It's a dry, sunny Saturday, so you roll the lawn mower out of the garage, grab the starting cord and pull several times, but although you hear the engine sputtering, nothing happens. Now you smell gas, which means the engine is flooded. Although it wasn't the original cause of the starting problem, flooding will now prevent your engine from starting. The remedy is to wait and address the original reason why your mower didn't start.
Turn the Choke Off
One of the most common causes of flooding is overuse of the choke. Consult your owner's manual -- usually available online at the manufacturer's website -- for the correct starting procedure for your model. In many cases, the recommended procedure is to engage the choke for a maximum of five pulls, then either set the choke to "Half" or "Off" before pulling again. If you leave the choke fully engaged and continue pulling, the carburetor and ignition chamber become saturated with fuel, which you can easily smell. Even so, you can often still start the engine if you turn the choke off.
Other Causes of Flooding
Overpriming the carburetor before pulling the starting cord is another cause of flooding of push mowers -- you don't have to worry about priming if you have a riding mower. Although the correct number of times to push the priming bulb varies from model to model, it's usually from three to five once the bulb has filled with fuel. Even if you use the proper starting procedure, your lawn mower or riding mower can flood if you're using old fuel. Old fuel tends to collect moisture that prevents ignition, and each pull pumps more of it into the carburetor until the smell of fuel becomes noticeable. The same thing can happen if the spark plug is old, corroded or encrusted with carbon and not producing sufficient spark.
The Engine Is Flooded ... Now What?
Once the engine has flooded, the lawn mower won't start until the fuel drains. Most manufacturers recommend letting the mower sit for 15 minutes, and it's important to place it on level ground to facilitate draining. If you're in a hurry, remove the air filter from your push mower to create a more direct air passage to the carburetor and increase the rate of evaporation. Remove the spark plug to ventilate the combustion chamber. After about five minutes, replace the spark plug -- but leave the air filter off -- and try starting the mower with the choke off. If it starts, stop it and replace the air filter. If you have a riding mower, just let it sit idle for the full 15 minutes.
Other Tips for a Flooded Engine
If the lawn mower's tank was full while it was in storage, replace the old fuel with fresh fuel. If you have a push mower, you can do this while you're waiting for the carburetor and combustion chamber to clear. Removing the spark plug gives you a good chance to examine it. Clean, regap or replace the plug if necessary. Sometimes, starting fluid can help start a flooded push mower; after you remove the air filter, spray a one-second squirt into the air intake port and crank the engine with the choke off. If the engine turns over, it will quickly burn off the excess fuel, which will produce a billow of white smoke for a short time.