Lawnmower gasoline engines are pretty basic setups so repairing them normally only takes a few basic tools and a little time and money.
When you are diagnosing a small engine with a no start condition there are three basic things that can cause this:
Air / Fuel Mixture
Electrical or Spark
Compression
The first two problems are the least expensive and easiest to fix. If you have a problem with low compression in the cylinder then this can point to physical damage or wear that may require a more expensive fix or total engine replacement.
The main way that I diagnose the difference between an Air / Fuel problem and an Electrical Problem is by first checking if there is any gas in the tank. Also is the gas good.. meaning that it hasn’t sat in your shed for a few seasons. Normally even gas that sits over the winter will start your engine if it was in a sealed container. If it sits longer than a year you might want to be skeptical about its strength and don’t test it with a match .. bad gas will still ignite.. its just not strong enough to let the engine run on it.
If you have gas and there aren’t any leaks around the carburetor / air filter then the next thing to do is check the air filter for extreme blockage. The air filter can be blocked with dirt and debris or it can be blocked with oil or even moisture. For the reason of testing clean around the carburetor so there isn’t any dirt and then just remove the air filter and try starting the engine.
If its not air or gas then the next thing to check is spark.
Diagnosing no spark on a Lawnmower engine
There are two different types of ignition and electrical systems on lawnmowers. There is the battery driven and the engines without batteries. If you are pulling a start cord you don’t have a battery.
If you have a engine that is rated under or about 20 horse power your spark is probably generated by what is called a Magneto. If you have a larger engine you may have actual coils like your car and a crank positioning sensor. Coils are not common on consumer grade equipment but they are out there on some engines so keep that in mind.
A Magneto is what supplies electricity to your spark plug. Although your gasoline engine might have a battery and starter the starter only turns the engine to induce the magneto just like you would if you had to hand pull start the engine.
Magnetos work by electrical induction. This is where a magnet that is placed on your flywheel passes by the magneto as the flywheel turns and the magnet causes electricity to be induced into the spark plug.
As you pull start the engine only a few rotations and sparks are necessary to start the combustion process which will take over and keep the engine turning. As long as the magnet is passing by the magneto it will cause a spark and no outside alternator and coil is needed.
It is a great way for small engines to get spark because it means they can run in pretty harsh conditions and a battery is not required.
So, how do you diagnose a bad spark?
The first thing you want to do before you are pulling your engine covers off is pull the spark plug and reattach it to the spark plug wire. Then place the bent L shaped electrode against the metal housing of the engine and see if you can see a spark when you pull the start cord or hit the key.
If you do see a spark it could still be a problem where not enough spark is being made to start the engine. This happens if you have oil on the spark plug or if the gap between the Ground L and center Electrode of the spark plug are too wide or eaten away.
If your spark plug is really wet and oily it will point to internal engine damage and you already should have seen signs of engine damage by blue gray smoke coming from your exhaust for some time.
If plug is dry but the ceramic cone is cracked or the gap on the spark plug is set wrong you should really just replace the plug. You can reset gaps by bending the L shaped Electrode with feeler gauges and a tool but a plug that is worn to that point is probably very old on a small engine and you might as well spend the $3 and get a fresh plug which is pre-gapped at the factory.
Reinstall the plug and see if the engine starts with the regapped or new plug.
If you don’t have a spark at all then your magneto could be bad. They don’t normally go bad but they can. It is not a part that you would normally replace like an air filter or spark plug but if your engine is about 10 years old it could be a magneto.
Unlike automotive engines the wire to the spark plug is normally attached to the magneto and is not replaceable if you have damage to your plug wire you will probably need to replace everything.
The cost of a Magneto is not that bad you can expect to pay $20 on push mowers to $50 on lawn tractors. If you have a Twin Cylinder engine you will have two magnetos and you should replace both of them at the same time if the replacement is due to age and not due to physical damage.
If you need to replace the magneto there are normally only two bolts that hold it in place but when you replace the magneto you must make sure to have a proper gap between the engine flywheel and the magneto fins. Some people place a business card between the fins and the flywheel before they tighten the bolts and this should work however it is best to read the directions or your service manual and set the gap with feeler gauges.
Replace everything and start your engine you should be ready to hit the lawn.
Final Note
Diagnosing engine problems is pretty easy if you follow basic steps and precautions. You should NEVER try to start your engine with the blade on and the lawn mower in any position other than its normal operating position. Never raise the wheels off the ground to take a look.. never place it on the side and pull the starter cord with a spark plug installed or the magneto wire not pulled and then covered with a rubber spark protector.
Just never take the chance because at every professional shop there are people hurt while working on lawnmower repairs.
Pulling the spark plug wire should be second nature. You should do it every time you work on your engine unless you return the mower to standard running position with all guards and safety devices in place.
If in doubt take it to a repair shop or many shops can come to you.
In other how to guides we will cover other no start conditions for lawnmowers and small engines.