I believe that the balance tube is there to help provide a smooth idle,...but how does it do that?
I happen to be replacing the Bing C/V's on my 1982 T140 with MK1 Amals. Now, since I'm completely redoing the intake manifolds, I have the option of providing the balance tube or not. No problem to do it, but what does it do and how does it do it? On the surface, it seems to be just an air leak to the other side that isn't doing anything at the time. And, of course singles don't need one and we have two singles here. Racing engines don't use it. (this is not a race bike)
OK... showing either my total ignorance; or deductive brilliance... here goes. My understanding is that the balance tube "equalises" the vacuum impulse of the two cylinders to improve cylinder filling and balance. I may have my head up my arse... but I believe this to be a tuning aid for low to moderate throttle settings enabling better side to side fuel distribution.
Racers don't generally care about smooth idle, EPA regulations or smooth transition from off throttle to part throttle. Their main focus is on the pilot/ main jet transition as revs generally will exceed the idle/low circuit on the track. Eliminating the balance tube allows each cylinder to be optimized seperately for mid to large throttle response.
If you pinch the balance hose shut, when the engine is idleing, the engine stops. It is because the manifold on intake stroke is also drawing air/fuel from the other carb idle circuit. It is also why it is best to re-adjust the idle mixture screws a couple of times when setting idle. To get it to idle with the hose pinched you must turn up the idle screws. By the time the needle and needle jet come into play the balance pipe has no effect. By blanking the hose, one can sometimes get a worn out set of carbs idle lower. Rick