Many aren't aware that there are two different venturis, or more specifically "venturi inserts" that have come with the carb on the Rotax Max engines.
There are two different venturi inserts that come with for the Dellorto VHSB QD/QS Carburetor as used in the Rotax Max engines. These are the "12.5" and the "8.5". Originally it was the 12.5, while now all engines should be delivered with a carb that has the 8.5 venturi in it.
EDIT: - Dell'Orto confirmed
The 12.5 or 8.5 actually refers to the size of two small holes in the venturi, and has nothing to do with the venturi choke size (which is 34mm).
The "12" in 12.5 means that the "progression hole" is of size "120" or more specifically 1.2mm... and the "5" means that the idle hole is of size "50" or 0.5mm.
In the 8.5 this means size 80 progression hole and size 50 idle hole.
This is the Venturi Insert when taken out of the Carburetor:
The only difference between the two types of Venturi is the size of one small hole, the progression port.
Here's a view from the top of the venturi in which you can see the difference in size of the progression port:
The progression port is located directly upstream(towards engine) of the main spray nozzle. In the picture above the direction of air and fuel-flow is from left to right. The role of the progression port comes into play as the throttle opens just a little bit.
Here's the explanation in the dell'orto manual:
As the throttle valve lifts the idle circuit air/fuel mixture decreases and the progression port begins to supply emulsified fuel. The air now comes from the main air intake. The progression port (circuit) is important to the system because of the increased amount of mixture required by the engine as the throttle leaves the closed position.
The transition circuit is activated when the driver begins to open the throttle and the vacuum decreases in the idle circuit. The mixture from the idle circuit decreases and it is necessary to introduce another circuit which can supply the fuel required for the transition from idle to the main circuit. This is called the progression.
At about ¼ throttle the vacuum begins to become constant and stops drawing the mixture from the idle port. At this point the vacuum is sufficient to draw fuel from the progression port fed by the idle jet in the float chamber.
and
When the valve is lifted slightly (up to about 1/4 throttle) the vacuum generated by the inducted airflow begins to be consistent, and stops
drawing fuel from the idle nozzle. Under these conditions, the vacuum
is sufficient; however, to draw fuel from the progression port, which is always fed by the idle jet placed in the float chamber.
What then is the difference between the two with regards to performance? While I'm not certain I think the reason Rotax changed from 12.5 to the 8.5 was because the engine was getting too much fuel on initial throttle application, which caused a poor response. The 8.5 gives a bit less fuel and I assume this should give better response when getting in the throttle. Personally I noticed very little difference between the two in back-to-back testing. There are some articles/comments on the net that claim the 12.5 is good for long tracks and the 8.5 is good for short tracks but I think these were made under the false pretense that the 8.5 vs 12.5 was the venturi choke size, which it is not. The size of the progression hole should have very little if any effect on wide open throttle.
Here's a view from the bottom of the venturi:
The smaller hole a little bit further upstream (towards engine) is the idle port (or hole).
Finally just to show that there is no difference in the shape or size of the Venturi here they are back-to-back:
[i reject your reality and substitute my own] dang! i'm such a nerd!
