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How Does A 4-Cycle Engine Work?

by Jeremy HIPA 31 Jan 2023 0 Comments

Small engine won’t start, runs unevenly, or won’t stay running? This blog provides information on how an four-cycle engine works and offers troubleshooting tips to assist you in diagnosing and repair.

While four-cycle engines can be used for different applications, engine operation will remain the same. In this blog we will address how a four cycle engine works as well as potential problems you may encounter.


Similar to an automobile engine, a four-cycle engine runs on gasoline and has a separate sump for oil. Large engines will have an electric start feature that allows the engine to be turned over by using an ignition key or a button. Smaller engines require the use of a starter rope, as the rope is pulled, the starter engages the drive cup on the flywheel and rotates the crankshaft. The rotating crankshaft connects to the piston which moves up and down within the cylinder and the ignition process begins.

The flywheel has permanent magnets built into it and as it rotates past the ignition coil, a magnetic field is created. The magnetic field induces electricity allowing the ignition coil to send voltage to the spark plug, meanwhile the intake valve opens. As the piston travels down the cylinder creating a vacuum which draws fuel and air through the carburetor where it mixes before entering the cylinder. This action is called the intake stroke of the engine.

Next, the compression stroke takes place as the piston travels back up to the top of the cylinder and the intake valve closes. The spark plug now fires and ignites the compressed fuel and air mixture which forces the piston down, creating the power stroke. The momentum of the spinning flywheel provides enough force to push the piston back up. The exhaust valve opens and the combustion gases exit through the muffler. This is called the exhaust stroke.
The engine will continue to run and repeat these four-cycles until it is shut off.

Choke and Primer Bulb

Some engines may have a choke or primer bulb on the carburetor to assist with starting, especially when the engine is cold. The choke will temporarily restrict airflow through the carburetor, so more fuel can enter the cylinder. Likewise, a primer bulb draws additional fuel through the carburetor which enters the cylinder before the engine is started.

If the engine does not start or it runs improperly, the most likely cause is a defective spark plug or a restricted carburetor. The carburetor can be cleaned but it often needs to be replaced.


Be aware that four-cycle engines are designed to use gasoline with no more than 10 percent ethanol. Gasoline with higher levels of ethanol can be corrosive and attract water which can cause starting or running problems and may damage the fuel system.
You should store the gasoline in a clean sealed plastic container approved for fuel storage. If equipped, close the vent when not in use and store the container away from direct sunlight. If you anticipate storing the fuel for longer than three months, consider adding a fuel stabilizer when you fill the container.

Hipa Repair Center has a solution for many of the problems you may be experiencing with your lawn and garden equipment. Enter the product’s full model number in our website search engine for a complete of compatible parts. also has an extensive selection of instructional blogs to assist you covering topics like part replacement. At, we make fixing things easier.

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Please let us know if this works and if you have any suggestions or comments. Or you can join HIPA DIY COMMUNITY to feature your passion for repair projects, share your stories with the Hipa family and get help from

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