Why would your engine race while in neutral?

Answer

Vacuum leak. Sensor going bad that will make a fuel adjustment that in not needed. It tells the computer to add fuel, when the computer does, other sensors say that there is too much fuel and tell the computer to back off, thus the idle fluctuation.

Answer

Could be insufficent free play in your throttle cable or a binding throttle cable.

Answer

if you have a carb, it could have been adjusted wrong, i need mine adjusted since the engine is new...usually needs to be retuned with new exhause, engine parts, and eveyrthing related to that.

Another Answer: The "AIS" (Automatic Idle control) is a step motor controlling a valve that bypasses the throttle valve or Throttle Body butterfly. At some point in time, the valve starts to fail and do not work properly, causing erratic or high idle speeds. Sometimes it can be cleaned, but frequently signals a worn mechanism and it is better to replace it, it should not be expensive. But check that one before purchasing the replacement.