Look for casting defects and corrosion inside and outside the bowl. Corrosion alone or in combination with a casting defect can open pinhole(s) allowing gas to leak out.
It's rare but does happen, especially in really old (decades) carburetors.
I know it's not good news, as it usually means replacing the carb rather then rebuilding.
Mr. Gasket is an example of cheap components ... meaning lower quality materials and less stringent quality control. I strongly dislike having to do a job more often than necessary. So, I prefer to pay more for things from reliable manufacturers. (I tend to prefer the same things NASCAR builders use. When's the last time you saw a Mr. Gasket sticker on a winning race car?)
I tend to prefer Fel-Pro and look for parts stores that carry that brand.
