Flange-mounted carburetors and maifolds tend to warp over time, from overtightening and gasket compression.
If a flange is leaking due to warpage, you can true it up.
Lay a piece of wetted wet-o-dry sandpaper on a surface plate (you can use a plate of glass) and lap the flange flat, using a figure-8 motion. Reverse the motion, and give an even number of strokes in each direction. You'll know when it's flat, when the whole surface of the flange is getting marked by the paper.