Zenith Carb Issue - What would you do?

[Mint2Me]

Trying to get the Zeniths on my 230S to run right has been a daunting task. Symptoms: Slight stumbling with part throttle while accelerating. Not fun. Full throttle and cruising seems fine.

The front carb seems to be ok. The plugs in the cylinder that it feeds look good. The rear carb runs too lean and nothing we do can get it to run any richer. Those 3 plugs are really light in color. The fuel tank has been boiled and re-lined, new fuel filter and the carbs have been cleaned twice. My mechanic suggests getting rid of them for a pair of new Webers at $1000 plus install. Yikes. What would you do? Rebuild just the rear carb? What do you think is wrong with it? And don't say get a new mechanic. He's one of the best.