Lawn Mower: Blows Black Smoke From Exhaust

If your lawnmower is blowing black smoke from the exhaust it might have a problem with the carburetor. The carburetor brings air and fuel together and mixes them in the perfect ratio for combustion. The fuel is very precisely added to the incoming air. Too little fuel and the mixture will be lean, which can lead to serious engine damage. Too much fuel will create a rich mixture. This will cause the fuel to not burn completely, leading to carbon buildup in the cylinder, on the valves, on the spark plug, on the exhaust system, and will create black smoke from the exhaust. The fix for this problem will vary depending on the design of the carburetor. Older carburetors typically have an adjustment needle that allows the flow of fuel to be regulated when the throttle is at full. With this style of carburetor, the needle can be adjusted slightly leaner so the fuel ratio is correct and the black exhaust and resulting carbon buildup will be corrected. If the engine is a newer model the fuel ratios will be metered by fixed jets. The jets are precisely machined nozzles that allow a specific amount of fuel through. They are fixed and are not adjustable. Most manufacturers of fixed jet carburetors offer “high-altitude” kits. Engine manufacturers typically set the carburetor to produce an air fuel mixture that is correct for an engine running at sea level. When an engine is used at a higher altitude the air density will be thinner so the fuel ratio will be on the rich side. These kits will consist of new jets that allow less fuel to pass through, which will make the fuel mixture leaner. This will correct the mixture so the engine will operate properly in the thinner air at higher altitudes. You must be careful after installing a high-altitude kit to not operate the engine at lower altitudes without reinstalling the original jets. At lower altitudes the high altitude kit will create a lean mixture that can cause severe engine damage. Installing a high-altitude kit is a pretty simple repair. The carburetor will need to be removed from the engine. The bowl is then removed, which will expose the main jet. The jet is unscrewed and the new jet installed. Replace the bowl and reattach the carburetor to the engine. If a carburetor is incorrectly cleaned, the main jet can be damaged, which will also cause the mixture to be rich, producing black exhaust smoke. The jets in the carburetor are very precise nozzles and care must be taken during cleaning so they are not damaged. If a metal pick or wire is inserted into a jet during cleaning it can enlarge the opening of the jet, which will allow more fuel through, causing the engine to run rich. A plugged jet should only be cleaned using something soft, such as a piece of fishing line. If you have cleaned your carburetor jets with a metal pick and the engine begins to run rich and produce black smoke, you will need to replace the jets with new ones.

If your lawnmower has black smoke in the exhaust you might need to service the air filter. The air filter cleans dirt, grass clippings, and other debris out of the air before they can enter the carburetor and the engine. The carburetor brings the incoming air together with fuel and mixes them into the perfect ratio for combustion. If there is too little fuel the mixture will be lean and the engine will be damaged. If there is too much fuel the mixture will be rich, which produces carbon build-up in the engine and creates black smoke in the exhaust. If an air filter is not regularly inspected and serviced it will begin to become plugged. The plugged air filter will reduce the amount of air entering the engine, which will cause the air/fuel mixture to become rich with fuel. This leads to black smoke in the exhaust. The air filter on your lawnmower should be inspected at least once per month and replaced at least once per year. If you mow in dusty conditions, such as weedy fields or areas of sparse grass, you will need to replace the filter more often. Replacing the air filter is simple to do. You will start by removing the air filter cover on the side of the engine. The cover is typically held in place by a couple of knobs or screws. Some covers will be secured using plastic tabs that you push in to release while pulling the cover off. Once the cover is removed, you can typically lift the filter away from the air filter base. Occasionally the filter will be secured with a couple of additional fasteners. If this is the case, remove them and then lift the filter away from the air filter base. Once the filter is removed, make sure the base is clean. Wipe away any dirt or debris on the base, being careful so nothing enters the carburetor throat. Now you can install the new air filter. Always use the exact filter recommended by the manufacturer to ensure the proper fit and filtering characteristics. Make sure the filter seals to the air filter base so no dirt or debris can leak past. With the filter installed you can finish up by reinstalling the air filter cover. Regular air filter maintenance will keep your mower’s engine running well and ensure a long engine life.