Small Engines (Lawn Mowers, etc.): mower quits after 1-2 hours of mowing, valve clearances, troybuilt

Question
twin cyl l head. b & S 18 hp I think.
Runs great for 1-2 hours then quits. acts like dead battery, checked battery at 12.5 or better not running, charging at 13.o or better when running. originally thought magneto, swapped with sten brand last summer, thought fixed. still quits, will start after cool down. motor off of different mower mounted on troybuilt bronco. pretty sure compression good, charging good, battery good... what is left, really puzzled.

Answer
Hello Jerry:

>>Have you Checked the Gas Cap and made Sure it is Venting Correctly?
>>Check the Valve Clearances.
>>How does the Engine Quit?
>>Is it like it is Slowly Losing Power or like the Ign Switch has been Turned Off?
>>Check the Fire to the Plugs as Soon as the Engine Quits.
>>If there is Fire to the Plugs and the Engine Turns Over like a Dead Battery, then
>>Check the Valve Clearances with the Engine Warm/Hot.
>>Sometimes the Valve Clearances will cause this Problem.
>>You can View a Breakdown/IPL of the Engine at this Site Addy; http://www.briggsandstratton.com/maint_repair/manual_and_more/ and Enter the Model Numbers  and the Type Numbers(XXXXXX-XXXX-XX) in the Appropriate Box and Click GO. Then Select Download PDF.
>>If you Hit a Snag or this Does Not Correct the Problem,  I Hope to be Here if You Require More Assistance. Hope this Helps. Let me Know What Happens, Please. May the All Mighty Bless You and Yours. Be Safe and Happy. Thanks.

Good Luck

Respectfully

John

PS: I am Allowed to Answer 5 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. Comments and Ratings are Greatly Appreciated. Thanks.

Respectfully

John