Small Engines (Lawn Mowers, etc.): primer bulb, carburator, gas tank

Question
I am trying to repair my mother-in-laws 3hp briggs engine on her lawnmower.  After the engine has been running 10 to 20 min. the primer bulb starts collasping then engine quits running.  After waiting a couple of minutes the bulb will return to its normal position.  What could be causing this.  Thanks for your help.

Answer
 Hello Charles:

 Check the Gas Tank Cap and Ensure it is Venting the Tank. It the Cap Doesnt Vent the Tank, a Vaccum will Occur in the Tank and Stop the Gas Flow to the Carburator. That is why the Primer Bulb is Collasping. If you have the Carburator Mounted on the Tank, then Check the Pickup Tube Screen and Clean the Tank. The Pickup Tube Screen will Slowly Stop Up and have the Same Effect. Lastly will be Trash in the Carburator Jet. Usually the Jet will cause a Problem all the Time, Not Just after 10 to 20 Minutes. Hope this Helps. I am here if you Require more Assistance. Let me know what Happens, Please. Thanks.

 Good Luck

 Respectfully

 John