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.
ww
actually, i have done no further modifications to the vacuum ports. i simply have not had the type of time that i need to accomplish. i am still running the modified slide with no ill effects and very strong mid range performance which is where this type of modification is going to tend to show out. i can comfortably say that i see no reason to go back to the stock slide but i do want to take these vacuum ports to the point that performance is adversely effected. in other words, i want to make them intentionally to big. i have an extra slide that i can then port to the best size once i figure it out.
actually, since i started thise vacuum port mod, i put an alba pipe on the rhino and wanted to get some seat time to learn the nature of the performance associated with the alba pipe.
will keep posted any results once i can get it done.
Modifying the resistance of the slide has been a common trick for a long time. One can do this by modifying/changing the spring tension and/or modifying the vacuum ports. They are basically edge modifications.
On the spring, some clip a few coils off of the stock spring and some change the spring out all together with a lighter wound spring available in some of the aftermarket jet kits. Either approach enables the slide to rise faster due to the lower resistance of the spring.
With respect to the vacuum ports, increasing the size of the ports allows for enhanced lift for the slide as it is not the vacuum itself over the slide pulling the slide up but the existing vacuum below the slide diaphragm pushing up on the diaphragm, pushing available air out through the vacuum ports and allowing the slide to rise. The vacuum ports are essentially an exhaust for the area between the top of the carb and the slide diaphragm. The smaller the ports, the more resistance to allow the slide to rise when vacuum is present below the diaphragm. The larger the ports, the less resistance to allow the slide to rise when vacuum is present below the diaphragm. Just because you change the spring does not result in an increase or decrease in available vacuum. It simply changes the way the slide functions with the existing vacuum.
Once you change one or the other, it is critical to assess your jetting circuit needs to insure optimum performance. It may be that in your examples, the a/f mixture, pilot jet circuit and jet needle settings were within the realm of the new engine a/f requirements once the modifications were done and did not require apparent changes. This is why they are edge modifications. These types of mods aren't going to turn your motor into a fire breathing monster but they will definitely give you an edge to someone who has not tuned properly. Anytime and I mean anytime you change the a/f demand for an engine, a thorough jetting assessment should be done. So while the snowmobiles are responding better, they are not responding as well as they could. To each his own.
Installing a camshaft, essentially changes your a/f demand profile. Ideally, a cam allows as much fuel to enter the combustion chamber and keeps it in there as long as possible before ignition. What do you have to do when you install a camshaft? You have to assess your jetting circuits to compensate for that altered demand. Simple as that. I might add that adding a camshaft with a different profile is also going to change the exhaust profile a good bit so a cam/exhaust combo is likely where you will achieve best performance characteristics as a performance cam may render the existing exhaust components ineffective and inefficient. ww