I've runnig this setup in alcohol with direct linkage and with no choke at all. Nothing, None, Zero.
Even in the coldest winter days, what i used to do is inject some gasoline on the center carb and run. I can't forget to mension my intake manifold is water heated. But anyway if you want to use a choke, i suggest you to use only on the center carb. I think it'll works perfect.
What makes me believe this is the reason when the car is idle if you cover the center carb it turns off while if you cover the others doesn't
Good luck
