Troubleshooting Small Engines

When an engine won’t start or runs poorly, here’s what to look for:

If the Engine Is Hard to Start

  1. The vent hole on top of the gas cap is plugged, creating a vacuum in the fuel tank. If the engine starts when the gas cap is removed, a plugged vent hole is the problem. Clean out the vent hole.
  2. The air filter is clogged. Remove the air filter unit and try starting the engine. If it starts, clean and replace the filter.
  3. The fuel line is partially plugged. Remove it and blow it out.
  4. The spark plug is fouled. Remove and inspect it (see Analyzing Spark Plugs). If the electrodes are worn or very dirty, replace the plug.
  5. The spark plug wire is worn out. This wire carries high-voltage electricity from the coil, and any breaks in it may cause a loss of power or a short circuit. Wrap any breaks with electrician’s tape or replace the wire.
  6. The choke isn’t operating properly. With a cold engine, the choke should close completely to make the fuel mixture richer. Remove the cover on the carburetor and inspect the choke in both the open and closed positions. Clean by spraying with carburetor solvent. If you close the choke when starting a warm engine, the combustion chamber will have too much gas for ignition to occur. This is called “flooding” the engine. If you can smell gas when the engine won’t start, let it rest 10 minutes and try again, this time with the choke open.
  7. There is water in the gasoline. This is a common problem in an engine that has been stored with gas in the tank during the winter. Condensation collects and forms a puddle at the bottom of the tank. To remedy drain the tank and fuel line and put in fresh gasoline.
  8. The kill switch is malfunctioning. Engines are shut off with a kill switch that grounds all the electricity. If an engine won’t fire at all, check to make sure that the kill switch is not accidentally closed. You can also remove the wire from the kill switch to check that the problem isn’t in the wire itself.
  9. The points are worn. If the problem can’t be traced to any other cause, inspect the points and replace if necessary, as outlined in Adjusting Breaker Points.

If the Engine Performs Poorly

  1. The air filter is clogged. Not enough air is reaching the carburetor to mix properly with the fuel. Remove the filter unit and see whether the engine’s performance improves. If it does, clean and replace the filter.
  2. The choke hasn’t been opened after the engine is running. As the engine warms up, it needs more air for a leaner fuel mixture.
  3. There’s a clogged vent in the fuel cap. This causes a partial vacuum in the tank, which restricts the flow of gasoline. Remove the cap to see if this is the problem.
  4. The fuel line is clogged. Remove and blow out the foreign material.
  5. There’s water in the gasoline. Gas that’s been left in a tank for more than six months may collect condensation water. Drain the tank and fuel line and install fresh gasoline.
  6. The carburetor is dirty. Remove the cover of the carburetor and look inside. Spray thoroughly with a carburetor solvent.
  7. The carburetor is adjusted improperly See information on carburetor needle valves in Adjusting Carburators.
  8. The engine won’t reach normal speed. Check the linkage to the throttle. If the engine speeds up when you turn the throttle by hand, readjust the linkage.
  9. The engine is running too hot. Check to make sure that the cooling fins around the engine are not filled with grass or other debris. If they aren’t—and if yours is a four-cycle engine—check the oil level. The oil should reach at least to the bottom part of the threads on the oil cap.

If the Engine Vibrates Excessively

  1. The engine mounting bolts may be loose. Shut down the engine and check the bolts.
  2. The cutting blade is out of balance. This is due to incorrect sharpening or a very large nick. See Lawn Mower Maintenance.
  3. There’s a problem in the flywheel. This problem may be a worn retaining key, a cracked flywheel, or a broken flywheel tooth. Inspect and replace if necessary.