Small gas engines serve us in many ways. They power lawn mowers, tillers, cultivators, trimmers, edgers, snowblowers, chain saws, pumps, generators, air compressors, and other useful home tools. They also power our fun: outboard boats, snowmobiles, motorcycles, all-terrain vehicles, ultralight aircraft, and other toys. To keep them operating efficiently, an owner of these tools and toys should know about small engines: how they work and what to do when they don’t.
Small gas engines are made up of individual systems that work together to produce power. Each system has many components. Internal combustion gasoline-powered engines require six systems: fuel, exhaust, ignition, combustion, cooling, and lubrication. In this article, we will discuss the systems and components that make small engines work.
Fuel and Exhaust
The fuel and exhaust systems are critical to operation. They furnish the fuel for combustion and remove exhaust gases. The following are components of a fuel and exhaust system.
Gasoline: Gasoline is a combustible liquid that burns relatively slowly. However, when sprayed as a mist and mixed with air, it is quite explosive. All it needs is a spark. Two-stroke engines require that oil be mixed with the gasoline to lubricate internal parts. Four-stroke engines use a fuel-air mixture.
Fuel Tank: The fuel tank stores fuel in preparation for mixing by the carburetor and use by the engine. Some fuel tanks are pressurized with air to help deliver fuel to the carburetor. Other tanks are non-pressurized and depend on a fuel pump to deliver fuel to the carburetor.
Fuel Line: Fuel is moved from the tank to the pump and/or carburetor through a fuel line. Pressurized fuel systems often have a squeeze bulb in the fuel line for building pressure.
Filter: A carburetor jet has a small opening that can easily become clogged. A fuel filter traps dirt and sediment from the gas before it is delivered to the carburetor.
Pump: A fuel pump produces a vacuum that pulls the fuel from an unpressurized tank, then delivers it to the carburetor.
Carburetor: The carburetor has one job: to mix the correct proportion of gasoline and air for the engine. Too much gasoline in the mixture makes it rich; too little gas makes it lean.
Throttle: The throttle controls the amount of fuel-air mixture that enters the engine from the carburetor. The throttle thus controls the speed of the engine.
Primer: A primer injects a small amount of gasoline into the carburetor throat to make the initial fuel-air mixture rich. A primer is used to help start a cold engine.
Choke: Some engines control the richness of the fuel-air mixture at startup by controlling the air rather than the fuel. A choke reduces the amount of air in the fuel-air mixture.
Governor: A governor is a device that automatically opens the engine’s throttle when more power is needed and closes it when the load is light.
Muffler: Small gas engines, especially two-stroke engines, are noisy when they operate. A muffler reduces the sound of the exhaust gases by passing them through baffles.
Spark Arrestor: A spark can exit the exhaust port of a small gas engine, potentially starting a fire on nearby combustibles. A spark arrestor on the exhaust port can reduce the chances of such a fire. Spark arrestors are especially important on chain saws, motorcycles, and all-terrain vehicles operated in dry woodlands.
Ignition
The ignition is a primary system within all small gas engines. It produces and delivers the high-voltage spark that ignites the fuel-air mixture to cause combustion. No spark means no combustion, which means your engine doesn’t run. Below are the components found in small engine ignition systems. Some systems will include breaker point ignitions while others depend on solid-state ignitions.
Magneto-Powered Ignition System: A magneto uses magnetism to supply electricity in ignitions where there is no battery. The magneto is turned by the crankshaft, which rotates when the manual recoil starter is pulled. The three types of magneto ignition systems are mechanical-breaker, capacitor-discharge, and transistor-controlled.
Battery-Powered Ignition System: If your small engine includes a battery for starting, the ignition coil will also use it to supply spark to the spark plugs. A battery stores electrical energy until needed. Battery ignition systems also use mechanical-breaker, capacitor-discharge, and transistor-controlled ignitions.
Mechanical-Breaker Ignitions: High-voltage electricity must be sent to the spark plug at the appropriate time. In mechanical-breaker ignitions, this job is performed through the contact points and a condenser.
