Basic Small Engine No Start Diagnostics

If you have ever woken up and decided to cut grass, only to have your lawn mower not start, you can probably appreciate how frustrating this feeling can be.

For some, no start diagnostics can seem very confusing, but like most things mechanical, if you break it up into parts, finding and diagnosing the problem is much simpler.

This article describes the basic no start diagnostics that should be preformed on a small engine.

The images(which will be added shortly) are of a Briggs and Stratton Engine, which is one of the most common types of small engines used on lawn mowers, but the principals can be applied to other types of small engines.

What is the Purpose of No Start Diagnostics

When preforming small engine no start diagnostics, you are checking the three main systems that make a small engine work. Specifically, you are checking for: compression, fuel, and fire.

Small engine no start diagnostics helps pinpoint which system is malfunctioning, as well as discovering some simple problems.

Important Safety Concerns

  • Before beginning, check your fluids, making sure that there is sufficient oil and gasoline.
  • Engines can be very dangerous and has many moving parts.
  • Pay special attention to the blades of the lawnmower and ensure that you are not at risk for cutting yourself.
  • When preforming tests on the engine, remove the spark plug and spark plug wire. Even though the engine isn’t working right now, one of the tests could cause the engine to unexpectedly start, causing injury.
  • When checking the spark plug, be careful, as it can provide a minor shock. You could also cause damage to the ignition system if done improperly.

Tip: Find the Engines Diagram

You should be able to find a diagram of how your lawnmower and engine work online. Locate the engine code, which is usually on the flywheel cover, as well as the model number of the lawnmower itself. Find the diagram, so you can see how the different systems are laid out on your lawnmower.

Compression System

The piston on an engine moves up and down as the motor runs. Compression is caused when the piston in the engine moves upwards, compressing a mixture of gas and oxygen. A typical engine requires 60 psi of cranking compression, but to check for this, you would need a special too.

Instead to start with, you can usually assume that if the engines flywheel is spinning, you have compression. The exception to this rule is in situations where you may have caused significant damage to the engine, such as running it for an extended period of time without oil.

First, remove the spark plug wire and spark plug. This will prevent the engine from accidentally starting.

First, try to manually spin the flywheel. You should be able to spin it freely, but not necessarily easily. If you pay attention, you can often feel and hear the compression.

Next, try to start the motor and make sure that the flywheel turns freely. For those with an electric starter, turn the engine over for about 5 or 10 seconds. Those with a pushing lawnmower can use the pull cord. In both types of lawnmowers, the flywheel should turn easily.

If the flywheel turns much more harder than before, does not turn over as easily as you remember, or simply does not move, this could indicate a problem with compression.

As stated above, just because the engine is turning over doesn’t mean you have compression for certain, but, barring a major engine problem, this is a safe assumption at first.

Fuel System

During compression, gas and air is compressed by the piston. This gas and air forms a fuel that is required for the engine to function properly. It is necessary to check that the engine is receiving both of these fuels.

Check to make sure that the air filter is clean and that the air can freely flow from the air filter to the engine. A dirty air filter or clogged air intake will prohibit the engine from starting. You can often clean a dirty air filter by gently banging it against a piece of wood.

Next, check and make sure the small engine is receiving gasoline. Gasoline should travel from the gas tank, through a fuel filter, into the carburetor, and to the engine. The carburetor is used to limit the gas flow to the engine. The compression of the engine sucks the gas through the carburetor, which can then regulate the flow with the throttle.

Check that the fuel is good and has not been contaminated with water. Gasoline that is over a year old can go bad. It is also necessary to check that gas is freely flowing from the gas tank to the engine. Starting at the gas tank, follow the fuel lines to the fuel filter and then the carburetor. Check that the fuel lines are unclogged and that the fuel filter is clean.

Next, preform a preliminary check of the carburetor. Inspect the throttle to make sure it is not stuck. Then, after turning the engine over for 5 to 10 seconds, remove the air filter and check to see if fuel is visible carburetor. You can also check where the spark plug is placed for gasoline. However, never try to start an engine with the air filter removed. You may want to ensure use some aerosol carburetor cleaner, especially if the engine hasn’t been run in awhile.

Fire(Ignition System)

Fire is the last ingredient required to make a small engine work and is controlled by the ignition system. The ignition system is responsible for causing a spark in the spark plug, which causes combustion. Without the spark, the gas in the engine will not ignite.

First take the spark plug out and inspect its condition. If it is very dirty and old, it may need to be replaced.

Next, check to make sure that the spark plug is actually creating a spark as the engine turns over. There is a special tool available to check for a spark, but there is another simple way to test for a spark. However, you must be careful not to shock yourself or damage the ignition system.

Remove the spark plug and reattach the spark plug wire. With the spark plug out of the engine and the spark plug wire attached, position the end of the spark plug against a grounded piece of metal on the engine. It is important that the spark plug is close to the metal, as if it is too far it can damage the solenoid.

Now, turn over the engine and watch for a spark to jump between the spark plug and the grounded piece of metal. If you have an electric starter, activate it and you should see a rapid spark. If you have a pull cord, you may need someone to pull and someone to watch for the spark.

If you have to physically hold the spark plug in place, make sure to use a pair of pliers with insulated handles to avoid shock.

Understanding the Results

Hopefully, by checking the three main systems, you will have been able to have pinpointed which system is causing the problem and possibly the cause. It is important, however, to remember that these three systems all work together, so a malfunction in one could cause a problem in another. For example, if you do not have compression, than this could cause the fuel system to malfunction as well.

If all systems appear to be functioning, you would begin further breaking down the systems. For instance, the next thing you would need to check on the ignition system would be the timing or the engine, which controls when the spark plug fires. It would also be a good idea to preform a more thorough test of the compression system.

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