The reason you disconnect your vacuum to set timing is because you're setting BASE timing. Base timing means timing with no mechanical or vacuum advance. Also even under WOT as your engine rpms increase so does your manifold vacuum. That is why your accelerator pump only needs to give you a short momentary boost of gas, then as your engine builds up rpms fuel is drawn through the jets because of the engines intakevacuum. If you have a vacuum advance can set to provide advance at a low vacuum signal you will see this advance even under heavy acceleration. Maybe you should read this link HERE
were it tells you to add
your vacuum advance to your mechanical advance to see your total ignition advance. At 40mph when you step on the gas to pass someone your engine is seeing your distributors mechanical+vacuum advance. Too much of this combined advance will cause that annoying little 'ping,ping,ping'.