Small Engines (Lawn Mowers, etc.): B&S loses power when warm, briggs stratton, head gasket

Question
My Briggs & Stratton 17.5 hp (model number 31C707) lawn tractor runs fine when first started but after about 20 minutes it begins to lose power, growing progressively worse until I shut it off. When cooled down, it runs fine at first and the same thing happens when it warms up. I have replaced the air filter, fuel filter and spark plug but with no change. What could it be?

Answer
Hello Brad:

Check the Gas Tank Gap and Ensure it is Venting the Tank Properly. This can be the OHV Requiring Adjustment, a Bad Ign Module/Coil/Magneto or a Bad Head Gasket. If the Gas Cap is Venting Properly, then I Suggest you Reset the Valve Clearance and then Check/Replace the Head Gasket if the Valve Clearance Resetting Does Not Correct the Problem. You can View a Breakdown/IPL of the Engine at this Site Addy; http://www.briggsandstratton.com/maint_repair/manual_and_more/doclist.aspx?categ... and Enter the Model Numbers (XXXXXX) and Type Numbers (XXXX) in the Appropriate Boxes and Click GO. Then Select Download PDF. If the Gas Cap, Valve Clearance and Head Gasket are All Good, then the Ign Module/Coil/Magneto is Most Likely Breaking Down as it Heats Up and is Causing the Power Loss by Not Firing the Plug Properly. If you Hit a Snag or this Does Not Correct the Problem,  I am Here if You Require More Assistance. Hope this Helps. Let me Know What Happens, Please. May the All Mighty Bless You and Yours. Thanks.

Good Luck

Respectfully

John

PS: I am Allowed to Answer 10 Questions a Day. If you See I am Maxxed Out, then Try Submitting your Question at or Just After 8pm EST (US). My New Day Starts then. Thanks.

Respectfully

John