Lawn Mower Won't Start? - Top 10 Small Engine Troubleshooting Tips

1. Use fresh petrol (gasoline). Old petrol can cause difficult starting. Make sure there is enough fuel in the tank and check the vent in the tank cap isn'tclogged
2.  The spark lead should be firmly attached to the plug, and the plug tightly screwed into the engine block. Try replacing the plug with a new one if the engine won't start
3. Most modern mowers have a control handle which needs to be pushed forwards/upwards and held in place to keep the engine running. Make sure this control is mower is held fully against the mower handle while starting.
4. If a manual choke is fitted, turn it on. Don't turn on the choke if the engine has been running in the last 10 minutes
5. If the engine has a primer bulb, press it about 5 times. If the mower runs out of petrol during cutting, it will need to be primed again
6. Check the air filter isn't dirty. Wash and dry foam type air filters in warm,soapy water. A paper filter can be blown out with an air compressor, but will eventually need to be replaced
7. Check the cable connecting the "dead man's handle" (the control which keeps the engine running when held in place) to the engine is not damaged or snapped
8. The carburettor should be tightly screwed or bolted to the engine
9. Make sure there isn't any water collected at the bottom of the petrol tank. If there is, drain the tank and use a piece of stick or long screwdriver with a piece of rag tied on (with a twisty tie) to soak up water
10. If the starter cord is hard to pull, check the underside of the deck and remove any clumps of grass which may be jamming the blade. Disconnect the spark lead before attempting to move the blade to remove clippings!

    

    Paper air filter

    
    

    

    Foam air filter

Why Won't my Lawn Mower Start?

Lawn Mower Won't Start?

As the days lengthen, and temperatures rise, the grass cutting season isn't far away. So you drag your mower out of storage over winter, fill the tank and pull the cord..., and pull it again....and again....and of course nothing happens - Well that's typical!

If you leave fuel in the tank over winter, the carburetor can get gummed up with varnish deposits, jets get clogged and small delicate parts can seize. Ideally you should run the engine dry before overwintering and/or use a fuel stabilisation product such as Stabil.

Engines need fuel and a spark to run, plus good compression (the engine needs to hold pressure when mixture is compressed)

Checking the Spark Plug

First check the lead is pushed onto the plug properly and this wasn't the cause of non-starting. Move the mower to the shade so that it's easier to see a spark, remove the spark plug and re-attach the lead. Leave the threaded part of the plug in contact with the engine block and  pull the starter cord. The chances are that you won't be able to see the plug from your vantage point behind the mower, so an assistant may be able to aid you by pressing the plug against the block (using a plastic bottle or whatever to avoid shock if the plug lead is damp). The spark should be bright and blue.

When replacing a plug, make sure there's nothing on the threads such as dirt or grit, and make sure it isn't cross threaded by screwing in by hand before tightening with a plug wrench. If the plug is cross threaded, it will be difficult to turn by hand.

Fuel  Starvation

This means the engine isn't getting fuel. On engines with the tank above the carburetor, fuel first flows to a device called a float bowl. This is like a toilet cistern and prevents fuel just running into the carburetor. The bowl fills and once its full, a float (which functions just like the ballcock in a toilet cistern) cuts off flow. Sometimes the intake to the bowl can get clogged or the filter in the fuel line can be blocked, either of which can cause fuel starvation. When you remove the bowl, petrol should flow freely and stop flowing when you push up the float (use a jar or tin to catch fuel).

Another cause of fuel starvation is a clogged main jet. You can get carburetor cleaning aerosols which have a straw just like on a can of WD40. You insert the straw into the jet and press the button on the aerosol, the spray flushes the jet. If you have an air compressor, you could try using the a blow gun attachment to blast air through the jet. Avoid poking nails, wires or other objects through it, which could damage the jet which is made from brass.

Compression

Check the plug is screwed in tightly. Don't over tighten as you can damage the threads.

Read my full version of this blog post on DenGarden.com. It has lots more info on lawn mower troubleshooting with loads of pictures and goes into much more detail:

 

https://dengarden.com/landscaping/Why-Wont-That-Lawnmower-Start