Points: As the crankshaft rotates, a cam opens and closes a set of contact points. These points function as an on/off switch: Closed is on, and open is off.
Condenser: Because the spark moving across points can damage their surfaces, the condenser stores voltage to reduce arcing between points.
Capacitor-Discharge Ignitions (CDI): A capacitor is a large condenser. A CDI stores and delivers voltage to the coil using magnets, diodes, and a capacitor
Transistor-Controlled Ignitions (TCI): Transistors are electronic controllers. A TCI uses transistors, resistors, and diodes to control the timing of the spark.
Coil: An ignition coil is simply two coils of wire wrapped around an iron core. The coil changes low voltage (6 or 12 volts) into the high voltage (15,000 to 30,000 volts) needed by the spark plug.
Spark Plug: A spark plug is an insulated electrode that is screwed into the top of the engine cylinder. High-voltage timed electricity from the magneto travels by wire to the spark plug. The base of the plug has an air gap of about 0.030 inch (30 thousandths of an inch), which the current must jump.
Wires: The primary wire from the coil to the breaker point and secondary wire from the coil to the spark plug(s) deliver electricity to the ignition components.
Distributor: A distributor is an ignition system for engines with more than one cylinder and spark plug. It distributes the spark to the appropriate cylinder using a rotor, cap, and individual spark plug wires.
Combustion
The combustion system of a small gas engine is where the work gets done. Components of the combustion system include the cylinder block, cylinder head, camshaft, valves, piston, connecting rod, crankshaft, timing gears, and flywheel. To better understand small gas engines, let’s look at how this vital system works.
Cylinder Block: The largest single part in a small gas engine is the cylinder block. It is a piece of metal in which the cylinder hole is bored or placed.
Cylinder Head: The cylinder head is the top, or ceiling, of the cylinder and is attached to the block with bolts. Depending on the type of engine, the head may or may not include valves.
Piston: A piston is the movable floor in the combustion chamber. Its upward movement compresses the fuel-air mixture. After combustion, its downward movement rotates the crankshaft.
Crankshaft: An engine’s crankshaft is a metal shaft with an offset section onto which the connecting rod is attached. Rotation of the crankshaft moves the piston up in the cylinder. Movement of the piston down in the cylinder then rotates the crankshaft.
Connecting Rod: Between the piston and the crankshaft is a connecting rod. At the larger end of the connecting rod is a bearing that allows rotation around the moving crankshaft. The small end is attached to the piston pin.
Valves: Valves simply open and close passages. A reed valve in a two-stroke engine is activated by changes in air pressure.
Flywheel: At the end of the crankshaft is a circular weighted wheel called a flywheel. The flywheel delivers the engine’s power to devices (wheels, blades, etc.) and helps keep the crankshaft turning smoothly.
Cooling and Lubrication
Combustion and friction produce heat. Heat and friction — if not controlled — can quickly damage an engine’s components. Small gas engines are typically cooled by air. Friction is reduced using movable bearings and lubricants.
Air-Cooling Fins: For simplicity, most smaller gas engines are cooled by air. Metal fins around the outside of the combustion chamber help dissipate the internal heat.
Friction: Friction is resistance that occurs when one surface rubs against another. Friction causes wear. In an engine with many moving parts, friction is reduced with bearings and lubricants.
Bearings: A bearing is a replaceable part that takes the brunt of the friction. A friction bearing relies on lubricants to minimize friction. A nonfriction bearing uses hard steel rollers or balls to prevent wear, though it too requires some lubrication.
Lubricants: Lubricants such as oil and grease reduce surface friction by coating parts with a film. Lubricants in two-stroke engines are applied to surfaces by mixing oil with fuel.
Viscosity: An oil’s viscosity is its resistance to flow. The thicker a lubricating oil or grease is, the higher its viscosity number.
