If you're using vacuum advance on a Chevy, you need to hook the vacuum hose to ported vacuum (above the throttle plates), NOT to manifold vacuum (below the throttle plates).

On most Holley carbs, there will be two ports on the base plate at the right front of the carb about a half inch apart. The front is prted, the back is manifold.

With the carb off the engine, you can tell by just looking at the passages in the baseplate under the carb.

By the way, were you checking timing with the vacuum hose removed AND plugged?

Check out this article:

http://www.chevyhiperformance.com/ho...438/index.html