Did you check the manifold for being straight. I always put manifolds on a belt sander before I install them. A chunk out of the manifold where it joins the exhaust would create an exhaust leak, not a vacuum leak. Have you tried a different carb. It may be junk in your metering jet. I have sometimes got carbs to straighten out by revving up the engine and then putting my hand over the carb. It creates a heck of a vacuum and sometimes sucks stuff on through. Don't even think of torching it yet.