I have a 408/727 in my Duster with 3.23 gears. The carb is a 750 VS (80508-10). The engine performs well at mid & upper RPM's but at low speeds (around 30 MPH and 1500 rpm or so) has a noticeable surge. Am using the stock 6.5 PV. Am wondering if the IFR needs to be opened? Never done that before.