Sounds like the float in the Carburetor is stuck
This results in the fuel not filling the bulb correcctly which is why it will start and run for about 10 secs then slow down and stop.
Pretty common fault in small engines after being dropped.
You will need to pull the carbi and reseat the float.
Note - If it started there is nothing wrong with the magneto
laserguidedbrick, January 2009