I can see that the elbow flange is bowed, Paul. It may be leaking dirty air into your carburetor, and should be fixed in due course, but that is not the immediate problem.

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I've copied your picture of the carburetor flange. The red circle is around the idle air bleed. I think you've said it idles properly even when the elbow is in place, and if so, this hole is not being blocked. The green circle is around what seems to be another air bleed: it connects to the inside of the carburetor but only passes air slowly. If that one is nearly blocked, but allowing a bit of air in each time suction builds up, it could cause your problem. That is the cut-out that you put the yellow oval around in the picture of the insulator. Note that the moulded-on rubber on that metal insulator will be crushed when you install it, and it may be squeezed into that recess. Can you try running it with the insulator and gasket omitted, so the elbow bolts directly onto the carburetor?