I'd start with the float level - make sure it's within spec. I used this reference:
Mercedes Benz Zenith Carb Manual | JaimeKop.com
Open up the carbs and make sure that the passageways and jets are clean, the throttles close and the float valve is working smoothly. Set the choke and fast idle on the front carburetor to spec and install.
If you have no vacuum leaks, it should start and run ok. Have you run through these checks?