Ok i have a problem with my Briggs & Stratton 16HP v-twin vanguard engine.When i start the engine and rev it up when i idles back down it sits there and rev's up and down then it straightens out and runs normal.what could be causing this?

It appears that you have a slight mis-adjustment in the idle speed and idle mixture. To adjust setup, start with at least a half tank of fuel, and have engine at operating temperature (That is warm but not overly Hot). The Briggs & Stratton Website manuals section should show the locations of the adjusting screws if you are not sure of them.
To adjust idle speed set Throttle control to minimum and adjust screw on throttle butterfly to maintain a good idle speed. To adjust idle mixture, screw in idle mixture screw (Lean) until motor falters and then screw out (Rich) until it falters again. Correct setting is the halfway point of these two positions. You may have to re-adjust idle speed again and also the mixture to get best running.
If you are not comfortable doing this, take it to a B & S dealer.