A lawnmower can be a significant expense so it pays to be able to troubleshoot the mower when it's not working. Even more important is to do routine maintenance.

I had a mower that was just two seasons old and it was hard to start when it got hot. I learned a lot about modern lawn mowers. They have changed since I was in high school. For a number of years I had a small lawn so I used a rechargeable.

Engine Essentials

For a gasoline powered mower to operate you need four things: air, fuel, spark, and compression.

An engine works by burning a fuel, gasoline. Not it's not the liquid gasoline that burns but a mixture of gasoline vapor and air (specifically the oxygen in the air), this is called combustion. The chemical reaction of a hydrocarbon and oxygen producing carbon dioxide and water (OK there's more to it but this will suffice).

This chemical reaction needs some help to get started. You need an ignition source or spark. Your spark plug is the ignition source that starts the fuel to burn.

Compression happens when you squeeze or compress the air and fuel into a smaller space. This helps produce power by making the fuel burn faster in a small space thus expanding and producing a force that we can use.


So for you engine to work you need all four of these, air, fuel, spark, and compression. You need to figure out what is missing and why.


Check to see if you air filter is dirty. It will either be a block of foam or a paper element. Go ahead and replace it, don't try to clean it.


Is there gasoline in the tank? Make sure the lines are connected and not leaking. Make sure the rubber priming bulb is not torn. The primer squirts extra fuel to get the engine going when it's cold.


Remove the spark plug and have a helper pull the cord on the mower to turn the engine over. While the cord is pulled, put your thumb over the spark plug hole. You should feel air pushing out. If not, you have a problem that is bigger than you can fix.


Take a look at the spark plug you removed. Is it oily or caked with carbon soot? Go ahead and replace it.

Here is where most engines have trouble. The spark plug is not getting voltage to make a spark. You need to check to see that the magneto is working.

Digging Deeper - Electronic Ignition

New engines have solid state ignition. Old engines had points, a type of mechanical switch that was prone to wear and corrosion. New engines have electronic ignitions that have no moving parts but they are more expensive.

With the spark plug out of the engine, connect the spark plug to the spark plug wire. Now with a pair of insulated pliers hold the spark plug by the white insulator and touch the threaded metal part to the metal of the engine. Hold it so that you can see the gap of the spark plug,

Now get your helper to pull the cord again. You should see a spark jumping the gap. You may need to turn out the lights.

If there is no spark, you magneto is probably bad. It will probably cost around $40. You will have to remove the engine cover to get to the flywheel. Remove the nut for the flywheel and it should come off without too much trouble. A light tap with a hammer usually does the trick.

The magneto is a big chuck of metal with the spark plug wire attached. The new magneto will have instructions for setting the spacing between the flywheel and the magneto.

Git 'Er Done

Put it all back together and let 'er rip.

If your mower has a clean air filter, full tank of gas, good compression, and new spark plug that is getting spark it should run. If not it's must be the carburetor that is gummed up. You did drain the gas from last season … right? At this point you need professional help.