Lawn Mower: Engine Rpm's Surge Up And Down

If your lawnmower’s engine is surging it could be a problem with the carburetor. The carburetor mixes air and fuel to the perfect ratio for combustion. There are passages and nozzles (called jets) that route the fuel through the carburetor. The jets meter the precise amount of fuel that is needed for combustion. The jets have tiny openings that can easily be clogged by dirt or from corrosion caused by old fuel sitting in the carburetor. In the center of the carburetor, near the bottom of the carburetor bowl is the main jet. The main jet supplies fuel to the engine when it is at full throttle. When the main jet becomes blocked, the engine will stall at full throttle. Once stalled, the governor will close the throttle. At this point the carburetor’s idle circuit will begin to supply fuel and the engine will fire once again and the throttle will return to full. This cycle repeats over and over, causing the engine to die and surge. Fixing a surging engine is usually a matter of cleaning the carburetor. You will need to remove the carburetor from the engine and disassemble it into its various components. Remove the bowl, float, and metering needle. Pay close attention to the area around the opening for the main jet. Remove the main jet and the emulsion tube above it if possible. Not all emulsion tubes are removable. On the outside of the carburetor, remove the pilot jet or pilot jet cover screw. Now you can clean the carb body and each of the components you removed. Use carburetor cleaner to spray each piece clean. Be sure to spray carb cleaner through all of the tiny openings in the carb body as when as the main jet, the pilot jet, and the emulsion tube. Never insert anything made of metal into the openings to assist cleaning. These openings are very precise and can easily be damaged. It is okay to use something soft such as fishing line to clean the openings and passages. An alternative method to clean the carburetor is an ultrasonic cleaner. The ultrasonic does an amazing job of cleaning all the nooks and crannies of a carburetor. If you do a lot of do-it-yourself repairs, a small ultrasonic cleaner might be a good investment. Inspect the condition of the bowl gasket and the bowl screw seal and replace them as needed. Reassemble the carburetor and reattach it to the engine.

The engine’s governor maintains constant RPMs when a load is applied to the engine, as well as limiting the engine’s top speed. There are two styles of governors typically used on lawnmowers: pneumatic and mechanical. The pneumatic governor uses an air vane that reacts to air flow from the flywheel and adjusts the throttle plate. The mechanical governor is gear-driven by the engine. As the RPMs vary, a shaft moves the governor arm to adjust the throttle plate. On both governor styles there is a governor spring that applies tension to the air vane or the governor arm. This spring is adjustable, so the proper tension can be set. If the spring is set in the wrong position it can cause the engine RPMs to surge. Over time the spring can stretch out, causing the same engine surging. Often surging caused by the governor can be fixed by simply adjusting the governor spring. It usually only takes a small adjustment to correct the surging. If the spring has stretched out it will need to be replaced. If the engine has a mechanical governor the spring will be easily accessible on the side of the engine, near the carburetor. The spring is a little more difficult to access on an air vane governor. If will be located under the engine shroud near the flywheel. Depending on the design of the engine, the starter may need to be removed before the shroud can be removed. Once accessed, the spring is easy to replace on either style of governor. The spring ends will simply slide away from their mounting locations. When replacing the spring, it is important to only use the exact spring specified by the manufacturer. Mount the spring in the same location as the old spring and test. Adjust as needed to prevent surging and to keep the engine running at the correct RPMs. The manufacturer’s service manual is a helpful reference when replacing or adjusting a governor spring. It will have information to help you get the right spring, mount it in the right location, and will have the proper RPM range for the engine’s design and application.