My lawn mower starts, but won't stay running. I just used it a few days ago. tryed changing spark plug. checked oil, and full tank of gas

After you check airflow (i.e. cleaning up the air filter), check fuel flow.

I'm assuming the gas is clean and fresh.

Is the gas flowing from the tank?
Is the gas getting into the carb?

If yes, then the next step is to remove the carb.
Check the diaphram between the carb and the tank, this acts as your fuel pump. Is this diaphram in good condition? If not replace it.

While the carb is removed, clean all the little ports (jets) into the carb with WD-40 or carb spray-on carb/choke cleaner (automotive section of your local auto store). Blow it dry with compressed air (canned or from a air compressor). Replace any gasket so the carb doesn't have seepage when reinstalled.

It may sound a bit daunting, but its an easy job and may stores carry the necessary parts. Try WalMart, or auto stores, if they don't then a good hardware store should. Or, a lawn mower shop should have it.

Good luck, I'm rebuilding doing the same thing to one of my mowers this week. Piece of cake.

If it fires and seems like it has hesitation you can also try to sand the brushes just a little bit. A good rule of thumb is to place them a match book width from the magnets. Good luck Im gonna buy a new one :)

If your engine is a newer Briggs that has what appears to be a 1/2" nut at the bottom of the carburetor bowl, first shut off the fuel valve (if equipped) or clamp the fuel line to the carb. Then remove the 1/2" nut, which is actually the main jet, and pull off the bowl. Blast the holes in the jet with carb cleaner. It's a good idea to make sure that none of the holes is pointing toward you when you do that. Next, clean the bowl. It'll probably have some sediment and water in it. Then put it back together, making sure the bowl O-ring is clean and in good condition. Only just snug down the jet. Do not overtighten. Turn the fuel back on and get yer dang yard mowed before the neighbors start complaining. (In my experience this solves the start/stall issue on Briggs mowers every time, which is why I love the Briggs carb design.)

Most problems with small engines these days is using gasoline with alcohol in it. Most of the small engines are not designed to use this kind of gasoline and the alcohol is very corrosive. The smallest circuit in the carb is the idle circuit and it is often blocked by the corrosion and needs to be cleaned out typically with a very small wire to open it. After that purchase gasoline without alcohol in it. I typically only put enough fuel in the tank to do the particular job that I am doing and then let the engine run until all the fuel is gone out ob the carb. Mostly I do this in my weed eater and my leaf blower and do not leave any fuel in my generators.

What engine is it??

If after that checking still no better ..suspect the coil

This is likely to be a fuel starvation problem.
Fuel is fed from a reservoir called the float bowl. This works pretty much like a toilet cistern with a float to cut off flow once it fills. Once the bowl empties and doesn't refill due to a blockage, the engine will cut out.
Clamp off the fuel line or turn off the fuel tap. Remove the bowl, and check fuel flows freely. If it doesn't, there is a blockage in the fuel line which you need to trace.


Another scenario is that the main fuel jet is blocked or semi-blocked. This can cause the engine to run ok when idling (since the idle jet is ok and not much fuel is required), but cut out as you throttle up. This is because more air is sucked in, but not enough fuel is drawn from the main jet due to the blockage. So the mixture is too weak and the engine stalls. The jet can be cleaned with compressed air or ideally a carburetor cleaning aerosol.

See my comprehensive guide for getting stubborn lawn mower or other small engines to start:

Why Won That Lawn Mower Start Small Engine Troubleshooting

Sounds like it's not getting fuel. It starts initially because you have the choke on, but when you open the choke you are getting inadequate fuel. I've found most of the time this is a carb problem - there is a tiny particle lodged in the needle valve which keeps it from opening. For me, most of the time, taking the carb off the machine, take the bowl off, and poking all the holes you can find with small thin wire (and spraying with carb cleaner) will solve the problem.

If it starts and stops it sounds like it is running out of fuel so try something even simpler check the filter in the tank and clean it out also check the tank cap vent sometimes it will need a poke with a piece of wire if it is simply offset holes so the fuel does not create a vacuum & stop fuel running later model caps have valves in them clean and then check the valve operation

Check the air filter for being stopped up also check the fuel filter for blockage .