Here is an install manual on the carb - it appears that is the vacuum advance port, but it seems that getting the port to work right may be tricky: http://www.racetep.com/weber/INS-WK601-34.pdf . There is a ton of info on the jeep forums about this carb and how hard it is to get tuned right. Also, this carb is not made any more, but jets and parts are still available.