Lawn mower problems!?

Ok im only 14 i couldent find anywhere that would let a 14 year old work and i found a lawn mower our neighbors threw out so i decided to go around mowing yards. i changed the oil, carburator, gas, air cleaner, fuel line, spark plug, and pull rope. It was running good but yesterday i was working on it and the only way to get it to start is to set it to medium throttle and as soon as it starts it shuts off. So i put it at medium throttle again and as soon as it starts you have to switch it to full throttle. i dont get it, it shouldent be run ning like this, BTW its a craftsman 22" mulcher push mower with a 5.5 horse teseh motor.