Lawm mower stalls

I have an MTD, self propelled lawm mower with a 5.5 HP Briggs and Stratton engine. Over the last several weeks, it has developed a problem where it starts right up, runs for about 20 minutes and stops. Sometimes it coughs and sputters for awhile before stalling, other times it's as if it was turned off. There have been times it's done that and would restart, other times, I have to wait until the next day. I have checked the air filter, checked the gas line, and checked the spark plug cable. What I haven't done is replaced the spark plug. If this helps, I thought it started moving alot faster about the time it happened but for the life of me, I can't see how that would be connected to the problem.
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If it ran faster just before it died , it may be an indication that it is being starved of fuel. Most small engines will rev. somewhat when they run out of fuel. Is there a fuel filter to check? Is there dirt getting into the needle valve? Is the float sticking closed? Is the carburetor dirty? Is there water or dirt in the tank?

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rile,
As gas is drawn from the gas tank air must come in through a valve, typically a cheap reed valve. These valves can fail. So you run the mower until a vacuum builds up in the gas tank and stops the flow of gas. Next time it stops take the gas cap off, put it back on, and try to start it. If it starts up then suspect the reed valve.
Dave M.
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the only thing using a reed valve is a two stroke, and that would have nothing to do with building a vacuum in the tank.
s

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The cap usually has a vent. It may be plugged.

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Rile,
You've already done the test by taking the cap off. You have a bad valve or vent. Look at the cap, is it vented? Drain the tank, get a flashlight, and look for a valve. This is a really cheap fix, you owe me a cup of coffee.
Dave M.
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Run with the gas cap loosened about 1/4 turn, see if that helps.
My couple thoughts were * Bad motor oil, as strange as that sounds. * Valve clearance problem, if it's an older motor * Ignition coil going bad
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Christopher A. Young
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5/5 HB Briggs don't have reed valves. Only found on two strokes.
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If the engine is more than a few years old, it may have valve clearance problems. To check this isn't all that hard, with a socket set and a couple of feeler gages.
To fix it requires dissembling the engine, and not many people have the tools. I tried to make a living of small engine repairs, and I do have the tools.
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