Lately, my lawn mower has had the very annoying habit of running for awhile and then inexplicably dying (where I have to wait a long time before it starts again).  WHEN the mower is running, it runs strong so I don't think it's the points (do small engines still have these?) and while I can't rule out bad gas, if it WERE bad gas, wouldn't it refuse to start at all?

 

My only thought is that gas tank is not venting properly and a vacuum is forming in the tank and not allowing fuel to get to the carburetor.  Is this a reasonable guess?

You can try a new gas cap, but it sounds like the carb needs rebuilding/replacement.  I replaced the one on my rider and now it works like a champ.  Was doing the same thing you are complaining about.

Vapor lock

I takes fuel, ignition and air, all delivered at the right time and proper ratio, so all you do is isolate which one is missing.

The two most common causes of your problem is fuel delivery or bad magneto/coil. 

The first thing you want to do is run it until it cuts off, check and make sure you have a good blue spark to the plug.  Easiest way to do this is take the plug out in a low light area, put the plug wire on it and hold the base against the metal block.  If the spark is red and not blue, or if there is no spark, good chance the coil is bad, but you need to make sure the kill wire is not shorting to ground first.  

 

If you have a good blue spark and it's not wanting to restart, then it's probably a fuel delivery problem.  To help with that, I would need to know what kind of motor and carburetor.

Seafoam it pour about 2 oz in a full tank of fuel let it run for a while then shut it off.it will get in carb and clean any sludge in the needles and jets ..and if that dont work the most likly is carb or your sparkplug is burning cold BTW go with a autolite spark plug if u get a new one and no gas over a week old gas has ethenal in it and that soaks up moisture so if gas is old over a week or 2 its getting moisture from the night to day change in temp.