Lawn Mower: Won't Start

If your lawnmower won’t start, you could have a problem with the ignition switch. Most lawnmowers have a lever or bail mounted to the handle that is depressed to start the engine. This bail attaches to a cable that attaches to the ignition switch. Lawnmower ignition switches work the opposite of how most would think. When the mower is running the switch is off. When the mower is stopped the switch is on. The switch can be thought of as a kill switch. When the mower is running, the switch is being held in the off position by the bail lever. The ignition coil produces electricity that passes through the plug wire to the spark plug. The engine’s frame is the ground in the circuit, so the electricity passes from the plug to the frame to complete the circuit. When the bail is released while the engine is running, the ignition switch is turned on. The closed switch completes a direct circuit from the ignition coil to ground. Because electricity always follows the path of least resistance, the spark plug gets bypassed and the engine stops. There are a couple of types of ignition switches commonly used on lawnmowers. One type is a micro switch. The micro switch has a plastic body with a post that extends from it. There is a hinged lever attached to the body that when depressed it pushes down the post which turns the switch on. The other type of switch is a much simpler design. It consists of a metal bracket mounted to the mower with a hinged lever that is insulated from the bracket. The control cable attaches to the lever to activate the switch. When the bail is released, a spring pulls the lever in contact with the bracket, turning the switch on. Diagnosing a bad ignition switch is easy to do. If the switch is the micro switch type, a multi-meter can be used to check for continuity and function of the switch. You can also simply remove the wires from the switch. If the mower starts without the wires connected, then the switch is stuck in the closed position and needs to be replaced. The bracket and lever switch can usually be diagnosed visually. Make sure that the contacts fully separate when the switch is opened. If they don’t, you can bend the contacts so they only make contact when bail lever is released. Check the insulator bushing where the lever is attached to the bracket. If the bushing is worn, the lever will be contacting the bracket and the engine won’t start. Replacing or repairing the ignition switch is an easy repair. The switch is usually easily accessible. The micro switch style is usually held in place by a couple of screws. Remove the screws and the wires from the terminals. Install the new switch in the reverse order. The bracket and lever switch can become damaged by low tree branches or shrubs. It can usually be bent back into place with pliers.

If the engine on your lawnmower won’t start, the carburetor could be the cause. The carburetor brings air and fuel together in a perfect mixture for combustion. During a 4-cycle engine’s intake stroke, the intake valve opens and the piston creates suction as it moves down in the cylinder. This suction draws air into the carburetor’s venturi or throat. The venturi is a restriction inside the throat of the carburetor. This restriction increases the speed of the air passing through the carburetor, which creates an area of low pressure, similar to how an airplane wing works. This low pressure draws fuel into the airstream and into the engine. On the bottom of the carburetor is a bowl that holds a supply of fuel in the carburetor. Fuel is metered into the bowl by a float and needle. As fuel is used the float drops and the needle opens allowing fuel to flow into the bowl until the float once again rises and closes the needle. There is a column that extends from the base of the carburetor body down into the carburetor bowl. At the base of this column is the main jet. The main jet is a nozzle that is preset to meter the proper amount of fuel into the engine. Above the main jet is the emulsion tube, which extends into the carburetor’s venturi. The emulsion tube atomizes the fuel and then adds it into the airflow passing through the venturi and into the engine. The carburetor has a throttle plate at the rear of its throat to control the amount of air entering the engine, which controls the engine’s speed. At the front of the carburetor’s throat there is often a choke plate. The choke plate allows the carburetor to draw extra fuel into the engine during a cold start. There is another series of passages in the carburetor’s body called the idle circuit. The idle circuit is made up of small passages, jets, and needles. The idle circuit allows a small amount of fuel to enter the engine when the throttle plate is at idle and when the throttle plate is transitioned from idle to full throttle. During this time little or no fuel is being drawn through the main jet. The most common problem with carburetors is clogging of the tiny internal passages by dirt or other debris. A missing air filter or a hole in the filter can allow dirt to enter the carburetor. A missing fuel filter or a crack in the fuel line will let dirt into the carburetor along with the fuel. Another major problem for the carburetor is damage caused by fuel that contains ethanol. Ethanol is an alcohol that is blended with gasoline. The ethanol helps the fuel to burn hotter and cleaner, reducing emissions. The problem occurs when fuel with ethanol sits unused for a period of time. Alcohol naturally attracts humidity in the air. When water from humidity bonds with ethanol acetic acid is formed. The acetic acid will cause corrosion in the carburetor, which will quickly clog the tiny passages inside. This is especially true in older carburetors, which were produced before the widespread use of ethanol in fuel. These carburetors were not manufactured with coatings and materials resistant to acetic acid. Most engine manufacturers advise that any fuel over 30 days old is considered bad and should not be used. Most carburetor problems are easily fixable. Usually a good cleaning is all that is required to get fuel and air properly flowing again. To begin cleaning your carburetor, remove it from the lawnmower and carefully disassemble its components. Remove the bowl and drain any fuel out of it. Notice that there is a gasket that seals the bowl to the carburetor as well as one to seal the bowl screw. If either gasket is damaged or cracked it should be replaced. Remove the pin that the float pivots on, and remove the metering needle. The metering needle should be inspected for corrosion and wear on the tip. Dirt or corrosion on the tip or the needle seat will prevent the needle from closing completely. This will cause fuel to continue to flow into the bowl and spill out of the carburetor’s throat. Remove the main jet and the emulsion tube. Dirt or corrosion can clog the tiny opening in the jet or the series of holes cross drilled in the emulsion tube. These tiny openings can be cleaned with carburetor cleaner and compressed air. Fishing line can be used, if needed, to help clean debris out of the openings. Never use anything metal when cleaning a carburetor. Metal picks, wires, or other tools can easily alter the size of the openings, changing the amount of fuel that can flow through them. Next is the pilot jet. There will often be a plug or screw on the carburetor’s body that covers the pilot jet. Access the pilot jet and clean it with carb cleaner, compressed air, and fishing line if needed. Once the carburetor has been stripped down, you can clean the carb body and bowl. Use carb cleaner to thoroughly clean each tiny passage in the carburetor. Also clean the bowl, main jet, pilot jet, and emulsion tube with the carb cleaner. Use compressed air to remove any residual carb cleaner from each component. If you have an ultrasonic cleaner in your arsenal of tools it will work great to clean your carburetor. Ultrasonic cleaners do an amazing job of cleaning the tiny openings and passages in a carburetor. Often times an ultrasonic is the only way to save a badly plugged carburetor. Replace any part in the carburetor that was too dirty to clean or that was damaged by corrosion. Carefully reassemble the carburetor after cleaning and reinstall it onto the mower. If you are unsure about disassembling a carburetor for cleaning, you can always just replace the entire carb. It is always a good idea to check the price of a new carburetor before tackling a cleaning project. Some carburetors are very affordable making it hardly worth the time required to clean it versus simply bolting a new one in place.

