Many Briggs & Stratton small engines can be found on water pumps, lawnmowers, small tractors and tillers. Simple in design and function, the engines have small carburettors that store and meter fuel for the purpose of combustion. Almost all Briggs & Stratton engines have a provision to adjust the idle mixture, which controls the amount of raw fuel and air that flows down through the carburettor throat into the combustion chamber. A rich-running engine that allows too much fuel will run rough. An engine that has too much air will run lean and stall, becoming overly hot. A few simple adjustments can get your Briggs & Stratton engine running perfectly.
Fill the gas tank of the engine to at least 50 per cent capacity with fresh gas. Use a screwdriver or socket, depending upon the model, to loosen the air cleaner housing top. Remove the air cleaner element and soak it in a bucket of soap and water, then ring it out and dry it with a clean towel. If your engine has a paper air cleaner element, replace it or clean it thoroughly with compressed air.
Reinstall the air cleaner element and housing; tighten the housing with a screwdriver or socket. Refer to your repair manual for the location of your idle mixture and speed screws. The idle speed will sit directly under the throttle linkage arm, and the mixture screw will screw into the side of the carburettor base. The main mixture screw will screw into the base of the carburettor bowel or near it.
Use a screwdriver to turn the mixture screw clockwise until it lightly seats. Use the screwdriver to turn the main mixture screw clockwise until it lightly seats. Turn both screws out -- counterclockwise -- 1 1/2 turns for an initial adjustment. Run the engine at half throttle for five minutes until it warms up. Turn the idle mixture screw in clockwise until the engine begins to slow. Turn it in the opposite direction until the engine begins to slow.
Turn the idle mixture screw in about half way between both points where it slowed the engine. This will be the optimum adjustment. Perform the same procedure on the main idle adjustment screw -- screwing it in until engine slows, and out until engine slows. Turn the main idle adjustment screw in halfway between both low engine speeds. Shut the engine off.
Hook up a tachometer lead to the plug wire, and ground the other lead to a bare metal source. Refer to your owner's manual for the correct connection procedure, depending upon a battery or magneto configuration. Use a screwdriver to adjust the idle speed within specifications. On aluminium engines, idle speed runs 1,750rpm, while on older, cast iron engines the idle speed runs at 1,200rpm.
If either of your idle mixture screws fail to adjust, you must remove them both with a screwdriver and clean the needle tips very gently with some solvent and a rag. Screw them back into the carburettor until they seat, then turn them both out 1 1/2 turns. Complete the adjustment from there.