See your other thread,
http://www.peachparts.com/shopforum/vintage-mercedes/367299-w111-m180-zenith-air-corrector-jet-mystery.html
Posted a link to some good tech info.
My heat risers were so rusted and stuck in the partially open position that I finally gave up trying to free them up and slightly bent the plates closed. I made some steel block off plates to prevent heat getting to the carbs. I live in NC so extreme cold is not an issue, I do see it would be better to have functioning risers further north though.
I think the Zeniths are a nice carb. You do read lots of complaints and recommendations to switch to Webers but once you do the homework and understand all the intricacies the carbs can perform very well. My car idles and runs very smoothly, good power, no stumbles, decent fuel economy... it will be even better once I get the mixture a bit more even between the carbs. My father had a 67 230 W110 back in the 70s and when he tuned the carbs you could balance a nickel on end on the valve cover at idle. I am almost there.