I'ts because the New throttle has more pull than the stock one. I Did the same swap to my 670. It got me the pull I needed to open the 42's.
What you need to do is slack the cable off so that it's not tight. There will be slack between the flipper and the mount when at idle, but, this helps as the flipper now can be set to be WOT at the bar, AND your thumb does not have to extend to much to reach the throttle at low speed positions.