What is Wrong if My Briggs & Stratton 15 HP Emptied All the Gas into the Engine?

Larger engines on riding mowers improve not only mowing power but also their ability to power accessories such as snowplows -- and the Briggs & Stratton 15-horsepower engine is used by several lawn tractor manufacturers. This workhorse engine, like others, requires regular maintenance for smooth performance. Many parts can contribute to limiting flow of fuel into the engine, but repeated emptying of gas into the engine typically traces back to the carburetor.

About the Carburetor

The carburetor manages fuel as it flows from tank to engine, mixing air and fuel to supply the explosive vapor necessary for combustion. Fuel moves from the tank by gravity and the vacuum formed in the bowl as gas leaves the carburetor for the engine. Exiting fuel moves through a tiny orifice regulated by a pointed pin, called a needle, mounted on the hinge end of a float that rises and falls as fuel enters the carburetor from the fuel tank. On a 15-horsepower Briggs & Stratton engine, the carburetor sits behind the air filter, next to the engine.

What Can Go Wrong

Clogged fuel lines, dirty fuel and air filters and faulty starters each cause stalling, but the carburetor controls the volume of fuel sent to the engine. Worn or broken gaskets, one around the rim of the carburetor's bowl and the other that seals the opening for the nut on its bottom, can produce leakage under the carburetor. A worn shutter between the carburetor and the engine -- or one that is stuck open due to dirt or gunky petroleum solids from old gas -- allows gas to flow directly through from the fuel tank to the engine, flooding and stalling the piston.

Needle in the Valve Kit

The collection of tiny parts known as the valve controls the flow, provided all else is working properly. The doughnut-shaped float sits in the bowl, rising and falling on a hinge as fuel flows in and out. The needle valve sits in a slot on the float and meters the correct amount of fuel. The needle rises and falls inside a small passage against a tiny gasket. This motion allows just the right amount of gasoline to rise into the carburetor's main chamber where it mixes with air, and then vaporizes and shoots into the engine through the shutter. If the needle slides out of the passage, the gasket shrinks or breaks, or the hinge pin on the float breaks or slides out, there is no control on the flow of gas into the main chamber, and liquid gas will run, unimpeded, from the fuel tank into the engine.

Danger, Danger

Liquid gasoline running into and over a hot engine poses a hazard of fire, so keeping your mower's carburetor clean and in proper condition is important. Cleaning the interior with carburetor cleaner and replacing worn gaskets should be part of annual maintenance. Rather than using worn or dirty valve parts, replacement of the hinge pin, needle and gasket will ensure proper functioning. All necessary parts are available in inexpensive valve kits.

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