Edelbrock 1406 whistling

Johnny Dart said
No disrespect, but why are you using a adapter plate ?
Your carb is made for your intake !
Give the 9266 a try.
No other gaskets necessary.
This will seal your carb to the intake. If you still have whistling, its somewhere in the carb. In that case talk to Edelbrock. In fact send them your video.

He is bolting a square bore carb onto a spread bore intake.

And no dirt will not cause a vacuum leak. It is missing on the one bank because it is sucking air. More than likly your metering rods are wide open because of the vacuum leak. You really need to take a closer look at your carb flange and intake flange, set the carb on the intake without gaskets and start the bolts to not run them down , pick up slightly on the carb and put the corner of a piece of paper under the intake in the center of the bolts, only the tip of the paper, do not shove the paper under from bolt to bolt Only the least bit between the bolts. While holding slight dow pressure on the carb see if you can pull the paper out. Do this between all bolts. this will tell you if there is a straight sealing surface between the two.. Did you use two gaskets with the spacer plate. Take a picture of what you put on under the carb gasket and plate. I would have to see at this point. this isn't that hard to correct a leak between the carb and intake. I wish I was closer I would come over and bring what you need with me. Show me the plate and gaskets before you post anything else. Steve