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Most small engine shops will sell you a membrane and there are four screws that hold the membrane to the carburetor. The membrane can only go on one way, so you should have no problem changing it out. Below I have written some tips for you on other Briggs engine with fuel problembs.

Ride-on and walk behind mowers.... Chainsaws and weed-eaters are below that.

Underneath and attached to the carburettor is the float bowel, held on with a brass nut, which also acts as the main jet. If no fuel filter is in the fuel line, often detritus from the fuel tank can enter the carburettor and clog up the main jet, as well as the idle jet, of course, but idle jet suffer their own problems, as we shall see later. Often, when starting the engine, the engine will run for a few seconds then die. This is a classic case of a clogged main jet. Unscrewing the main jet and blowing out the hole in the jet and releasing a little fuel will cure the problem.

Gumming is a major problem, for fuel is often mixed with ethanol, and, if left to stand in the carburettor for any length, some of the volatiles will come out of solution and a gum residue will be left to form in the fine idle jet and the float needle making it stick in the closed position. The only solution is to take apart the carburetor and let it soak for 24 hours in a solution of Chem-Dip. This will remove all the gum deposits in the idle jet and off needle and its seating. Always use Sea-Foam in your fuel or use a fuel additive such as Sta-Bil. The use of these stops the formation of gumming by stopping the volatiles in the fuel from separating out and forming a gum in your caburetor, thus blocking the carburetors jets. The same tip goes for chainsaws and weed-eaters, too.

On chainsaws, however, do not have the same restrictions as rid-on or walk behind mowers, and are free to run as fast as the engine will allow, torque permitting, so in this case they are self governing. The choke on a chainsaw usually comes off automatically when the throttle is used as it spring loaded, at least on some models, on other models the choke has to be adjusted and taken off manually. However, the carburetor is operating "normally" at full throttle. In this case, the throttle plate is parallel to the length of the tube, allowing maximum air to flow through the carburetor. The air-flow creates a nice vacuum in the venturi and this vacuum draws in a metered amount of fuel through the jet. Some carburetors have two adjustment screws, one on these screws (labeled "Hi" on the case of the chain saw) controls how much fuel flows into the venturi at full throttle and the other the idle.

When the engine is idling, the throttle plate is nearly closed. There is not enough air flowing through the venturi to create a vacuum. However, on the back side of the throttle plate there is a lot of vacuum (because the throttle plate is restricting the airflow). A tiny hole is drilled into the side of the carburetor's tube just behind the throttle plate; the throttle vacuum can now draw fuel into the tube. This tiny hole we call the idle jet. The other screw of the pair "Lo" controls the amount of fuel that flows through the idle jet.

Both the Hi and Lo screws are simply needle valves. By turning them in or out you allow more or less fuel to flow past the needle. When you adjust them, you are directly controlling how much fuel flows through the idle jet and the main jet

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Here are some pictures that may help, but if you go to the Briggs & Stratton web site and enter your mowers model number, you may be able to download a repair manual. http://www.lawn-mower-shop.com/carb2.htm Good Luck