Popping sound in exhaust

It's a Holley 750 vacuum secondary with manual choke. I bought this used and the weird thing is it has a secondary metering block instead of a metering plate like every other Holley vacuum secondary I've ever seen. I did check the floats and their right on. I did go through this carb before installing it.
A long time ago, Holley used to put the block on the secondary side too. Or somebody retrofitted it... there are kits to add it.