Use a delay-flash timing light to set the total timing at 2500 RPM to 36-37 degrees BTDC with the vacuum advance disconnected. With the advance connected, no-load advance at 2500 should be 42-43 degrees. Set this way, the advance at idle with the vacuum disconnected should be somewhere in the 6 to 8 degree BTDC range at normal idle speed (500-600 RPM). If you want more initial advance to help low-speed acceleration, you'll need to restrict the centrifugal advance to prevent over-advancing at high speed. On the race engines with a radical camshaft, I up the initial timing to about 12 degrees BTDC, and restrict the centrifugal advance enough to give a max of 37 degrees at speed. One degree over-advanced drops as much horsepower as 5 degrees late; I've proved it repeatedly on the dyno. It sounds like you're over-advanced on the initial timing, and the engine is trying to run backwards on acceleration.
Jerry
