holley carbs

The previuos people summed up the difference between vacuum and mechanical secondaries pretty well but there are some other important differences:

4150 vs. 4160 series carbs.

Almost all Holleys have a primary metering block with 2 jets, one for each primary barrel plus a power valve that adds additional fuel when the manifold vacuum drops below a certain value. There will also be two idle mixture screws that (obviously) control fuel added at idle. Again, there is one for each barrel.

The 4160 style carbs use a metering plate (not to be confused with a metering block) that has pre-drilled holes that meter fuel into the secondary barrels once the throttle valves starts to open.

The 4150 style carbs use a metering block with interchangeable jets. Some Holleys use a power valve in the secondary metering block and some don't. Those that don't stagger the jets with jet sizes being about 6 sizes larger in the rear metering block vs. the front.

There are also single and dual feed, center pivot and side pivot floats, with or without choke towers, mechanical or electrical chokes, four and two idle circuit carbs and those with interchangeable air bleeds.

Any questions? :)