Small Engines (Lawn Mowers, etc.): 8 hp briggs stall after about 20 minutes, hp briggs, fly wheel

Question
Hi Michael,
1st,  once a tool and die maker, always a tool and die maker.
What up with this ex stuff?
I have inherited a 25 yr old John Deer with an 8 horse briggs.  It starts well and runs for 15 to 20 minutes and then starts missing and puffing blue smoke.  Then usually stalls and back fires.  If I wait 10 minutes it usually starts again but in a couple of minutes the same thing happens.  I have carb cleaner in the gas and I removed the bowl and sprayed that area but it didn't seem to dirty.  I also took the fly wheel off to clean any rust.  Very little found.  Other notes;  sometimes if I disingage the blade I can keep it from stalling for a little longer.  It will stall imediatley going up my minor hill in the back yard.
I would appreciate your input.  
thank you,
was, is and always a t&d maker,
Gary L  

Answer
Hi Gary,

Yes, you're right.  Not once but always a Tool & Die Maker.

The smoke and backfiring indicates a valve or ignition problem.  The valve clearance may be too small or nonexistant, causing improper sealing when the engine gets hot.  Also check the ignition points.  
Other than that, I can't think of what else to suggest.

Let me know how you make out.
Michael