Lawn Mower: Hard To Start

If your lawnmower won’t start, one of the first parts you should inspect is the carburetor. The carburetor brings fuel and air together and blends them to the perfect ratio for combustion. After passing through the air filter, air enters the front of the carburetor. Fuel is regulated into the carburetor bowl by a metering needle, which is open and closed by the float. When the bowl is full of fuel the float rises, lifting the needle to the needle seat, which stops the flow of fuel into the bowl. The emulsion tube extends from the center of the carburetor, down into the bowl. At the base of the emulsion tube is the main jet. The main jet regulates the amount of fuel entering the engine when the throttle is wide open. After fuel passes through the main jet it enters the emulsion tube, where air mixes with the fuel, causing it to atomize. The emulsion tube passes through the carburetor body from the bowl area into the venturi. The venturi is a smaller-diameter area in the passageway the air flows through. This smaller restriction creates an area of lower pressure, which lifts fuel out of the bowl through the emulsion tube. It is the same principle that an airplane wing uses to create lift. The atomized fuel mixes with air passing through the air passage and enters the engine for combustion. There is a second fuel system in the carburetor called the idle circuit. The idle circuit delivers fuel to the air stream when the throttle is closed and the engine is idling. It also works with the choke plate to deliver extra fuel to the engine for a cold start. Most carburetor hard-to-start issues are the result of dirt, varnish, or other debris blocking the tiny fuel passages in the carburetor. Many of these passages, especially the ones in the idle circuit, are smaller than the diameter of a pin. It doesn’t take much dirt to completely clog them. Another fuel issue that is becoming more of a problem is blockages due to ethanol in gasoline. Many areas of the country now have ethanol mixed into the fuel supplies. Ethanol creates problems when it is allowed to sit unused for long periods. Damage can start to occur after it has been sitting for 30 days. Ethanol is an alcohol. Alcohol naturally attracts and binds with water. When fuel sits, the ethanol will bind with humidity in the air. Once water binds with the ethanol, it forms an acid called acetic acid. The acid will corrode the carburetor’s metal components and harden the rubber parts. An easy way to diagnose a carburetor hard-starting issue is to spray a small amount of carburetor cleaner or starting fluid into the carburetor throat and then try to start the engine. If the engine fires and then dies a couple of seconds later you will know that you are dealing with a fuel delivery issue. The engine will run on the starting fluid, but will die once the fluid is gone. Most of the time a good cleaning is all that is required to fix a plugged carburetor. You will need to remove the carburetor from the engine first. This part is usually straightforward, often requiring you to only remove a couple of nuts or bolts. Make note of how the choke and throttle linkages attach to the carburetor and then remove them. Once the carburetor is free from the engine you can begin disassembling it. Start by loosening the bowl screw and allow the fuel to drain from the bowl. Then remove the screw and the bowl and its gasket. Now remove the float hinge pin, the float, and the metering needle. There will be a tube extending from the base of the carburetor. Remove the main jet from the bottom of the tube. The emulsion tube is above the main jet. It is removable on some carbs and fixed on others. Remove it if possible. On the top of some carburetors there will be a screw or a plastic plug that covers the pilot jet. If so, remove it. If the pilot jet is covered with a screw, the jet beneath it is likely fixed and will not be removed. If it is a plastic plug the jet is actually part of the plug and will come out with it. At this point you can start cleaning. Use carburetor cleaner and compressed air. Be sure to spray carb cleaner into each of the small passages until they flow clear and freely. If any of the jets are badly clogged, you can use fishing line to clean them. Never insert anything metal into any of the tiny jets or passages. These openings are very precisely sized and can be easily ruined. Use the carb cleaner to clean each of the parts you previously removed from the carburetor. Another method you can use to clean the carburetor is an ultrasonic cleaner. The ultrasonic does an amazing job of cleaning all the tiny internal passages in the carburetor. If you do lots of repair work it might be worth it to invest in a small ultrasonic cleaner. Many models suitable for cleaning a carburetor can be found for under $100. Once everything is clean you can begin reassembling the carb. Carefully inspect the bowl O-ring, bowl screw gasket, and the tip of the metering needle. If any of these parts are hardened, cracked, or if the metering needle tip is worn, they should be replaced. Once reassembled, you can reattach the linkages to the carb and bolt the carb to the engine. If cleaning the carburetor seems too complicated, you can simply replace it. Removing the carb and bolting a new one in place is a very easy repair that will quickly solve a hard-starting condition caused by the carburetor. Some carburetors are very expensive, which makes cleaning them more attractive. Some carbs are so inexpensive that cleaning hardly makes sense. In either case, cleaning or replacing a plugged carburetor should get your engine running again.

