[Formulajones]

I think some confuse the 3310 carbs as "factory" carbs. I know that's a general term thrown around but many of the holley carbs that are actually OEM and appear like the 3310 vacuum secondary are actually 4053's and are of the 780 variety and actually do have a rear metering block and factory equipped with a rear power valve.

Chevrolet used them heavily on all their performance models.

DZ carbs come to mind, as well as L78's and L-72's


That carb gets mixed up with the basic 3310 all the time as being the same thing. Sometimes these posts don't make it very clear exactly what we are dealing with.

Then there were the over the counter replacement 780's referred to as 3310's that throws in more confusion when dealing with questions on the internet.

In a nut shell, some came factory equipped with rear power valves and some didn't. I think the confusion starts with the OP's description having a 3310 780 and not knowing which one it is.