Lawn Tractor: Engine Misfires

A badly clogged air filter could impact the engine and cause a misfire and will need to be replaced. Cleaning and inspecting your air filter should be done as a part of routine maintenance of your lawn tractor, and can help with overall engine performance.

The purpose of a gasket is to create a seal between two joining surfaces, and prevent leaks of any kind. A leak in the intake gasket or head gasket can cause the engine to misfire. Let’s start with the intake gasket. Leaky intake gaskets can be hard to diagnose, because even the smallest leak can affect the engine greatly. To inspect and replace the gasket, first detach the engine cover, and remove the air filter. Next, remove the blower housing’s mounting bolts, detach the oil drain tube, and lift it off the blower housing. Next, remove the nuts securing the air inlet pipe and the breather tube and set the pipe aside. Remove the carburetor and pull back and remove the intake gasket. Now you can inspect the gasket for damage and replace it. With the new gasket installed, you can re-install the carburetor. Make sure the O-ring is still in place on the inlet pipe and re-install it on the carburetor. Re-attach the breather tube to the inlet pipe, place the blower housing back onto the engine, and make sure it is seated properly on any air diverters. Tighten down and attach the oil drain tube, reinstall the air filter assembly, and put the engine cover back onto unit. A damaged head gasket can also cause poor engine performance. The head gasket provides a tight seal between the engine block and the cylinder head. If a head gasket fails, it can cause very poor engine performance and in some cases, internal damage. If the engine is liquid cool you can check for an external leak around the head gasket. Next you will want to inspect the spark plug. A bright or shiny plug indicates an internal leak. If the engine is air cooled, you can check the outside of the engine first for an oil leak at the head gasket. At this point, pull the oil dip stick and check for exhaust coming out. A blown head gasket can increase the pressure inside the engine, forcing the oil out in these areas. Check the vent on the fuel pump and the air filter for oil as well, as this can indicate a leak. If you see any of these signs, you have a leaky head gasket and it will need to be replaced. To do this, first remove the engine cover, fan, and fuel pump if your engine has one. Then, remove the carburetor and muffler from the cylinder head and pull the valve cover off. If you see more signs of exhaust smoke when removing the cover, it is another indication of a blown head gasket. Remove all the bolts holding the cylinder head to the engine block, remove the head and pull off the old head gasket. Clean both the surfaces of the engine block and cylinder head with a soft brush, being careful not to damage these parts. Now, install a new gasket and re-assemble. It is best to replace all the cylinder head bolts with new ones, in order to prevent further engine damage. You will want to check the manufacturer’s specifications for the bolt tightening sequence for the head bolts. This helps to provide even compression for the gasket. These bolts also must be torqued for a proper seal. When the bolts have been torqued, re-install the carburetor, muffler, valve cover with new valve cover gasket, fuel pump, fan, and cover.

The muffler’s job is to decrease the sound of the engine’s operation. Misfires in a muffler are usually related to overheating. The muffler can easily become over-heated when being worked too hard. If grass is too tall and thick, or if the tractor’s blades are not sharp enough, it will cause the piston in the engine to slow down and not keep up with the firing cycle. When this happens, not all the fuel burns properly and it can exit through the exhaust valve and into the muffler, creating a build-up of fuel. You can prevent this by letting the engine cool down if it is overheating. Regular sharpening and replacing of the blades can prevent the engine from becoming overworked as well. You can also keep your air filter clean and replace a dirty spark plug to avoid an engine misfire. If the carburetor is allowing too much fuel flow it will cause a build up of fuel in the muffler. When this occurs, you should adjust the carburetor idle screws to lean out the mixture. To do this, first remove the air filter and the air filter assembly. Locate the screws on carburetor and adjust by turning them in all the way then backing off a turn and a half. If the carburetor has a main jet adjustment screw, turn it in until it touches the seat inside the emulsion tube, then back the screw off 1 to 1 ½ turns. At this point, you can re-assemble the air filter unit. Now, start the engine and allow it to idle for a few minutes, bringing it up to operating temperature. Turn the idle mixture screw slowly clockwise until it begins to slow, then turn the screw in the opposite direction until it does the same. Make note of the screw’s position, then turn it back to the midpoint. Next, set the idle speed with a tachometer. You will want to set the idle speed screw to bring the RPM to 1,750 for aluminum engines, and 1,200 for cast iron. By holding the throttle lever to the idle speed screw, it will bring the engine speed to true idle. Then, fine tune by setting the idle speed again.

The job of the spark plug is to ignite the air/fuel mixture that runs the engine. A worn, dirty, oily, or improperly gapped spark plug can cause a weak spark, and this can lead to a misfire. A quick way to check that the spark plug is functioning properly is to carefully remove the spark plug wire while the engine is running. If you hear a drop in the engine rpm, then the cylinder is fine. If you hear no change, you have determined that the spark plug is causing your misfire. Once you have determined that the spark plug is the problem, replace the old plug and gap it to the manufacturer’s specifications. Do not over-tighten, as spark plugs are fragile.

The push rod rides on lobes on the camshaft, and transfers power from a rotary motion to a reciprocating motion. The top of the push rod pushes on the rocker arm, which opens and closes the valves in the cylinder head to allow fuel to enter and exhaust to escape. If a push rod becomes bent, it will affect how much these valves open. If the valves do not open enough, it can result in decreased engine performance. If a push rod is bent, you will usually hear a ticking noise, and if it is bent badly enough or is broken completely, the engine can misfire. To diagnose, remove the engine cover then locate and remove the valve cover. Undo and remove the bolts holding down the rocker arms, and remove the push rods to check for a noticeably bent rod. If the rod is bent, replace it with a new one, adding some engine oil when you are installing. Re-install the rocker arm and bolts to the manufacturer’s specifications, and re-gap to 3-5 thousandths of an inch for the intake and 5-7 thousandths of an inch for the exhaust. Finally, you should replace the valve cover gasket with a new one, and re-install the valve cover.

The ignition module, also known as the ignition coil, provides electricity to the spark plug, which runs the engine. Sensors on the coil create a charge using magnets that pass by coil on the flywheel. If the timing of these magnets passing by is off, it can cause a misfire and you will need to set the timing.