Rough idling with proper rpm both engine cold and warm

[motozine]

Hello,
Just purchased 86 560SL with 114k miles.
Trying to tackle first annoying issue with rough idling. Rpm range seems fine with about 700.
Doesn't mater when engine is cold or after running for a while. Every time when I stop/idle in traffic light engine runs very rough. When it goes above 1k rpm engine runs very smooth.

One thing to note, my economy gauge is all the way right all the time. appears to be stuck doing nothing.

I suspect something to do with vacuum. Any idea what to check first? thanks.