Lawn Tractor: Blows Black Smoke From Exhaust

Carburetors blend air and fuel to the right ratio for combustion. If a carburetor is running rich (too much fuel), it will cause black smoke. Over time, carburetors tend to gum up and may require cleaning. You should clean them with carburetor cleaner and a soft brush for the inside. To adjust the fuel mix on your new or cleaned carburetor, first turn both the L screw (low) and H screw (high) inwards to shut off fuel completely, then turn them back 2 turns. Now you may start the engine, though it may not run effectively. However, it should still start at these settings. Next, let the engine idle for 5 minutes, which allows it to reach operating temperature. Once the engine is running for 5 minutes, you can start by turning the L screw clockwise to add more fuel and set the idle correctly. Next, at full throttle turn the H screw clockwise to set the high idle. When you have figured out the ideal setting for your carb, your lawn tractor should be running properly again. Depending on your model, you may have a fixed carburetor. This means that it will not have the screws that allow you to adjust the mixture. In this case, simply clean out or replace the carb, and it should be back to normal. Setting up a carburetor usually takes experience, and it can be very tricky to get it just right, so you may want to take it to a small engine mechanic to set it properly, or at the very least be prepared for a lot of trial and error when you attempt this repair for the first time.

The air filter is designed to provide clean air to the engine for combustion. A badly clogged air filter will starve the engine of air. If you overfill your engine with oil, it can force some of the excess oil into the carburetor, soaking the air filter. This will not only add oil to the air/fuel mixture but can clog your air filter as well. To fix this problem, drain the excess oil and remove the air cleaner housing. Clean the housing out well, and install a new filter. You may also want to spray the carburetor with carburetor cleaner and use a soft-bristled brush on the inside. This can prevent the air filter from clogging again in the future. Proper maintenance of the air filter will prevent your lawn tractor from smoking.

The head gasket creates the seal between the engine block and the cylinder head for compression. The gasket requires a tight seal to prevent oil from leaking. To check for a blown head gasket, simply perform a compression test. If you pull out your spark plug and notice that it is oily, it is an indication that you have oil entering the cylinder. You will need to replace the head gasket if this is the case. To do so, you will first need to drain all fluids. Next, remove the engine cover, fan, and fuel pump, then remove the carburetor and muffler from the cylinder head. Remove the valve cover bolts and pull the valve cover off. You will need to locate the bolts holding the cylinder head to the engine block, then remove the cylinder head. Now you can remove the old head gasket. You will need to clean both engine block and cylinder head surfaces with a soft brush, being careful not to damage these areas. This is crucial, as a damaged engine block or cylinder head may prevent a tight seal from forming with the new gasket. At this point you can install the new gasket and re-assemble. Due to the amount of pressure and abuse the cylinder head bolts are under, it’s best to replace all the cylinder head bolts with new ones. This will help prevent the new gasket from experiencing premature failure. Check your manufacturer’s specifications for the bolt tightening sequence, to ensure even compression of this gasket. These bolts must be torqued for a proper seal to 150 foot-pounds, then 250 foot-pounds. Now you can re-install the carburetor, muffler, and valve cover with new valve cover gasket, fuel pump, fan, and cover.

The job of the piston ring is to provide a seal between the piston and the cylinder wall, which allows for compression. The piston ring also helps to keep oil out of the combustion chamber. When these rings become worn, oil can enter the combustion chamber, and the burning oil can create black smoke. Worn or damaged piston rings will need to be replaced, or it can cause further problems for your lawn tractor. A compression test will tell you if you have low compression, and this indicates that the rings are bad. After you have determined the rings are bad, start by removing the hood and draining all fluids. Remove the air filter, then the blower housing and oil drain tube from the blower housing. Remove wires from starter and disconnect the spark plug wire and spark plug. Next, remove the exhaust manifold and the carburetor from the cylinder head. Remove the engine mounts, disconnect the engine, and place it on a work bench. With the engine on a stable work surface, pull the valve cover off and rotate the piston to the bottom. Remove the rocker arm nuts and push rods, then unscrew all the bolts holding the cylinder head to the engine block and remove the head. Pull off the old head gasket. Clean both the surfaces of the engine block and the cylinder head with a soft brush. Be careful not to damage this area, or you will not be able to achieve a tight seal. Next, remove the crankcase bolts and wiggle it off. You may have to tap it off lightly with a hammer and punch wherever you can make contact on the case. There may even be a key way on the crankshaft that will have to be removed as well, depending on your specific model. Next, remove the camshaft by simply pulling it out. Then, remove the camshaft tappets, the connecting rod bolts, and the bottom part of the rod bearing. Rotate the crankshaft to the top then reach in and push the piston out with your fingers. Clean any parts that you intend to re-use with parts cleaner and dry them thoroughly. Install new rings on the piston and make sure the oil ring is on the bottom. Using a piston ring installer, gently tap the piston into the cylinder, making sure to lubricate the cylinder with fresh motor oil beforehand. It’s worth nothing that all bots must be torqued to the manufacturer’s specifications. With the piston at top dead center, and touching the crankshaft, rotate both together by pushing down on the piston so you can re-attach the connecting rod bearing. Lubricate the camshaft tappets and re-install, then install the camshaft by making sure the timing marks are in line. Be sure to lubricate all moving parts with motor oil or white grease first. Re-install the cover with a new gasket then re-install the cylinder head with a new gasket. Install the connecting rods and bolts, as well as the cover with a new gasket. Finally, re-attach the carburetor, muffler, fuel pump, fan, and engine cover.