This has been mentioned before and it bears repeating: When rebuilding the carb. did you notice there is a small port in the base of the throttle body? Were you careful to align the new gasket so this passage is not blocked? This is the vacuum port for the power valve, which accounts for maybe 10-15% of your total mixture volume.
If the engine is tuned to run well when this port is blocked, there won't be sufficient fuel enrichment on hard acceleration. If the port is open and has a vacuum signal, the valve will be closed (cruising position). If you open the throttle quickly, there will be no vacuum (or almost) and the power piston will pop-up, adding the necessary fuel mixture required.
I've rebuilt so many carbs, I hope I am remembering this correctly!!
:o
