Troubleshooting

• Examine various parts of the lawn mower while it is running. Pull the throttle cable back to determine if it came loose. A loose throttle indicates the engine is not getting enough fuel. Check the governor if your mower has one. The governor is located at the motor end of the throttle cable. If the governor is moving, it may indicate the spring holding it is loose or has become unsprung. Locate the carburetor -- usually found underneath the air filter. Remove the air filter and spray the carburetor with carburetor cleaner. If the engine still surges, professional repair may be required.
  
  Examine various parts of the mower with the engine off. Disconnect the spark plug and examine it for debris or a dark coating. Either could indicate buildup of oil around the engine. Buildup can force the engine to surge while trying to work fuel through the debris. Turn the mower on its side. Observe the mower blades for blockage of debris underneath the housing. Debris and grass can block the blade, causing a surge when the motor attempts to push the blade through the debris.
  
  

Mechanics

• Tightening and cleaning the mower's moving parts may eliminate surging. Turn off the mower before performing maintenance. Unplug the spark plug to ensure no accidental starts or sparks occur. Tighten the cable between the throttle and the motor. Adjust the spring at the governor. If the cable or spring cannot be adjusted, replace them. Clean the air filter, mower blades and carburetor to remove any debris. If the air filter or carburetor is clean and the engine still surges, the pistons or motor itself may be the issue; this requires professional attention beyond the scope of home repair.
  
  

Other Considerations

• Cutting thick or tall grass may cause accelerated buildup of debris. Wet grass also causes buildup because moisture makes the grass stick to the underside of the mower. Using old fuel or oil, or improper amounts of either, causes the motor to work harder, which can also lead to surging. Constant and continuous wear on the machine without regular maintenance causes parts to age quicker; the aging process causes spark plugs to go bad, cords and springs to loosen and motor parts to lose their agility due to viscosity from fuel and oil.
  
  

Prevention

• Maintaining a routine schedule for cleaning is the best preventative measure against surging. Clean the moving parts of the mower after each use. Check the parts as you clean for debris and damage. Remove the air filter and spark plug for cleaning semi-annually. Drain the fuel from the mower at the end of each season by running the mower until it is out of gas. Drain the oil into a container by tipping the mower on its side before storing the mower for winter. Change the fuel and oil annually.