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

Air-to-Fuel Ratio

• If the lawnmower was blowing out black smoke and now it doesn't start, the problem is likely a problem with the air. For the engine to run properly, the ratio of air to fuel should be 14-to-1. If there's too much air and not enough fuel, the lawnmower is hard to start, misfires and idles erratically. However, if there is too much fuel and not enough air, there will be black smoke in the exhaust, and the lawn mower will not start.

Air Filter

• An air-to-fuel ratio imbalance is probably the result of a dirty or blocked air filer. Remove the air filter and see if the engine starts; if it does, the air filter needs to be replaced. Remember that you are removing the air filter only as a troubleshooting step and you should not operate the lawn mower without the filter, because this will allow dirt to enter the carburetor. If you have a foam air filter, it can be cleaned with water and dishwashing liquid and then re-used, but paper filters should be replaced.

Choke

• Another air-to-fuel ratio problem that would cause black smoke and result in the lawn mower not starting is an issue with the choke, which may be stuck in the closed position. The choke should be closed when you are initially starting the lawn mower, but it should be open while the mower is in use. However, it may have gotten jammed in the closed position. Some lawn mowers have a throttle cable that operates the choke. If your mower has this, adjust the cable accordingly.

Carburetor

• If the lawnmower has been tilted on its side, oil may have leaked into the carburetor, which would result in black smoke and cause the lawn mower not to start. If the carburetor has accumulated too much debris, this can cause the same problem. You can purchase carburetor cleaner from an auto parts store and follow the instructions in the owner's manual for taking the carburetor apart. If you don't have instructions and you're not familiar with this process, it may be best to take the lawnmower to a repair shop.