Thing with the Bing cars is that they are very simple and easy to work with, but non-forgiving if you make even the tiniest misstake...
You need to check:
- if the stubs are propperly seated and sealing (rings in the heads on which you attach the rubbe hose from the carbs)
- that there are NO cracks in the rubber hoses between the carbs and heads; and that they are soft and flexible.
- the steel-lid at the top of the carb-cover/dome is fully seated and that there is no play nor any leakage
- Which version of the covers that you have, with or without a pressure releef(?) hole drilled between the throttle shaft and the vaccuum chamber.
- The choke needs to be fully disassemble and a cast reidue grinded off; it interefers with choke-return
- the paper gasket to the chokes needs to be well greased with good quallity bearing greas to seal proppely.
- The vaccuum membrane must be in good condition, and made of -rubber- (I tested Volvo/Saab semi-plastic replacement membranes... didn't work so well...) - i.e. you are stuck with BMW originals for a fortune each!
The carbs needs to be cleaned with some detergent (petrol, diesel will do ok) and blown Clean with compressed air, especially all holes and jets...
The butterfly valve is not installed propperly from factory, so you may need to re-set them - it may cost you two new shafts and four new screws... the brass screws are loctited in the brash-shaft...
Makes wonders for the operation to get the butterfly-valves operation correctly
As for jets and needle settings - look at the german site:
BING Power Systems GmbH | Motoren sind unsere Leidenschaft
Datenblätter | BING Power Systems GmbH
You may have to change jettings slightly due to climate, wear and tear. To get a good idea of what you may have to do you need the US Bing tuning manal:
It is highly recommended that you do Clean the air-intake as well with detergent and blow it dry. A new air-filer or re-oiled K&N is also highly recommended.
As for the tuning.
I'm used to vaccuum-gauges, but the counter-balansing hydraulic gauge is by far better!
Manometer, Throttle, Bodies, Carbs, Balancer, balancing carburettors, throttle, synchronization - YouTube
If you experience a problem such as the one you described - something is jamming.
A jet, a wire, a valve - someting is not operating correctly.
If you experence a problem with instable idle, rushing Engine as it gets warm, etc - there is an air leakage (9/10 it will be the rubber hose/manifold; 9/10/1 will be the steel-lid and 1/10/1 will be loose stub).
It happens that it is the choke housing that causes all the problems... Darn rare, but can happen.
I have a couple of good articles on carb-tuning and how to build the gadget - how do I up-load them?