I would check to see if there was something close to the throttle linkage (like the clamp for the air cleaner) that it may have got caught on the linkage holding the throttle open. A stuck slide will not do this, it will only rev
up if the throttle plate (on the back side of the slide) is opened. A stuck slide alone will allow more fuel into the carb
but not more air. Both are needed to rev the motor. If the throttle plate is stuck open it will open the slide giving you more air and fuel. The result will be the engine reving like you described.