It's a heat riser port that runs exhaust gas under the carburetor during warmup when the flap valve in the exhaust manifold is closed on the passenger's side manifold. It's impossible to keep that area painted- - - -maybe powder coat would work but I doubt it. Make sure the heat riser isn't stuck closed all the time.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!