Small Engines (Lawn Mowers, etc.): Briggs & stratton on Sears mower, briggs stratton, sears riding mower

Question
I have a Sears riding mower with a Briggs %26 Stratton engine.  The engine runs perfectly for about 20-30 minutes then it starts to miss as if out of gas.  It stutters for a few seconds than runs perfectly for another 20-30 minutes.  It has a new filter, new oil and air filter.  It may be that the gas is set to rich.  If so, what screw do I turn, which direction and is a quarter turn a lot.

Answer
Hi Harlan
 It sounds like to me that you may have trash in your fuel tank that is restricting the fuel flow. If the fuel mixture is too rich, you would notice black smoke coming from the muffler and the engine would be sputtering all the time. Hope this helps.