I have a Craftsman 21 in. self-propelled mower, model No. 247.374900, Briggs & Straton engine. It will run just fine for a while - 5 min., 15 min., maybe even 45 min. But then it dies. It will start right up and then dies within a few seconds. I replace the spark plug every year, the air filter is new, oil level is fine, gasoline is the best of the options and is two weeks old. I have taken it to three mechanics and each cleaned the carburetor, checked the gas line and...whatever else. Each time they pronounced it fixed, and each time I had to take it back. The last mechanic said he let it run for over an hour and it was fine. Any ideas what the problem might be? When it isn't dying, it runs great!
Martha

 

The next time it starts to die loosen the fuel cap it may not be venting properly, if it continues to run either clean the cap vent or get a new cap. Have a good one. Geo

 

I just started it with the gas cap off and it died immediately.

 

There is a cable that goes to the handle you have to hold to keep the engine running. Follow it down to the engine and see the mechanism that moves when you pull the handle. Hold the handle down and then with your finger, move that mechanism a little further int he direction the cable moves it and see if it runs longer.

 

Magneto is a possibility...???

 

Can you be more specific? Magneto is a comic.


I found it - under all the comic references. What would you expect to happen if the magneto goes bad?

I tried to start it but it won't start. It's interesting that whatever it is the mechanics are doing, they are getting it to start - for a bit. I'll get some brand new gas today and see if that works, and I'll pass on your suggestions to the mechanic. Thanks guys.

 

When it won't start, check for spark. magneto/coil is the ignition and symptoms especially on briggs of one going bad is loosing spark when they get warm.

 

The magneto creates the spark. There is a coil under the flywheel (what the spark plug wire connects to). There is a magnet in the flywheel. When the magnet passes by the coil, it induces a high voltage in the coil, which fires the spark plug. If the magneto is bad, you won't get any spark. There ususally is a set of points and condenser near the magneto. Possibly the point gap is too small and is changing when the engine warms up.

 

Thanks all. I'll pass all of this on to my mechanic. I just know he will be thrilled to get the mower back.
Martha

 

The nut that holds the bowl on the carb is the main jet it has a hole down the center and another across the bottom they may be getting clogged from junk in the bottom of the fuel tank or a deteriorating fuel line, remove the bowl and check for debris in the bowl if you find it to have black bits in it it is probably from bad fuel line, be very careful when replacing the bowl so a not to screw up the bowl gasket. Have a good one. Geo

 

Thanks, I'll mention it. That carb and bowl should have been cleaned, by my calculation, six times by now.

 

LOL you will not have points on that engine, the spark plug creates the spark, the flywheel magnet and magneto produce the voltage for the spark plug to produce spark
There is no coil under the flywheel on this engine, any coil under the flywheel is an alternator and produces voltage to charge a battery or run lights.

The magneto is a stack of iron plates that bolts next to and outside the flywheel, as the magnet on the flywheel rotates past the magneto stack, the opposite poles of the magnet create a voltage that is stepped up now by solid state components and released by the same, not points and condensor.