Lawn mower air filter?

I have a lawnmower that had been running fine. A week or so ago, it started stalling out, just after I'd started running it.
It'd start easy, run for about 20 seconds, then sputter out. It did this regardless of whether I was mowing anything or not. Load didn't seem to have anything to do with it.
I was thinking "gas or oxygen", so I removed the air filter and it ran fine. I then ordered a replacement air filter.
With the replacement air filter in place, I get the same behavior. It starts easily, runs for a bit, then stalls. Without the air filter, it runs fine.
Any ideas?
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Its running rich, maybe the choke is on, carb adjusted to rich, plug fouled, weak spark, float bowl is not completely shutting off, timing key is bad from hitting a rock, or its old and valves are bad, im sure there are other reasons.
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ransley wrote:

If rich sound dof engine and exhaust color will tell.
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On Wed, 02 Jul 2008 13:21:54 -0500, Jeff Dege wrote:

Most carb difficulties come from the main jet being obstructed either partially or completely and/or the needle valve/seat or float preventing the proper level of fuel being maintained in the bowl. Not knowing what motor/carb you have makes it difficult to speculate on why it runs with the filter off and not on. Not knowing if the engine stalls because it is getting too rich of a mixture or too lean makes it even more difficult. If you've never had a carb apart you might just want to take it to someone who has and can clean it, replace the needle and seat and gaskets.
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Jeff Dege wrote:

then choke out. It would stay flooded out from start. Something else is going on. Is it an automatic choke? Is it a dry or wet filter?
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Claude Hopper wrote:

Fuel tank venting rpoblem? If filter is suspect run it without it to prove. To test the fuel cap, loosen it up so air can pass and run the mower.
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If you don't have experience with engines (trouble shooting, repair), you might be better off in the long run to just take it to a service shop and let them deal with it. If you are handy with tools and know a bit about carbs and electrical systems and engines, you might try to tackle it yourself.
Do the basics first. Make sure the fuel tank is clean, and there is no debris in it. Check the spark plug. Might as well replace it, spark plugs are cheap. See if there is a fuel filter in line to the carb, check it to see if it is plugged. Check the plug wire. Look for cracks, and make sure it is clean. Inspect the throttle linkages and make sure they are all clean and moving freely. Check hoses and wiring for obvious problems, cracks, etc.
Does it run good before it dies? If it does, then you can probably eliminate mechanical failure. Make sure there is no grass build up underneath. If you mow wet grass, you can get a lot of grass accumulating there.
If it does not run good before it dies, then most anything could be wrong with it.
If it runs normally before it dies and is easily restarted, then it is probably not getting too rich. An engine that dies because it is too rich is not easily restarted. My first guess would be needle/seat is sticking, float bowl empties, engine stalls. It could be something in the float bowl that blocks the main jet, but does not become lodged in it so that when the engine dies, it falls out until you run it again. Even a leaking gasket could do that. Does it backfire through the carb before it dies? Backfiring through the carb/intake is a sign of a lean mixture.
Good time to take the carb apart and carefully clean it and replace all of the gaskets if you know how to do this. That would be the first thing I do. . If you don't, see your local friendly lawn mower service shop and let them deal with it.
If you know the carb is good and there are not leaks of fuel or air, and nothing else is obviously wrong, time to take it to a shop. Based on your description, my first guess would be a carb problem.

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