Most carbs have aux. vacuum ports for other engine components. The sell various size/diameter rubber/vinyl plugs that fit over the ports to close the vacuum when the port is not needed. They can rip or tear causing a vacuum leak.

I'm not familiar with the carb on a 215 I6 nor the plug as you described it. If the plug you have described is a vacuum port, it's best to use the proper plug if it is damaged. I would not use duck tape to seal a vacuum leak.

As other have stated, a vacuum leak will cause the engine to run poorly and your gas mileage will be terrible.