Small Engines (Lawn Mowers, etc.): Engine dies, internal passages, throttle plate

Question
We bought a go-cart.  the engine runs when idling, and runs alright when maxed out,  but dies as soon as you let off the gas.  it also dies sometimes when you first hit the gas.  we've siphoned and put new gas in, a new spark plug, sprayed throttle plate cleaner into intake, and played with taking air cleaner and muffler both on and off.  we've exausted all we know to do.  what's up?

Answer
I suspect the low speed circuit is either plugged or you need to open the low speed mixture screw a little more to provide adequate fuel at an off-idle condition.  Cleaning the carb involves pushing a piece of tag wire through all of the internal passages and following up with carb cleaner.  Hope this helps.