The tube you see connecting the vacuum ports on both carbs connects to the clean air system in the airbox. The vacuum is used to activate the clean air system solenoids to inject air into the exhaust upon decelleration, thereby igniting unburnt vapors in the exhaust (I think that's how it works).
If you've removed the clean air system, you should remove these vacuum lines and plug the ports with small screws. You can find the BMW part numbers for removing the clean air system and plugging the vacuum ports on the IBMWR web page, or contact me for the part numbers.
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