The valves in a Briggs & Stratton engine control the flow of fuel and air into the engine, and exhaust out of the engine. Improperly adjusted valves can lead to a variety of problems, including loss of power, stalling, excessive fuel consumption and backfiring. With some basic tools and a little mechanical aptitude, you can adjust the valve clearance on a Briggs & Stratton engine in about an hour. The procedure for adjusting the valves varies between models, but the basic steps are the same for all Briggs & Stratton engines.
Disconnect the rubber boot connecting the spark plug to the magneto. Use a socket wrench to unscrew the spark plug and remove it from the engine cylinder.
Remove the screws holding the plastic cowling on top of the engine. Lift the cowling and turn it to the side, being careful not to kink or break the fuel lines. Remove the air filter housing, carburetor, muffler and any other components blocking access to the cylinder head. This varies by model; some models require you to remove all of these engine components while other models don't require you to remove any of them. Remove the rocker-cover screws and pull the cover off to reveal the valves.
Turn the flywheel by hand until both valves are closed and the piston is at top dead center. Insert a dowel into the spark plug hole until it contacts the piston. Make a mark on the dowel even with the edge of the spark plug hole. Turn the flywheel until the piston is at its lowest point. Make another mark on the dowel. Turn the flywheel until the valves are closed and the piston is at top dead center.
Measure the distance between the two marks on the dowel. Make another mark on the dowel that is one-quarter of the distance from the first mark to the second. Insert the dowel in the spark plug hole and turn the flywheel past top dead center until the third mark on the dowel is even with the spark plug hole.
Check the manufacturer's documentation to determine the correct valve clearance for your engine. Slide a feeler gauge the thickness of the correct valve clearance between the rocker arm and the valve head.
Hold the rocker nut with an open-end wrench. Loosen the setscrew in the rocker nut with a star-head driver. Turn the rocker nut clockwise to decrease the valve clearance and counterclockwise to increase the clearance. Adjust the rocker nut until the feeler gauge slides between the rocker arm and valve head with slight resistance. Tighten the setscrew to the correct torque using a torque wrench with a star bit to lock in the valve clearance. Adjust the other valve using the same procedure.
Place the rocker cover on the cylinder head and screw it in place. Install any components that were removed to reach the cylinder head. Place the plastic cowling on the engine and install the retaining screws. Screw the spark plug into the cylinder head and tighten it securely. Place the spark plug boot on the end of the plug and snap it into place.