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latest truck came with a BB downdraft carb, as shown in the (hopefully) attached photo. i'm curious what the threaded port just above and left of the idle air screw is for. on mine, it's plumbed to one of the vent lines of the distributor. the other vent line is just cut off...i intend to route it into the air cleaner if i can find a way to do it. i might need to use rubber hose.

It is for the vacuum advance on civy distributors. You should plug it if you don't have one or it will run lean.

exactly what i needed to know. thanks!

any easy way to set up the distributor ventilation? I do have another military carb...it might be easiest to just switch back to that at some point.

I am running that same carb for now on my truck. I am running a civy dizzy because I can't come up with a good way to vent it. I just got a complete ETW setup that will go on the T245 engine as part of the rebuild. Then all the stuff gets easy.
I did buy an airhorn for an M715 hoping I could adapt it, but it wasn't worth the amount of work for a short term solution. You MIGHT be able to tap into that vacuum port with a needle valve to the dist and the other line into a tap on the feed to the PCV to get some airflow through the dizzy. But, it will still lean out the mixture.
I will be using a bunch of this "extra" stuff on the engine that comes out of the M37. We are going to hot rod it with cams, multiple carbs, headers and block work. It is going to go in a handmade lakester.