In doing some research on the subject of a balance tube I found the following on the Rotax Owners web site: An individual there wrote::

((Hope it's ok to post this: it's a probably a better explanation than I could write. I apologize if this is not proper etiquitte.))

"Built and added a 1 1/2"D. balance tube to my 912S and it is remarkable how much difference it made in the smoothness at low RPMs. This is not new technology and has been used on intakes and exhaust systems since the forties to smooth out low end vibration. It works.

I saw it advertised recently by an italian racing equipment firm called FLYGAS but they wanted something like $1200.00 for the package. (cost me about $100.00 to build). It does not increase HP, I don't know yet about fuel consumption. Its advantage is to reduce vibration and improve manifold airflow.

Here's what happens. Firing order on the Rotax, and most other boxer engines, is such that when one cylinder is firing, the other cylinder on the same side is on a compression stroke which means that NO air is flowing into that manifold. This inrush of fuel/air mix which has suddenly stopped has nowhere to go except through the balance tube and into the other manifold which is calling for air. The Rotax balance tube is woefully small for this task, even though it is great for idling. You can see this starting and stopping of airflow in the wildly bouncing of the vacuum gauge needles during carb balancing. Because all of this is happening in fractions of seconds, most of the air in the balance tube never actually goes anywhere, but bounces back and forth in the large tube. "

If this explanation is accurate, then the it appears that this modification is more a function of the style of the engine ( ie: boxer) rather than the displacement of the engine. The zipper kits only add to the cubic inches of the engine.

Got a text from Hal and Jay this morning. Cost of their conversion is $300.00 plus shipping plus your old manifolds. I have no connection with Hal or Jay, am just posting this for the information of our group.
Well, I did buy the 114HP Zipper Kit from Hal n Jay and I can report I'm very pleased with it.

Seem like every source I read on this subject reports a major improvement in idling performance of the engine.

Hope this sheds some light on the subject.

Regards
Rodney