Need help with this vacuum reading

The vacuum reading in N seems o.k,but in D your RPM drops alot.Have you played with your air mixture screws on carb?,with car running and watching vacuum guage?Could your timing be low?I have 2 little holes drilled in my primary flap,to give the carb more air because of my cam.That has helped me when tuning my car.Sounds like a big cam(robbing vacuum)?