I have a 750 vacuum secondary on my 66. I just don't think it's giving me what it should. I was running too lean. Plugs looked brand new after several hunderd break in miles. I removed the 72 jets and upped it to 74's and now the plugs look nice and light tan. I don't think i'm getting all of my secondary throttle. I tried to gauge it with a paper clip on the diaphram shaft but it fell off twice in a row. I'm ordering the secondary spring kit "holley 20-13". I'm curious what others are using for carburation on their 390.