Trimmer: Blows Black Smoke From Exhaust

The air filter keeps dirt and debris from getting into the engine for combustion. A badly clogged air filter will starve the engine of much needed air. If your engine is a 4-stroke then the following applies. 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 clog your air filter. Drain excess oil and remove air cleaner housing by undoing bolts. Clean it out and re-install with new filter. Spray the inside of the carburetor with carburetor cleaner and use a soft bristle brush on the inside to clean out any oil residue. If your engine is a 2-stroke, blowing too much black smoke from the exhaust, then you have added to much oil the oil/fuel mixture. Drain the tank and add the correct ratio of oil and fuel to burn.

The head gasket creates the seal between the engine block and cylinder head to keep oil from leaking and for compression. This procedure is for a 4-stroke engine. To check for a blown head gasket simply perform a compression test. Remove the spark plug. If the spark plug is oily you have oil entering the cylinder. The head gasket will also need to be replaced. Perform the compression test. A low reading indicates a leak inside the cylinder either through the head gasket or piston rings. Drain all fluids and remove the engine cover to gain access to the engine. Remove the air filter housing, air filter, and carburetor. Remove the fuel line and linkage from the carburetor. Next, remove the muffler from the cylinder head. Remove the valve cover bolts and cover. Locate the bolts holding the cylinder head to the engine block and remove the head. Remove the old head gasket. Clean the surfaces of the engine block and cylinder head with a soft brush – do not damage this area. It is important, as you want a tight seal for the new gasket. Install the new head gasket and reassemble. It’s best to replace all the cylinder head bolts with new ones as they should be used only once because of the pressure and abuse they take. This will help prevent the new gasket from premature failure. Check manufacturer specs for the bolt-tightening sequence to provide an even compression of this gasket. These bolts must be torque for a proper seal. Reinstall the valve cover with a new valve cover gasket. Reinstall the carburetor and hook the fuel line and linkage back up to the carburetor. Reinstall the muffler, air filter housing, filter, and engine cover.

The job of the piston ring is to provide a seal between the piston and the cylinder wall, creating a seal for compression as well as keeping oil out of the combustion chamber. When these rings become worn over time, oil can enter the combustion chamber and create black smoke. This problem and repair procedure is for 4-stroke engines only. At this point you will need to replace the piston rings. A compression test will tell you if you have low compression, indicating the rings are bad. After you have determined that the rings are bad, start by removing covers to the engine, then drain all fluids. Remove the air filter and air filter housing, then remove the spark plug wire and spark plug. Remove exhaust manifold and carburetor from the cylinder head. Pull valve cover off. Rotate the piston to the bottom, remove rocker arm nuts and the push rods. Remove all the bolts holding the cylinder head to the engine block, and pull the cylinder head off. Remove the old head gasket. Clean the surfaces of the engine block and cylinder head with a soft brush. You do not want to damage this area as you want a tight seal. Next, remove the crank case bolts; you may have to tap it off lightly with a hammer. Remove camshaft, simply by pulling it out. Remove camshaft tappets, remove the connecting rod bolts, and remove the bottom part of the rod bearing. Rotate the crankshaft to the top then reach in and push piston out with your fingers. Clean all parts with parts cleaner that you are going to re-use, and dry thoroughly. Install new rings on the piston. Make sure that the oil ring is on the bottom. Using a piston ring installer, line up everything and gently tap the piston into the cylinder. Lubricate the cylinder with fresh motor oil beforehand. With the piston at top dead center and making contact with the crankshaft, rotate together by pushing down on the piston so you can re-attach the connecting rod bearing. They only go in one way. Lubricate the camshaft tappets and re-install. Next, install the camshaft by making sure the timing marks are in line. Lubricate all moving parts with motor oil or white grease first. Re-install the crank case with a new gasket. Re-install the cylinder head with a new gasket and tighten down the head bolts in the proper alternating sequence for a proper seal on the head gasket. Install connecting rods and bolts then re-install the valve cover with a new valve cover gasket. Re-install the muffler and re-attach the carburetor, hooking the fuel line and control linkage back up and install the air filter housing with the air filter. Reinstall any covers that you may have taken off to gain access to the engine.