Lawn Mower: Only Runs With The Choke On

If your lawnmower will start and run with the choke on but dies when the choke is turned off, it may have a problem with the carburetor. The carburetor brings air and fuel together and mixes them in the perfect ratio for combustion before they enter the engine. There are several passages in the carburetor that allow fuel to enter the air stream depending how the throttle is set. The idle circuit, for when the throttle is nearly closed and the engine is idling. The transition circuit, for when the throttle is transitioning from idle to full. The main jet is used when the throttle is at full. The choke assists with providing additional fuel when the engine is first started. When closed, the choke plate restricts most of the incoming air from entering the carburetor’s venturi. This creates additional low pressure in the venturi and also forces air to be drawn through a port on the front of the carburetor body. This port leads to the air space at the top of the bowl. This air pressurizes the space above the fuel in the bowl, which forces fuel to flow out the main jet, into the venturi. This rich fuel mixture makes it easier to start the cold engine. Once the engine fires up and runs for few a few seconds, the choke is opened and the other fuel passages in the carburetor do their jobs as normal. An engine that will fire and run with the choke closed but dies when the choke is opened likely has debris at least partially blocking the fuel passages in the carburetor. In this condition the closed choke provides enough extra fuel to enter the engine to overcome the lack of fuel flowing through the other passages. The engine runs until the choke is opened. The fuel supply is then starved by the blocked fuel passages, and the engine dies. You have a couple of choices to solve this problem: clean the carburetor or replace it. Most carburetors can be cleaned and made to run like new again. Cleaning a carburetor isn’t difficult, but it does require the carb to be mostly disassembled. Replacing a carburetor is a fast and easy repair. In some cases, the new carburetor is so inexpensive that it really doesn’t make sense to clean the old one. Usually the cost of a new carburetor versus your amount of repair experience and skill will inform the decision to clean or replace. 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 the metering needle. The metering needle should be inspected for corrosion and wear on the tip of the needle. 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.