Most engine starting problems are due to stale old fuel, or dirt and debris in the fuel system. Another common cause (for lawnmowers) is making sure to hold on to the safety bail when starting/pulling the rewind cord.
Basic Repair (Advanced Repair information can be found here )
Repair projects should always begin with troubleshooting - the search for the source of a problem - starting with the most obvious or simple explanation and working toward the less obvious or more complex.
Typically, starting and/or rough running conditions are found in either Fuel, Air, Compression or Spark (Ignition) Systems. Visit "Troubleshooting for poor engine performance" for tips on a poorly running engine.
When you're troubleshooting a small engine problem, you need to rule out the various parts or systems as possible sources of the problem. It's important to work systematically to isolate the cause rather than skipping parts or systems that you believe are in good working order.
It's a lot like looking for a set of keys: often, they're in an "obvious" place that didn't seem worth checking. The solution is not to overlook things that seem obvious. They just might hold the keys.
For successful troubleshooting:
Make sure you consider all symptoms carefully.
Look for the cause, not just a cure for the symptoms.
Gather as much information as possible. Knowing whether an engine was at top speed when it stopped running or whether it simply failed to start may make a difference when you're trying to identify the problem.
Remember that a simple solution is not always the correct one, or may be only a partial solution. Replacing a worn spark plug may get an engine running, but the real culprit may be a carburetor that is partially blocked. In this case, the problem will probably turn up again soon.
Most four-stroke engine problems fall into one of these categories: