A lot of the tricked out vacuum secondary carbs today have an adjustment screw that looks like an idle mixture screw on the passenger side of the carb just ahead of the vacuum pod. This allows you to adjust the timing of the secondarys opening to prevent a bog and get max performance. I sent mine off to Quick Fuel Technology for this mod. This cut my test n tune time at the track down considerably. Instead of changing a spring all you do is turn the screw.