""Street Avenger 670 Carb (out of the box, no changes have been made yet)""
""Idle speed in drive is around 450-500. I checked the timing while in drive and it is solid at 24* initial.""

A solid 24 degrees in drive is a good start. 500rpm in D seems a little low, but if you can get a steady vacuum reading at that rpm, then good. If the vacuum reading is jumping around at that rpm, then I would suggest bumping up the idle rpms until you get a solid vacuum reading.

Have you tuned the carb to show the best vacuum reading in D with the brake applied? If not, try that first once the engine is up to running temp. Then see if the bog is still there. If it is, then try backing out the metering screws 1/4 of a turn and test for the bog again.