Lawn Tractor: Engine Rpm's Surge Up And Down

The springs on the carburetor allow the butterfly valves and the governor to operate correctly for proper engine performance. A broken, missing or improperly installed spring can affect the carburetor function. You will want to check your lawn tractor’s owner’s manual for the correct routing of these springs. You will first want to make sure the springs are installed correctly, then replace any broken, weak, or missing springs.

The carburetor’s function in your lawn tractor is to mix together fuel and air at the correct ratio to create combustion. You can find the carburetor for your lawn tractor underneath the air filter. To inspect it, first remove the air cleaner cap and air filter, at which time you should check the air filter for dirt and debris and replace if needed. There may also be small dirt particles in the carburetor, so purchasing a carburetor kit for cleaning and rebuilding your carburetor is a good idea. It is also advised to take pictures of the carburetor as you’re taking it apart and lay all pieces out on a bench in order, so you can remember the correct order for re-assembly. Once you have cleaned and rebuilt the carburetor, locate both idle screws. One will be marked L (low) and one will be marked H (high). Turn both screws in to shut off the fuel, then back off 2 turns in the opposite direction. Next, start the engine, and allow it to idle for about a minute. Next, turn the L screw clockwise to add more fuel and set the idle. Then, at full throttle, turn the H screw clockwise and set the high idle. When you have the engine running at its best you’re done (note this does require some experience and may take a few times to get it right). Now you can adjust the governor screw, the location of which may differ depending on your model. Now, loosen the bolt on the governor screw and turn the screw very slightly to get the engine to run around 3,400-3,600 RPM. This should prevent your RPMs from surging up and down. Some carburetors are fixed, meaning they have no adjustments, and therefore cleaning the carb should do the trick.

The air filter is located on top of the carburetor on your lawn tractor. It is designed to allow clean-flowing air into the engine and prevent small particles from damaging internal engine parts. Over time, the air filter will need to be replaced as it becomes clogged with small particles and debris starving the engine of air. A clogged air filter can cause your lawn tractor’s RPMs to surge up and down. To replace the air filter, first locate the air filter cover and remove it. Once the cover is off, simply remove the filter, replace it with a new one, and re-assemble.

The job of the fuel filter is to stop debris and dirt from entering the fuel system and causing any clogs. Clogs can limit the amount of fuel to the carburetor, causing the air/fuel mixture to be off, and occasionally resulting in surging RPMs. The fuel filter is found on the fuel line between the fuel tank and the carburetor. Simply twist the fuel line and pull the filter off, then pour any fuel out, looking for debris. Next, blow through the fuel lines and check for any blockage. Fuel filters should be replaced as part of regular maintenance. When re-installing the fuel filter, be sure to check for an arrow indicating the direction of the flow of fuel.

The gasket’s job is to seal two joining pieces tightly together, preventing air and fluids from entering areas where they don’t belong. A damaged or worn gasket below the carburetor will pull excess air into the engine causing the air fuel mixture to become lean (containing too much air). To inspect the gasket, remove the air filter and bolts, holding the carburetor down, and check the gasket for damage or leaks. Once you decide to inspect the gasket, it will need to be replaced regardless of the condition it is in, as gaskets are designed for one-time use only.