vacuum advance

If your throttle blades are open slightly at idle, the advance port may be getting a signal. There are a couple ways to correct this (assuming you aren't content to just live with it). Many Holley carbs have a secondary throttle plate stop screw, so you can crack the secondaries slightly allowing the primaries to be a little more closed removing vacuum signal from the advance port. The other way is to drill small holes in the primary throttle blades. If you go this route start small like 3/32". A lot of smog era OEM carbs went this route.

Also, most Holleys have their ported vaccum nipple on the right side of the primary metering block. If you have a 3310, you should have the secondary idle stop screw located onthe underside of the throttle body (or at leastt he older ones did.)