Small Engines (Lawn Mowers, etc.): Mower will not start, internal passages, air gap

Question
I have a Snapper rear engine mower with a B&S engine.  It has worked fine up until a few weeks ago.  Engine will turn but will not fire.  Thought spark plug may have been bad, so I replaced it.  It started up fine.  After mowing for about 30 minutes, had to stop and turn off engine.  When I came back, it would not start and has not started since.  Engine just turns over.  I know I probably need to do a tuneup as I have not done a proper one in some time.  Thanks for any advice.

Answer
First off, check for spark.  If the ignition system is working correctly, it should be able to produce enough current to jump a 1/8" gap to ground.  If you have no spark, you will need to go over the ignition system (if it's older, replace the points and condensor) and make sure the air gap between the coil and the flywheel magnets is 0.010".  If there is ample spark, you may need to thoroughly clean the carb, including pushing a piece of tag wire through all of the internal passages and following up with aerosol carb cleaner.  Another issue could be the fuel tank vent being blocked. Try loosening the cap if the vent and running. If it corrects the problem, replace the cap.  Another issue could be incorrect valve lash. You would need to adjust the valve lash to the correct clearance. Hope this helps.