Small Engines (Lawn Mowers, etc.): Mower stops running after 10-20 minutes, piece of dirt, circular plastic

Question
Hi,
I have a Craftsmen self propelled mower. It starts up and runs well for between 10-20 minutes, then it begins making a dry, throaty, coughing noise, then dies. If I wait a good 10 minutes or so, it will start back up and run for a while again, then the same thing happens. Last summer, I had lost the inside flat, circular plastic and metal pieces that go inside the gas cap, could that have anything to do with it? Thanks for your help!

Answer
Hi Alan,

As long as the gas cap is venting, you should be okay in that area.  
I would then suspect a piece of dirt in the float bowl or elsewhere in the carb.  I would remove the float bowl and start there, checking the float's needle and seat.  Also, check for a good fuel flow from the fuel line at the carb's inlet fitting.  Rebuilding the carb may be the next step.

Let me know how you make out.
Michael