Saturday, 27 February 2016

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.


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 It has lots more info on lawn mower troubleshooting with loads of pictures and goes into much more detail:

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