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I recently ordered an o.e.m. throttle slide to determine the impact of increasing the size of the vacuum ports which is the primary design component that enables a CV carburetor to do what it is intended to do. Those vacuum ports,...any vacuum port/s on a CV carb for that matter, are positioned so that they are exposed to that area in the venturi where incoming air is at its highest velocity and its lowest pressure. That area is the restriction in the venturi which is the throttle slide.
When you are cruising around and/or when you stomp it, the vacuum created by the intake stroke as a result of the position of the butterfly valve, first becomes apparent to the vacuum ports on the slide. The vacuum condition path is restricted by the throttle slide which is in the way whether you are at idle or somewhere up to about 3/4 throttle. As a result, the vacuum condition is channeled through the vacuum ports to the area above the diaphragm. This condition essentially creates an area of vacuum within the closed diaphragm which initiates the upward movement of the slide. Incoming air, which is at atmospheric pressure, senses that vacuum condition and seeks to fill the vacuum area but is blocked by the diaphragm. The only thing left to do is for the incoming air to push up on the diaphragm to try and fill that area with atmospheric pressure. Up goes the slide and off you go.
A tried and true modification, known especially to Harley owners, is to increase the size of the vacuum port. In theory, increasing the diameter of the vacuum ports will cause the slide to rise at a faster rate sooner. The problem is that in doing a modification such as this will likely result in lean condition on the pilot circuit primarily as well as the mid range circuit, which consists of the jet needle, needle jet and air jet.
So to make sure that I didn't screw everything up, I went ahead and bought a slide and micro drill bits from 2.8 mm to 3.5 mm in 0.1 mm increments. That way, if I overdid it, I would be able to put my other slide back on and not miss any riding time.
This will kind of be an ongoing thing as I will continually modify and shape the vacuum ports until I reach a point where performance suffers. In other words, I want to find that point that the existing pilot circuit cannot overcome the lean condition caused by the slide moving up sooner and at lower rpms.
I have done 1 drill test so far. In stock form, the ports are approximately 2.7 mm in diameter. The ports are stepped in that the first couple of mm's into the port, the diameter is larger than the actual restriction of 2.7 mm's. I measured the stepped area at 3.0 mm. My goal was to get to a size of 3.0 mm on each vacuum port which is an increase of 11.1%. I felt that this should be a substantial enough change to make a difference.
The result of the modification is a positive one. The pilot circuit and transition to needle jet/jet needle circuit is as seemeless as I have felt. It is subtle but a recognizeable difference. The motor gets on to making power sooner and enhances the low/mid range press back into the seat that we all like.
From this step in this process, i feel like increasing the size of the vacuum ports requires increasing the size of the pilot jet from the stock size of a #40 to a #45 with excellent performance.
I'll continue to update as i continue to increase the size or shape of the ports and comment on performance.
Since all jetting circuits interact, its important to realize that increasing the size of the vacuum ports, or any other modification for that matter is going to require an assessment of all jetting circuits including idle, pilot, mid range and main.
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Roostin' Rhino RAIS Power Tube
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