Small Engines (Lawn Mowers, etc.): lawn mower, valve job, gas hose

Question
I have a Rally self propeled mower with a 4hp briggs & straten motor. It starts fine then after cutting a path about 500 feet it lost power and died. When I restarted it there was no power, just enough to keep it going without cutting, kind of like running with the choke on but without the smoke. I cleaned and checked around the carb and entire mower and do not see a thing wrong. I started it back up after about 15 minutes and it ran for about 200-300 feet then it is like someone just turns the key off. It won't start until it sits for about 10 minutes than it goes like a pro for about the 200 to 300 feet and it dies again. I took the gas hose off and gas pours out the tank so there isn't any blockage there and once I started cutting with the fuel line shut off so I figure it does have enough gas in the carb. I also put a new spark plug in it and thouroughy cleaned the safty shut off. I tried to adjust the carb but it didn't do much. I have no way to take the flywheel off to see anything there but the magneto looks fine. Any ideas?

Answer
I suspect the problem is related to heat if you say it cuts good for a short distance and dies.  I would suspect the engine either needs a valve job (and restore proper tappet clearances) or there is an ignition problem.  When it quits immediately pull the spark plug and check for spark.  If it has spark, rotate the engine over slowly and listen for a whisping sound (usually in the exhaust) which indicates a valve(s) is leaking.  Hope this helps.