Lawn Mower: Engine Will Idle But Dies At Full Throttle

If your lawnmower will start up and idle, but then dies when you throttle it up, you might have a problem with the carburetor. The carburetor brings air and fuel together and mixes them in a perfect ratio for combustion. Fuel makes its way through the carburetor using several different passages. These passages all serve different purposes, depending on the state of the throttle setting, such as idle, part throttle, and full throttle. When the engine is at idle the throttle plate is almost completely closed, limiting the amount of air entering the carburetor’s venturi. In this condition, almost all the fuel is being delivered through the carburetor’s idle circuit. As the throttle plate opens, more air enters the venturi and therefore more fuel is required. At this point the transition circuit kicks in, providing additional fuel. When the throttle is opened all of the way the majority of the fuel flows through the main jet. Some fuel will also enter the carburetor through the idle and transition circuits at full throttle but not nearly enough to sustain combustion on their own. If the engine will idle but dies at full throttle the main jet in the carburetor may be plugged. You can repair a plugged main jet by cleaning it, replacing the jet, or replacing the carburetor. If you choose to clean or replace the main jet, you will also want to clean the rest of the carburetor. If the main jet has been plugged by dirt, corrosion, or old fuel, the rest of the carburetor will also be affected by similar conditions. To clean the carburetor, you will need to remove it from the engine. This part is usually pretty straight forward, 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 allowing the fuel to drain from the bowl. Next, 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 always 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 at full throttle again.

If your lawnmower will idle but dies at full throttle you might have a problem with the muffler or the muffler spark arrestor. The muffler on your lawnmower quiets down the sound created by the flow of exhaust exiting the combustion chamber. In some cases, the muffler will have a catalytic converter inside, which further burns the emissions that are created by the engine, making for cleaner exhaust. Some mowers are equipped with a spark arrestor. The spark arrestor is a screen that captures sparks that might be in the exhaust flow so they can’t exit the engine and potentially cause a fire. If the lawnmower is running rich, the exhaust will have a higher content of carbon and tar caused by fuel that wasn’t completely burned during combustion. These unburnt components will build up on the exhaust port, in the muffler, and on the spark arrestor over time. As the buildup increases, the exhaust flow exiting the engine will become restricted. Eventually the buildup can restrict the flow of exhaust gases enough that the back pressure in the exhaust system will stall the engine. The engine will often run fine at idle, but will die when it is throttled up. This is because there is less air moving through the engine at idle. When the engine is run at full throttle, especially under load, the air flow is much greater, which causes more back pressure, and results in the engine stalling. Fixing a restricted exhaust system is usually an easier repair. Start by removing the spark arrestor if your muffler is equipped with one. A plugged spark arrestor will be covered with carbon buildup. There won’t be any opening between the wires that make up the screen. There are two common styles of spark arrestors used on lawnmowers. One style is a round device attached to the end of the exhaust pipe with a screw. It is easily removed by removing the screw and pulling it off the pipe. The other style is a flat screen that will be sandwiched behind an exhaust deflector. The deflector and screen will be attached to the muffler using a few screws. Once removed, the spark arrestor can be replaced or cleaned. You can clean the spark arrestor using a small propane torch. Hold the arrestor screen with a pair of pliers and carefully burn the carbon out of the screen with the torch. Allow the screen to fully cool and then reattach it to the muffler. The next step is to remove the muffler from the engine. It will typically be held in place with a couple of bolts. Sometimes the bolts will pass completely through the muffler body. Some designs will have a short exhaust pipe between the exhaust port on the engine and the muffler. Once the muffler is removed you can inspect the exhaust port. The exhaust port should be free from carbon or tar build up. It should have a light coating of grey soot on it. If there is substantial carbon and tar buildup the exhaust port will need to be cleaned and the muffler replaced. Heavy carbon buildup on the exhaust port will indicate that the muffler is likely also plugged with carbon and tar. Carefully scrape the exhaust port clean using a small screwdriver or pick. Be careful to not scrape the exhaust valve stem at the rear of the port. Once the port is clean, install the new muffler onto the engine.