It's the brass bolt on the bottom of the fuel bowl (float bowl). Tip the mower up on it's right-side wheels so the carb is up. Remove the bolt and the bowl should stay in place. Clean the bolt and it's apertures, including one you might normally miss - it's a black speck at the top at the end of the threads. Take a wire tie like from a bread wrapper, strip the paper off and use it to rod out this bleed that goes down at an angle. Blast out with carb cleaner. Replace bolt and snug down, don't over-tighten. Set the mower down and check for a leak at the bolt. If it starts leaking, give it a 1/8 turn with your wrench - repeat if needed. This should get the mower back to work. You can forestall future problems by using a fuel stabilizer/fuel system cleaner like Sta-Bil or Seafoam all the time. Put it in your gas can when you buy gas. At the end of the season, you don't have to do anything with the fuel- just do the other year-end things for storage.

This is a common problem on lawnmowers produced during the past ten years. Emission engines are calibrated right on the edge and cannot tolerate any dirt in the fuel system. Additionally, fuel deteriorates in as little as 30 days. Fuel varnish forms on internal passages in the carburetor and reduces the amount of fuel delivered to the cylinder. The main fuel jet located on the bottom of the float bowl must be clean for the engine to run properly. A lean fuel mixture causes the surging and stalling you are describing. There are some good comments in the previous post. I assume this is a typical Quantum engine with the primer system. When servicing a mower, I clean the entire fuel system by removing the fuel tank and draining. Spray some carburetor cleaner in the tank to better clean the tank interior. Let the carburetor cleaner drain out and use air pressure to dry the tank interior. I recommend a new fuel line since your original line is probably deteriorated. When fuel lines are installed or pulled off, tiny particles of fuel line materials may be left in the fuel line so it is very important to flush the line. Cleaning the carburetor with a spray carburetor cleaner dissolves the fuel varnish. Attempting to clean with gasoline or kerosene will not be effective. When cleaning a carburetor with spray cleaner, the viton seat for the needle valve may swell up from the carburetor cleaner. This results in lower fuel level in the float bowl and the primer may not work as it should. Additionally the fuel may have a slightly greater distance to be drawn into the carburetor venturi area. Replacing the needle and seat is a good idea. The bolt that holds the float bowl in place also contains the main fuel jet. The jet must be cleaned with the spray carburetor cleaner to remove all varnish. Replace the o ring that seals the carburetor to the mounting plate. Finally, install a new gasket between the carburetor and the air filter housing. Sometimes the air filter mounting area warps and the gasket does not seal effectively - causing a weak prime.Get rid of the fuel in your spare fuel can. If there is evidence of rust, replace the can. Make sure there is no dirt or grass residue in the spare fuel can. Add about 1/2 ounce of fuel preserver to the can - then add 1 gallon of fuel. If this fuel is not consumed within 30 days, get rid of it by pouring in your car. While this step may seem extreme, I am sure lawnmower techs will verify the importance of using only fresh fuel that is treated with fuel preserver. The old B&S Quantum engine was a simple flat head engine that generally gave good service. Good luck.

Does it seem like the engine is being shut off? Typically the seat switch can cause this condition or the no mow in reverse wire is shorting to ground. A stuck blade/ blade spindle or a deck belt that has rusted to the pulleys will physically stop an engine as well. A full model # from under the seat could help should you report your findings.

>> Rule #1: Never assume the rocks you buried the previous year stayed buried. That's how I killed a perfectly good Honda pushmower. I sell those at the Farmers Market as potatoes. You'd be amazed how many people buy 'em