How to Tune-Up Your Gas-Powered Lawn Mower

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INTRODUCTION  

The engine that powers your lawn mower is constructed in the same fashion as the engine that powers older cars. It is normally aspirated, which means it has a carburetor that delivers fuel to the cylinder by means of an intake valve. 

Like all mechanical devices, it takes proper care and scheduled maintenance to keep this equipment in top operating order. The thought of maintaining the engine can be intimidating to some people, but, with my guidance and this article, you will see how simple it is to properly maintain your lawn mower. 

If you have the ability to follow simple instructions, some mechanical inclination, and a few hand tools, you will be able to maintain your gas-powered lawn equipment. I will walk you through the process with the help of this guide so your mower will run at peak performance all season long. 

Note: Because rototillers and small lawn tractors are powered by the same type of engine as your lawn mower, you can use these instructions to tune up those pieces of equipment as well. For ease of reading, however, we will reference only lawn mowers in this guide. 

4-STROKE GAS ENGINES

There are two types of 4-stroke gas engines: the horizontal shaft and the vertical shaft. The horizontal shaft engine is so named because the crankshaft runs horizontally through the engine block. This type of engine is most widely used to power equipment such as rototillers and other equipment where the device it powers isn't directly connected to the crankshaft.

The vertical shaft engine is so named because the crankshaft runs vertically though the engine block. This type of engine is most widely used to power lawn mowers. 

In either case, the operation of the engine is the same. They're called 4-stroke because the piston makes four complete strokes per cycle. The four strokes are:

• intake,
• compression,
• power, and
• exhaust. 

Now, let's examine the components that make up a typical 4-stroke gas-powered engine. 

COMPONENTS OF A 4-STROKE GAS ENGINE  

Crankshaft 

The crankshaft is the primary shaft connecting many of the components. It's the large shaft that runs through the engine block and has the blade, connecting rod for the piston, and flywheel directly connected to it on your lawn mower. It also indirectly turns the camshaft. 

Camshaft 

The purpose of the camshaft is to raise and lower the intake and exhaust valves. The camshaft has a gear at one end that engages with the gear on the crankshaft. When the camshaft is installed at the factory, timing marks located on the camshaft and crankshaft gears are aligned to set the engine timing.

Proper timing is necessary so that both the intake and exhaust valves open and close at the correct time, allowing smooth operation of the engine. 

Valves

A typical 4-stroke engine has two valves: the intake valve and the exhaust valve. A lifter that sits on the camshaft operates each one. There's a gap between the lifter and the valve stem on each valve when they are closed to allow for proper timing of the valves during the cycle. If you were to remove the cylinder head, you would be able to identify these valves quickly, as the exhaust valve is larger than the intake valve.

The purpose of the intake valve is to allow the air-fuel mixture to enter the combustion chamber during the first stroke of the cycle. The exhaust valve opens in the fourth stroke of the cycle to allow the combustion gases to be exhausted from the combustion chamber and exit via the muffler.

Connecting Rod and Piston 

The connecting rod and piston deliver power to the crankshaft. The connecting rod is attached to the crankshaft with a bearing. It also has a device called an oil slinger attached; this looks like a small paddle and distributes oil from the crankcase to lubricate the moving parts of the engine.

The piston is attached to the connecting rod. It's housed in the combustion cylinder and is surrounded by two compression rings and one oil ring. The compression rings maintain compression in the cylinder, and the oil ring wipes excess lubricating oil from the cylinder wall during the intake and power strokes. 

Spark Plug 

The spark plug delivers a high-voltage spark to the compressed fuel-air mixture during the power stroke. It's located in the cylinder head and is connected to the ignition coil by a high-tension wire. Gapping the spark plug properly—creating the gap between the electrode and the wire—is essential for delivering the maximum voltage, which is used to ignite the compressed fuel-air mixture.

Carburetor 

The carburetor draws fuel from the gas tank and mixes it with the proper amount of air for combustion. A needle valve, located in the carburetor meters the amount of air that is allowed to mix with the fuel. Turning the needle valve clockwise causes the mixture to become lean. Turning the needle valve counter-clockwise causes the mixture to become rich, meaning you will be burning more fuel and less air. The needle valve must be adjusted properly for best performance. 

An air filter is attached to the carburetor intake to keep contaminants from entering the fuel-air stream, which could damage the engine. 

Ignition Coil 

The ignition coil delivers high voltage to the spark plug. As the flywheel rotates, magnets attached to the flywheel pass by the ends of the ignition coil, energizing it. At the same time, the points close on the condenser, causing the circuit to close and allowing a high-voltage charge to be delivered to the spark plug. 

Points and Condenser 

The points and condenser are located under the flywheel and are protected by a metal cover that keeps oil and dirt from entering the chamber. A plunger that makes contact with the crankshaft activates them. As the crankshaft rotates, a small cam comes in contact with the plunger, causing it to be pushed out of the opening and forcing the points to open. A spring attached to the points keeps tension on the plunger.  

At the proper timing, the plunger retracts back into the opening and the points come in contact with the condenser, closing the circuit and allowing the coil to deliver the high voltage to the spark plug. Proper gapping of the points is critical for delivering the maximum voltage for ignition. 

 

 

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