The air filter is mounted ahead of the carburetor. It filters out any dirt or other debris before they can enter the carburetor and the engine. It doesn’t take much dirt to plug the tiny openings in a carburetor. Dirt that gets past the carburetor and into the engine will quickly do serious damage. Dirt in the cylinder acts like sandpaper, wearing away the cylinder wall and the piston rings. A worn piston and cylinder will diminish the engine’s compression to the point that it won’t start and run. Low compression requires a costly engine rebuild to repair or often, engine replacement. The air filter should be inspected often and replaced at least once per year, more often in very dusty conditions. Any air leaking past the air filter can cause engine damage, so look for tears in the filter or worn or cracked seals on the air filter. A cracked or damaged air filter base or cover can also allow unfiltered air past the air filter. Some air filters can be cleaned. Felt or paper filters can be carefully cleaned with compressed air one or two times. Blow air at low pressure from the back side of the filter. Be careful to not tear the filter. Foam type filters can be washed with dish soap and reused many times. As soon as a foam filter shows signs of crumbling or flaking it should be replaced. An air filter that has become plugged with dirt will limit the amount of air available for the engine. This will make the engine hard to start and will reduce the performance and power of the engine when it is running. The lack of air will cause the engine to run rich, so a plugged air filter can cause the engine to produce some black smoke from the exhaust, especially when under load. When replacing an air filter, make sure you use the exact filter called for. This ensures a good fit and seal. The wrong filter may not make a good seal and might not have the correct filtering properties required by the engine. Most foam filters will need to be oiled prior to installation. Your owner’s manual will give more details. Place a foam filter in a plastic bag and add a small amount of foam filter oil or engine oil to the bag. Squeeze the oil evenly into the filter. Remove the filter from the bag and remove any excess oil with a rag. The filter should be evenly coated with oil but not saturated. Inspecting and maintaining the air filters on your small engines is easy to do and will help to ensure that the engines last for many years.

The fuel filter on your lawnmower removes any dirt or debris in the fuel before it can make its way into the carburetor. Many of the fuel passages in the carburetor are very tiny and any debris can quickly plug them. There are two common types of fuel filters used by lawnmower manufacturers: in-tank and in-line. In-tank filters are located inside of the fuel tank. Many of these filters fit inside the end of the fuel line and then the fuel line is attached to the fuel tank, pushing the filter into the tank. Some in-tank filters are permanently attached inside the tank. In-line fuel filters attach in the middle of the fuel line. The fuel lines press onto either side of the filter and are secured by a couple of hose clamps. As the fuel filter becomes plugged with debris, less fuel will be able to flow to the carburetor. This can make the engine hard to start and can cause it to start and run for a few seconds and then die. It is a good idea to replace your mower’s fuel filter at least once per year as part of your annual mower maintenance. Replacing an in-line filter is easy to do. Start by either clamping the incoming fuel line, or by draining all the fuel from the gas tank. Now you can remove or simply slide back the hose clamps from either side of the filter. Next, remove the fuel lines from the filter. A pair of pliers can help loosen the fuel lines from the filter if they have become stuck. When installing the new fuel filter, look for an arrow or writing on the filter that would indicate the direction of fuel flow. Some filters must be oriented in a certain direction to properly filter and flow. Once the filter is properly oriented, reinstall the fuel lines and secure them with the clamps. When replacing an in-tank filter you will need to start by draining all the fuel from the tank. Once drained you can remove the fuel line from the tank. Remove the hose clamp and then pull the line from the tank fitting. If the filter is the style that is fit into the end of the fuel line, the filter will be protruding from the end of the line. Pull the filter out of the fuel line. Press the new filter into the line and reattach the line back to the fuel tank. If your model has the filter permanently attached to the fuel tank, your next step will be to inspect the filter closely. Use a flashlight so you can see the filter clearly. Look for debris covering and clogging the filter. Look for and tears or other damage to the filter. If the filter is just dirty you can use carburetor cleaner to clean the filter. Start by removing the fuel tank from the engine. Clean the filter with carb cleaner and pour the excess cleaner and any debris in the tank out. If the filter is torn or damaged, you will need to replace the fuel tank assembly. Once the filter is clean or you have the new tank if the filter is damaged, reinstall the tank to the engine. Reattach the fuel line to the tank and secure it with the hose clamp. Performing annual maintenance to your lawnmower’s fuel filter will keep it starting easily and running well for years to come.

