Mower start puttering after 20 minutes of running fine.

This one has me hornswoggled. I have a fairly new push mower your typical l ow-end briggs and stratton engine. The thing starts perfectly, runs perfec tly for about 20 minutes and then starts sputtering. Not enough to stop th e engine but like it is choking or something.
Any ideas?
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On 5/16/14, 7:08 PM, noname wrote:

One thing to check for is a clogged vent hole in the gas cap.
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On 5/16/2014 7:08 PM, noname wrote:

new push mower your typical low-end briggs and stratton engine. The thing starts perfectly, runs perfectly for about 20 minutes and then starts sputtering. Not enough to stop the engine but like it is choking or something.

Some of the possible causes:
Bad ignition coil, fails when hot Bad brand of motor oil Valve clearance Clogged vent on gas cap
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On Saturday, May 17, 2014 6:56:26 AM UTC-4, Stormin Mormon wrote:

That's a new one. How would that cause the symptom?
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On 5/17/2014 8:03 AM, trader_4 wrote:

Some years ago, I met a mechanic who assured me that cheap motor oil doesn't work properly in air cooled small engines. I am not sure the reason. I did put cheap oil in a lawn mower I sold, and it came back "doesn't run right". Drain it, and put in good oil, and the next customer was happy. Of course, that's not scientific. Beyond that, not sure.
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On Saturday, May 17, 2014 9:47:09 AM UTC-4, Stormin Mormon wrote:

I've tried everything. New gas, plug, changed the oil, took the air filter off, checked the gap cap for venting. Also, took off the engine cover whe re the flywheel is to make sure it wasn't clogged either.
With the air filter off I do notice something though, when the thing is spu ttering the little choke valve that is over the carb and is attached to the throttle and the little springs just keeps popping open and closed over an d over again. Make sense why it is sputtering then but I can't see anythin g wrong with it. I tried fiddling with the springs and experimenting with restricting it, but doesn't seem to help. My guess is either something is making the thing choke on and off over and over or it is hooked up wrong.
Further debugging ideas anyone? Thanks.
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On Sunday, May 18, 2014 8:09:36 PM UTC-4, noname wrote:

er off, checked the gap cap for venting. Also, took off the engine cover w here the flywheel is to make sure it wasn't clogged either.

puttering the little choke valve that is over the carb and is attached to t he throttle and the little springs just keeps popping open and closed over and over again. Make sense why it is sputtering then but I can't see anyth ing wrong with it. I tried fiddling with the springs and experimenting wit h restricting it, but doesn't seem to help. My guess is either something is making the thing choke on and off over and over or it is hooked up wrong .

Oh..jeez I think the long governor spring is attached to the wrong place. There isn't enough tension against the governor arm and that is why it is flapping. Too dark to go and try this fix out... does this make sense?
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On 5/18/2014 8:22 PM, noname wrote:

Could be. The governor springs sometimes get rusty and don't spring correctly.
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On 5/18/2014 8:09 PM, noname wrote:

oil, took the air filter off, checked the gap cap for venting. Also, took off the engine cover where the flywheel is to make sure it wasn't clogged either.

when the thing is sputtering the little choke valve that is over the carb and is attached to the throttle and the little springs just keeps popping open and closed over and over again. Make sense why it is sputtering then but I can't see anything wrong with it. I tried fiddling with the springs and experimenting with restricting it, but doesn't seem to help. My guess is either something is making the thing choke on and off over and over or it is hooked up wrong.

It occurs to me that the choke opening might be a symptom of faltering, not the cause of the faltering. ATM, my guess is bad ignition coil.
If it were valve clearance problem, it would fade and die until it's cooled down.
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Is it still under warranty?
Dave M.
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wrote:

I do notice something though, when the thing is sputtering the little choke valve that is over the carb and is attached to the throttle and the little springs just keeps popping open and closed over and over again. Make sense why it is sputtering then but I can't see anything wrong with it. I tried fiddling with the springs and experimenting with restricting it, but doesn't seem to help. My guess is either something is making the thing choke on and off over and over or it is hooked up wrong.

Hold the choke open or put a wire on it to make sure it stays open. If that dont fix it, check to make sure the gas filter or screen inside gas tank is not plugging up. Another thing, is the carb loose? Tighten carb screws or bolts incl. intake tube. (manifold).
Sometimes a magneto will begin to fail when its hot, due to expansion causing some high voltage leaking. Electronic ignition modules (if yiou have one) can get screwy when hot too, or the capacitor at the points can be dying.
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wrote:

I've had mowers do that when they have water in the gas. Makes no sense to me that they run right for a while and then run poorly but draining the carb bowl clears it up. I've also had this happen when there appears to have been something not even visible clogging the jet in the carb but taking the jet out, blowing it out and buttoning it back up made it run perfect afterward.
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On 05/18/2014 05:22 PM, noname wrote:

Yes, governor springs are incredibly touchy, but it wouldn't explain why it is running for 20 minutes before it starts causing trouble.
One possibility is that you have a bad coil, the symptoms of which do not show up until it warms up.
Jon
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