Why Does My Lawn Mower Blow Out Black Smoke & Now Doesn't Start?

Black smoke is not always a sign of a ruined mower.

Black smoke is not always a sign of a ruined mower.

Black smoke emitted from a lawn mower engine is typically an indication that the gasoline to air ratio is too rich. The mower is probably consuming far more gasoline than it should during operations. Black smoke is also an indicator that oil has found its way into the cylinder, and the mower is trying to burn it along with the fuel. In some cases, the mower will not restart.

Check the Choke

When you start a mower, the choke closes so that the carburetor sends a mostly gasoline mixture to the cylinder. Once then engine starts, the choke opens since that rich mixture is no longer necessary to keep the engine running. If the choke sticks, it fails to open and the gas mixture continues to be too rich. Check any linkages to the choke to make certain they are working correctly, and make sure the choke is free of dirt and debris.

Clogged Air Filter

A dirty air filter creates the same problem as a sticking choke. The carburetor cannot pull in the correct volume of air, so the mix is too rich. Most mower air filters are made of either foam or paper. Foam filters may be cleaned and recharged with a light coating of fresh engine oil before reinstalling. Paper filters, however, must be replaced instead of cleaned.

Carburetor Adjustments

A lawn mower carburetor typically has two adjustment screws mounted on the outside of the unit. One adjusts the idle, and the other adjusts the fuel mixture. Unless you are versed in carburetor adjustments and repairs, you may want to take the mower to a qualified person for servicing. Inside the carburetor bowl, a float rises and falls with the amount of gasoline in the carburetor. If the float is sticking, or has developed a hole, it cannot regulate how much gasoline enters the carburetor. The result is a too-rich gasoline mixture and a smoking engine that will no longer start.

Burning Oil

Oil can enter the cylinder from several sources. Overfilling the mower with engine oil can force some of the excess into the carburetor, soaking the air filter. The carburetor sends this oil, along with the gasoline and air mixture, on to the cylinders, where the engine attempts to burn the mix. A black smoke cloud follows. More difficult repairs lie ahead if the oil leaks into the engine cylinders through failed gaskets or damaged cylinder rings, both of which are designed to keep the lubricant out of the ignition cylinder.

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