lawnmower tune-ups may sound complicated, but they only require a few tools and the manual. Follow the instructions in your owner’s manual to adjust these settings, and you should be good to go – this may take some practice as there are several different ways to adjust them based on make/model.
Mower Needs Maintenance
Lawnmowers must be regularly maintained so that they run well and operate effectively. However, many owners neglect regular maintenance because they don’t know what each step entails (or how it is done).
Dispose of old gasoline properly: if you have more than a quarter tank of fuel left in your mower, drain it into another container using an oil-drain hose or by siphoning it out with a plastic hose.
If your mower is empty, disconnect the fuel line and run it until it’s dry before storing it for long periods.
Clean Air Filter
Make sure that you have a clean air filter. When changing these out, make sure to use only high-quality filters recommended by your manufacturer (if none are provided in your owner’s manual) and check your engine’s airflow through the carburetor with your hand before replacing them.
Locate any cracks or holes near where the side meets the back of the mower (in engines over 30cc). If necessary, apply epoxy or duct tape to seal up any leaks.
Check your spark plug and make sure it is securely in place. If it doesn’t appear to be, tighten it back into place with pliers.
If there are cracks in the area around the tip of the spark plug (where you screw on the wire), replace it as soon as possible; this is usually located in one of two places: along the side near where you pull out the cord to start or near where fuel enters your engine.
Make sure that all wires have been attached correctly and are not loose; if a wire has come disconnected from either terminal, reconnect them correctly (red/positive connects to red/positive and vice versa).
Replace anything that looks damaged or worn – most manufacturers recommend replacing the spark plug after every 100 hours of operation.
Make sure that the mower is not leaking fuel; if there are leaks around the carburetor or tank, run them until they dry up before storing them for long periods (if it is too close to winter, try using a silicone sealant to temporarily fix this hole to prevent any fuel from evaporating and condensing inside the engine).
This will also help you check how much gas is left in your tank – just remember not to store gasoline over the summer months!
Remove debris by hosing down your machine with water; make sure no grass clippings have made their way into where you would normally remove the spark plug.
If necessary, use a rag to wipe off any gasoline you don’t want getting on your skin.
If your mower has a pressurized oil tank, make sure to drain the remaining oil into another container before storing it for extended periods (more than two weeks).
It may be necessary to use an extra-long hose if you are unable to drain it.
With non-pressurized engines, remove the oil filter and run them until they are dry; at this point, most lawnmowers will stop running – so if yours does not, disconnect the spark plug first!