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,...
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.
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