Why wont my lawn mower keep running?

We just had the same problem with ours. my husband removed the plastic cover on the top and discovered that the spring that holds the throttle open was rusty. it lost it's strength to hold the throttle open to keep the gas getting to the engine. he used a zip tie to fix it until we can get a spring. open the cover and have someone move the throttle bar up and down and you will see where the throttle is from it's movement near the engine.