So much misinformation about the STVA flying around these days about the STVA function, and failure repair that I thought I'd step in and shine some light. In case you're wondering, yes, I am TheGeek.
First, let's start with why the STVA is there in the first place. The intake runners on these bikes are pretty small. That means not much volume. This makes them very sensitive to changes in the vacuum. Without getting too technical, a rapid decrease in vacuum is going to throw off the fuel mix for a very short time and cause dips in the power curve. Not a good thing when you're getting back on the throttle mid corner. Remember that small volume I spoke of? The air mass in there moving toward the intake valve is important. Google mototune and the 8 cycle engine to learn a little more about precompression intake. By controlling the vacuum, you can also control this air speed.
The whole point is pretty simple. Maximum smooth possible power. I don't care what your butt dyno says, removing the secondary plates doesn't help your torque or peak HP. The intake runners are tappered and the lower valve is more restrictive than the upper valve fully open. In fact, having the STVA working improves low to midrange torque by allowing the bike to measure and maintain optimal vacuum.
The STVA between the k4-5 and the k6-7 generations are essentially the same. Both are manufactured by Mikuni and both use a Mineba-Matsushita bi-polar stepper motor to drive the system. There are a few small differences.
The K4-K5 was the first to use this particular motor. As such, there were bound to be problems. What it all boils down to is that this motor was never intended to have a hard connection to a plug. Combine that will cheap production, and a high vibration application and something is going to break. The k6-k7 is slightly different. They changed how it's mounted to free up some space, but the motor was also changed. Mukuni and M-M both changed a couple things to help solve the problem. It's much stronger, but it still suffers from poor implementation. The small, but very important difference is the STPS. On all of these, the STPS is spring loaded. On the k4-5, it's spring loaded to the fully closed position. On the k6-7, it's spring loaded to the fully open position.
Removal of the STVA is easy, but installation requires knowledge of how to get everything setup. If you do happen to have an STVA to install, use a small screwdriver to turn the action to the correct position so that the sensor is lined up. On the k4-5, you'll turn the action fully clockwise, and on the k6-7 fully counter clockwise. When you do this, don't insert the screwdriver more than 3/8". If you engage the STPS while turning the action, you can easily damage it.
So what do you do with the failure. Honestly, hire someone to fix it. All of the units I've seen that couldn't be fixed are because someone without the proper skills made some serious mistakes. A few years ago, this wasn't that big of a problem. Now, parts are getting difficult to find.
Before you go searching ebay for STVA fixes/replacements, do some testing. From an electrical point of view, the STVA is pretty simple. I’ve attached two zip files with testing instructions for both generations. Regardless of which generation you have, I suggest you read both when you go to do your tests. The pictures in both documents will help increase your understanding of the tests. The procedure and results should be the same regardless of generation of your bike, but things just look a little different. The main reason to test is because the more recent C28 codes on the k6-7 bikes have been caused by the ECU. My theory is that with the known regulator/rectifier issues, there's some spikes or dips that are damaging the stepper control on the ECU. And, if you're lucky, your fix will be a little effort and $0.20 worth of electrical tape to correct a broken wire.