The spark plug works with the ignition coil and the flywheel to deliver a perfectly timed spark into the combustion chamber for ignition. Spark plugs are simple parts, largely consisting of a couple of electrodes, a metal body, and an insulator. The spark plug carries extremely high voltage, usually over 10,000 volts, to create the ignition spark. The electrode end of the spark plug is inside of the combustion chamber, where temperatures can exceed 1200 degrees F. Eventually, the electrodes will wear and the gap between them will open causing a weak or inconsistent spark. The spark plug in your lawnmower should be replaced at least once per year to ensure trouble free operation. While the plug itself can go bad, many spark plug problems are the result of outside factors. An engine running too rich will foul the plug with carbon deposits or excess fuel. If the engine is too lean, the electrodes can be damaged or the ceramic insulator may chip. Diagnosing a bad spark plug is best done through a combination of visual inspection and elimination of other possible problems. If you suspect a bad plug is making your engine hard to start, begin your diagnosis by removing the plug. Look at the electrode end of the plug. It should have a thin, medium-gray coating on the metal surfaces. If the plug is coated in a heavy, black carbon or tar, the engine is running rich and the plug may be fouled. If it has a white coating on the electrodes the engine may be running lean. A lean-running engine can quickly damage other engine parts such as the cylinder and piston, leading to costly repairs. The electrodes should have crisp edges and not show any signs of being melted. The ceramic insulator should not be chipped or cracked. If the plug looks okay visually, you will want to do some additional tests. Use a feeler gauge or gap-measuring tool to measure the gap between the two electrodes. The correct distance between the electrodes is important so the plug can produce a strong spark. Most lawnmower engines call for a gap of .030” – refer to your owner’s manual for the specific measurement of your engine’s spark plug gap. If the gap is correct, you will next want to test the ignition coil. The ignition coil along with the flywheel produce the electricity that is sent to the spark plug. To test the ignition coil you will need a spark tester. The spark tester is installed between the plug wire and the spark plug installed in the engine. You then attempt to start the engine and watch for a strong spark in the window of the tester. A common method that has been used for years to test an ignition coil is to remove the plug from the engine and then ground the plug to the engine frame. The engine is then turned over while watching the plug for a spark. The problem with this method is it doesn’t create the same conditions that exist inside of the engine’s cylinder. It takes much more power to create a spark in a pressurized combustion chamber than it does in ambient air. So a weak ignition coil may create a spark with the plug removed from the engine but not with it installed. If the ignition coil checks out using the spark tester, then you likely have a bad spark plug. Replacing the plug is simple. Remove the old plug from the engine. Use a feeler gauge or gap tool to set the proper electrode gap on the new plug. Screw the new plug into the engine and firmly secure it with a socket wrench. Reattach the plug wire and your mower should start and run great.

Some lawnmower engines use a primer bulb to assist with starting the engine cold. The primer bulb is pressed about three times before starting the engine. When the bulb is pressed it pumps air into the bowl on the carburetor. This pressurizes the bowl, which pushes fuel through the main jet, through the emulsion tube, and into the carburetor’s venturi where it can enter the engine. This extra fuel makes the engine easier to start when it is cold. The most common problem with primer bulbs is cracks forming in the bulb itself. As the bulb ages, the material it is made from will begin to harden, which will lead to the cracks. Once the cracks form, the air will leak out of the cracks rather than being pumped into the carburetor bowl. Occasionally, the primer base can be cracked or damaged and the hose leading from the primer bulb to the carburetor can also crack over time. When the air can’t be pumped into the carburetor due to damage to any of these parts, the extra fuel won’t be pumped through the carburetor, which will make it difficult to start a cold engine. The flexible part of most primer bulbs can be replaced separately from the base. Some models are replaced as an entire assembly. To replace the primer bulb, start by removing the base from the engine. It is typically held in place by a couple of plastic tabs. Use a screwdriver or pliers to carefully bend the tabs back so the base can be pulled away from the engine. Remove the hose from the base. If your model allows you to replace the flexible bulb separately from the base, go ahead and pull or cut the old primer bulb away from the base. Some designs require a retainer to be removed before the bulb can be removed. Use a pick or small screwdriver to remove the retainer. If the retainer is bent during removal you will want to replace it, too. If the primer assembly uses a retainer you can now place the new bulb into the base and secure it by pressing the retainer back in place. If the bulb fits over the top of the base, ease the new bulb onto the base. A small screwdriver is usually helpful to stretch the bulb onto the base. Be careful that you don’t slip and puncture the primer bulb with the screwdriver. Sometimes it is helpful to apply a small amount of oil around the primer base to make installing the bulb a little easier. Once the bulb is reassembled or if you are installing a new primer assembly, reattach the primer hose to the primer base and snap the base back onto the engine.