Need some help with a 750

Ok so me and my dad have been trying to set my carb and we cant get it right.
Its a 750 holley DP we set the idle with the 508 purple shaft at about 850-900 rpm you go and drive it and stop at a red light the idle is between 1100 to 1400 rpm why is it doing this we have tried every witch way to fix it and still nothing.