The spring pulls the metering rod OUT of the jet so the mixture can richen up under load/full throttle.
Vacuum works against the spring so vacuum at idle/cruise sucks the metering rod down INTO the jet, leaning the mix out for cruise.
You could try leaving the spring #38 out of the carb, put it together and try it. Now, the metering rod will fall in to the fully lean condition. Don't go out and hammer on it as it will be too lean under load - but at least you could see if it leans things out at idle. If so, it would tell me that the vacuum piston is leaking or something and cannot pull down and overcome the spring.
Otherwise, over-rich usually means a blocked air bleed somewhere (check all air bleed passages with a wire) or high float level.
