concours
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- Dec 29, 2011
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comnoz said:The balance tube on a Norton or most 4 stroke engines is only there for idle and off idle regions. It does not affect anything at higher RPM. It really does not have anything to do with pulsing or ram effects like the resonance bottle on some two strokes.
The main thing it does is it allows a higher RPM for the same slide position because each cylinder can draw a little extra air [mixture] from the opposite cylinder during it's intake stroke.
When you are tuning a single cylinder engine you will often find that the transition from the idle circuit to the needle circuit is the hardest to get right. On a single the transition happens at a low RPM as you first apply the throttle and unless it is just right a single will often cough or be rough right off idle.
When you have a twin with a balance tube then the engine will be able to run faster for the same throttle setting because of the extra air supplied by the balance tube. That means the transition from the idle circuit to the needle circuit is going to happen at a higher RPM where it is going to be less noticeable and easier to tune.
The transition is also going to be less noticeable at a higher RPM when the ambient conditions change enough to throw the mixture off a bit.
If you pinch the balance tube on an idling Norton engine you will find the RPM will drop. When you raise the slides to get the RPM back to where it was you also pull the needle farther from the jet and change the air pressure under the slide. Then to get the mixture correct again you will usually need to adjust the idle mixture screws or jet and change the slide cutaway and sometimes a different needle or jet will be needed.
Getting the transition from the idle to the needle just right is critical when the engine speed is only 1500 rpm. It is a lot easier if the transition does not happen until 2000 or higher. Jim
got it, thanks