273 intake on a 318

I'm assuming you're using the '66-7 single plane manifold? If so, Try dribbling a tiny stream of gasoline in the carb on the side that has the problem. If it smooths out, you have an idle circuit on that side that's partially clogged. I make a primer can out of an old plastic oil bottle by drilling a tiny hole in the cap. You can use a squirt can also but it's a little harder to control volume. Just keep a fire extinguisher handy in the odd chance you have a backfire or bad plug wire somewhere.
Thanks, Mark