Filters: Friction happens. Moving parts wear, even with the best lubricants. The resulting metal as well as carbon from the combustion process must be cleaned from the oil to ensure long lubrication. Some small engines use oil filters to remove contaminants from the circulating oil.
Regularly servicing your small engine will ultimately save you money and time. In the next section, we’ll review how, where, and when to service this engine.
The Benefits of Regular Small-Engine Maintenance
Purchasing a small engine-driven implement can make a dent in your budget. Tools and toys powered by small engines can cost anywhere from $100 to $10,000. That’s why it’s a good idea to invest in periodic servicing of your small engine. Replacing an engine every couple of years is an annoying and needless expense. Below we will review detailed information on how to service two-stroke gas engines. Following these procedures could help you put more money in the bank and less into your mechanic’s pocket.
Benefits of Regular Service
Servicing your small engine tool or toy on a regular basis offers many advantages over the Wait-Until-It-Breaks Maintenance Program.
- By establishing a service schedule, you will gain confidence that whenever you need the unit it will be ready for use.
- By performing a number of service functions together, you will save time. You can pick up all needed parts and lubricants in one trip to the parts store. Then you need to disassemble a component only once to perform numerous service procedures rather than taking it apart many times.
- Regular service gives you a chance to visually inspect the entire engine and related components for damage, wear, and other potential problems.
How, Where, and When to Service Knowing how to service is as important as knowing when. Some service procedures can be performed wherever you store your tool or toy: in a garage, storage shed, or tool shed. If the unit is heavy, you can build a ramp up to a sturdy table that is at a handy height for working. Or you can use a ratchet winch to lift the engine. Units weighing less than 40 pounds may be lifted to a workbench or table as long as you lift with your legs rather than with your back. Get help if you need it, and make sure that the unit will remain sturdily in place as you service it. Remember to always put safety first!Servicing a small engine is easy once you know what to do and when to do it. A service chart can help you determine common service requirements as well as track what service has been done. Your engine-powered unit may have a service chart in the owner’s manual or service manual. Typical recommendations include changing engine oil every 25 hours of use and tuning up the engine at least once a year.The purpose of ongoing service, also known as preventive maintenance, is to keep your engine-driven tool or toy in good operating condition. Ongoing service procedures include air cleaner service, crankcase breather service, cooling system service, muffler service, lubrication, and tune-up.Lubrication service means making sure that all moving parts have sufficient lubrication (oil and/or grease) to minimize wear. Lubrication service procedures include mixing oil with fuel in two-stroke engines, and lubricating other moving parts.A tune-up consists of the adjustment and/or replacement of parts critical to smooth and efficient engine operation. Those parts include components in all engine systems: fuel, exhaust, ignition, combustion, cooling, and lubrication. Ignition tune-ups are more important for mechanical-breaker ignitions than they are for self-contained solid-state ignitions. Regular tune-ups will keep your small engine running smoothly and reduce the need for repairs.In addition, you should check other systems and make adjustments as needed to keep them operating smoothly. This includes adjusting the throttle, choke, and governor linkage, and cleaning off debris.Engine-driven tools and toys usually come with an owner’s manual. While some manufacturers’ manuals are more complete and better written than others, most manuals include basic information on safe operation and service. Unfortunately, product manuals are often written to reduce the manufacturer’s liability for accidental misuse rather than to help the owner service the product. In addition, manuals for engine-driven products typically show how to service the nonengine components: the grass catcher, wheel adjustments, blades, chains, and other parts. Service information for the engine may be minimal or nonexistent in the owner’s manual.
What can you do about this lack of information? Fortunately, there are numerous after-market publishers of service manuals for specific models of small engines. If you don’t have an owner’s manual, you can contact the manufacturer directly to purchase one; manufacturers also sell service manuals. Most manufacturers keep product manuals for up to 20 years. If they only have one original copy left, you can often request a photocopy for a small charge.
Knowing how to service the fuel system is an important part of caring for a small engine. Learn how to care for fuel filters, carburetors, and other major fuel system parts in the next section.
Grease n Gasoline 
