It could be missing, or it might be a fuel injection problem.
Check your spark plugs and wires and replace them if they are old and worn out.
You can run some fuel-injector cleaner in your gas to remove the shellac that builds up in the injectors (careful when you put it in - it removes paint too).
Replace your air filter, breather filter, and pcv valve too.
If none of this fixes it, you might need to find a mechanic to troubleshoot it for you.