Your lawnmower engine is made up of several systems that must function properly for the engine to start and run. One of those systems is the compression system. Compression is the pressure inside the cylinder. It is both the pressurizing of the air/fuel mixture in the cylinder before combustion as well as the pressure of the quickly expanding gasses after combustion that drive the piston. There are many parts that contribute to compression inside the engine. Two that often lead to a lack of compression are the piston and its rings, and the cylinder. The piston has a series of free-floating rings around it. These rings seal the piston to the cylinder and control the application of engine oil to the cylinder walls. In proper operation, the piston rings don’t contact the cylinder wall. The rings ride on a thin film of oil between them and the cylinder. This film is amazingly thin, usually only a few molecules thick. Over time, dirt and the breakdown of the oil as it is used will cause wear to the piston rings and the cylinder wall. Therefore, it is important to regularly change the engine oil following the engine manufacturer’s guide lines. Frequent oil changes are cheap insurance to ensure long engine life. If the engine oil is allowed to run low or if the engine is run with no oil at all the piston rings will rub against the cylinder and serious damage will occur very quickly, usually in just a matter of seconds or minutes. Testing for proper compression can be done a couple of ways, both of which require some special tools. The simplest way is with a compression tester. The spark plug is removed from the engine and the compression tester is attached. You then pull the starter rope several times until the gauge stops climbing. The gauge will now show the amount of compression pressure in the cylinder. Normal compression in a lawnmower engine will vary by manufacturer and even by model, so you will need to consult a repair manual for these values. The compression tester won’t work in every case. Some engines have decompression systems on them that make it easier to pull the starter rope when starting. Engines with one of these systems will always read low on a compression tester. Also, the compression tester will tell you that compression is low, but it won’t localize the problem. A more comprehensive test is the leak-down test. The leak-down tester also attaches to the spark plug hole. The blower housing and starter is removed to get access to the flywheel. The engine is rotated so the piston is at top dead center on the compression stroke. The flywheel is then locked so the crankshaft can’t move. The leak-down tester is connected to a compressed air line. The tester is calibrated and then a valve is opened allowing air into the cylinder. The gauge on the tester shows the percentage of compressed air that is leaking from the cylinder. The leak-down test is more comprehensive because you can track where the air is leaking from. Air escaping from the carburetor indicates a leaking intake valve. Air from the muffler would be a bad exhaust valve. Air leaking from the oil fill or the crankcase breather indicates wear or damage to the cylinder and piston. Repairing a damaged cylinder and piston is an advanced repair. The repair requires a complete teardown of the engine. If it is a repair you are feeling up to, it is a good idea to obtain the service manual for the engine you are working on. The manual will give testing instructions and standards. It will show best order of teardown and re-assembly. It will have important specs such as torque values, clearances, and engine timing.

If the engine on your lawnmower won’t start, the fix could be as simple as replacing the spark plug. Spark plugs mount to the engine’s cylinder at the top of the combustion chamber. A wire from the ignition coil attaches to the top of the spark plug. The ignition coil sends very high voltage electricity to the spark plug at the proper time. The electricity travels through a conductor in the center of the plug called the central electrode. At the tip of the spark plug there is a gap between the central electrode and a second electrode called the ground electrode. The high voltage electricity jumps the gap between the two electrodes, creating a spark that ignites the compressed air and fuel mixture in the cylinder. Diagnosing and replacing a spark plug is easy to do. Because spark plugs are cheap, the easiest way to test for a bad plug is to simply replace it. Otherwise you can use a spark tester to test for spark coming from the ignition coil and then by process of elimination, diagnose a bad spark plug. If the spark tester shows that the ignition coil is producing a good, strong spark, you can then assume that the problem is the spark plug itself. Many people test spark plugs by removing the plug from the cylinder and then grounding the plug to the engine’s frame while attempting to start the engine. If a spark is seen, they assume that the plug is good. The problem with this method is the conditions inside the cylinder are very different from the conditions in open air. Inside the cylinder, the air and fuel are under compression, which greatly increases the amount of voltage required to jump the spark plug’s gap. A good spark tester simulates what happens inside the cylinder. Some testers even allow you to increase the size of the gap, which stresses the ignition coil to produce more voltage than would normally be required to jump a spark plug gap. If the spark tester shows a strong spark you can assume the problem is the spark plug. If the tester shows no spark then the problem is elsewhere in the ignition system, such as the switch or the ignition coil. To replace the spark plug, start by removing the spark plug wire boot from the plug. Use a socket wrench to remove the plug from the engine. The new plug will need to have the gap between its two electrodes checked and set before the plug is installed. The gap can be checked using a gapping tool or a set of feeler gauges. Refer to the mower’s owner’s manual to find the correct gap for the plug. Once gapped, screw the plug back into the engine and tighten firmly with the socket wrench. Replace the plug wire. It is a good idea to replace the spark plug once per year as part of your normal preventative maintenance of your lawnmower.

A lawnmower that won’t start might have a bad starter. The recoil starter mounts to the top of the engine. When the starter rope is pulled, pawls inside the starter extend and engage to a hub, which rotates the flywheel to start the engine. When the rope is released, a spring inside the starter recoils the rope back around the starter pulley. Common recoil starter issues are: broken ropes, broken recoil springs, damaged or worn pawls, and wear to the pulley where the spring mounts. Repairing a recoil starter is usually pretty easy. The starter is attached to the mower with a few screws. On some models the starter will be riveted to the blower housing. In this case remove the entire blower housing from the engine. The first step when disassembling any starter is to remove the tension from the recoil spring. If you don’t, the spring will explode out of the starter housing as the pulley is removed. To take the tension off of the spring, remove the grip from the end of the starter rope and let the rope retract back into the starter housing. The pulley will spin inside the housing until all the tension is released from the spring. Now you can begin disassembling the starter assembly. Remove the fastener in the center of the starter pulley. The pulley can now be removed along with the pawls. The pawls will sometimes have small springs with them so be careful to not lose them. With the pulley removed, you will see the recoil spring sitting in the starter housing. The majority of the tension will be released from the spring but it may still expand some when it is removed. Wear gloves while removing it and be ready for it to expand once removed. Inspect each component of the starter. Look at the ends of the recoil spring where it mounts to the housing and the pulley. Replace if the ends are broken or badly bent out of shape. Look at the pulley where the spring attaches. If the mounting location is worn or broken the pulley will need to be replaced. The starter pawls will wear over time and not make a solid connection with the hub on the flywheel. Replace them if they seem worn or if you have noticed them slipping while starting the mower. Begin reassembly by installing the recoil spring back into the starter housing. Next install the pulley, making sure it is lined up with and properly attached to the recoil spring. Install the pawls and pawl springs if the starter has them. Secure the pulley with the hardware. Now the pulley needs to be tensioned. Rotate the pulley in the direction so when released it will wind the rope back into the starter. You can test this by rotating the pulley about a half turn and then letting go. The pulley should spring back in in the direction that will pull the rope into the starter, if not wind the pulley the opposite direction. Carefully wind the pulley to tension the recoil spring. The number of turns varies for different starters. You need enough turns to recoil the entire length of the rope into the starter. Once tensioned, you can often insert a small screw driver through an opening between the ribs of the pulley and the vent holes in the starter housing to hold the pulley while you install the rope. If the design of the starter doesn’t allow for this, you will need to hold the tension back by hand. Now thread the starter rope through the opening in the housing and then through the hole in the starter pulley. Tie a knot in the end of the rope at the pulley and tuck the knot down into the pulley. Now you can carefully allow all but about a foot of the rope to recoil into the starter. Temporarily tie a simple knot in the rope so the last foot of line doesn’t recoil into the housing. Insert the rope through the grip and tie a knot on the end of the rope to retain the grip. You can now untie the temporary knot and reinstall the starter onto the mower. Most starters are also offered as a complete assembly. If rebuilding the starter seems too complicated or too time consuming, you can always just bolt a new starter onto your mower’s engine and quickly get back to making your lawn look great again.