Most likely when you re-assembled the tank assy back on to the engine, the governor mechanism got jammed, or the spring is twisted/trapped somewhere.
You only have to remove the air filter to see and chack for free movement.
On some carbs, there is enough free movement in the throttle butterfly for it to go 'over-centre' when re-assembling, resulting in the throttle jamming wide open. Check for this.
phil_saunders(a)bigfoot.com
Phil Saunders, September 2012
