A Little Scratch Will Do You

Last night I went hunting for a vacuum leak. It took me a while, but I think I’ve found the source - a scratch across the carb mounting surface of the head. I used some high-temp RTV and bolted everything back together and let it sit overnight. This morning I was able to sync the carbs and get the idle down to about 700 rpm without the engine dying, but I still think I hear hissing around where I sealed the crack. I drove the car to work and there’s still something amiss (a slight miss at full throttle), but, for the most part it goes down the road without issue. It currently has one race carb and one original carb. I’d like to swap out the race one for the second original.