I shared this information with some friends a short while ago, so I thought it might be helpful to share here as well:
This post is intended to give an overview of how the Fly vibrato system works and provide one approach to setting it up; this a complement to a guide created by members of the old Official Parker Guitars forum. As described in the Fly owner’s manual, the Fly bridge has three modes: 1) bend-down only, 2) floating (bend up or down), and 3) and fixed, which is effectively the same as bend-down only mode, but with a different use-case. If you don’t intend to use the vibrato system, you can tighten the wheel until the bridge can no longer bend up at all; this is as “fixed” as the bridge can become short of blocking the bridge by replacing the flat spring with an inflexible material.
If you intend to use the vibrato system, one useful setup of this system is that, with your guitar is in tune, the step-stop and the three-ridge spring plate just slightly touch eachother so that 1) when the step-stop is down, the bridge is in a fixed, non-floating, state, and 2) when the step-stop is in the up position, the guitar retains the same tuning in its “floating bridge” state that it had when the bridge was touching the step-stop.
Basic concepts of the Fly vibrato bridge:
-setting up the vibrato system relies on finding a single balancing point so that the tension of the strings (when they’re in tune) is being counteracted by the tension being placed on the spring; the tools you have to work with in this regard are 1) the tuning pegs to tighten/loosen the strings to pitch, 2) the step-stop to move the bridge into a fixed position or a floating position, and 3) the tension wheel (a.k.a. balance wheel) (exposed or otherwise) which controls the amount of tension on the spring and, ultimately, the bridge.
-the wheel moves a threaded rod (labeled the “Long Tension Screw” in the image above) that is 1-1/2 inches long; there is a cavity in the Fly where the threaded rod sits (a cut notch in the screw aligns with a protruding notch in the cavity, and this prevents the rod from rotating in the cavity). When you turn the wheel up, loosening it, the threaded rod recedes into the cavity and lessens the tension placed on the spring; when you turn the wheel down, tightening it, more of the threaded rod is pulled from within the cavity it's sitting in. This pushes the screw against the T-bar (which holds one end of the flat spring), which, in turn, puts more tension on the spring, which, ultimately, pushes the three-ridge spring plate (which is attached to the bridge) against the step-stop. If the spring is too tight or too loose, moving the step-stop between its two states will bump the guitar out of tune. Again: when the step-stop and the three-ridge spring plate touch, the bridge is stable enough to tune just as if you're tuning a guitar with a traditional fixed bridge; the goal of the setup procedures below is to retain this stability when the step-stop is down and add the ability to move the step-stop out of place (in the up position) without having the bridge move anywhere (which would, of course, throw the guitar out of tune). In order to do this, we'll need to do a balancing act!
-the springs have been rated to compensate for the tension placed on the bridge by a specific gauge of strings; a spring labeled “9”, for example, is intended to be used with a set of strings where the high E string is .009. Long term: you’re asking for trouble if you don’t observe this.
-you want the step-stop, when down, to just barely touch the three-ridge spring plate, so that when you move the step-stop up to floating mode, you retain the same tuning as you did in step-stop mode
-if the step-stop and the three-ridge spring are not touching when the step-stop is down then you are effectively always in floating mode; you can eventually find an equilibrium point where the tuning will be stable, but you will lose the benefits of the step-stop keeping the bridge straight and the tuning stable while it’s in the down position.
Common mistake: it’s easy to assume that the step-stop is working because it’s in the down position, but, again: the step-stop is only working if it is making contact with the three-ridge spring plate; otherwise you are actually in floating mode even if the step stop is down, and, at that point, switching the step-stop up or down will make no change to the function of the bridge. It's not always easy to see if contact between these two points is being made.
-tuning in a floating-bridge mode on any guitar can be a nightmare, which is why Ken invented the step-stop; unlike your typical non-Fly floating bridge guitar, if a string on your Fly breaks during a performance while you’re in floating bridge mode, the bridge will go way out of tune, but you can just flip the step-stop switch down and you’ll be back in the game...if you can get this balancing act (described herein) down!
-when you loosen a string by tuning down, the spring will have less tension to counteract and the three-ridge spring plate will move forward toward the step-stop (touching); when you tighten a string, the three-ridge spring plate will move back and away from the step-stop (not touching).
-the wheel on an original Fly (which is exposed) functions the same way on a refined Fly: it is tightened or loosened to apply more or less tension to the spring, which puts more or less force on the bridge, which puts the three-ridge spring plate closer or or further away from touching the step-stop.
-on Refined Flys, the bearing, arm posts, and vibrato arm bushing are different than on an original Fly, but the operation concept of both bridges is exactly the same, and you can follow these steps to successfully set up either bridge.
Setting up the Bridge
In the following steps we will effectively be doing two things: 1) tuning the guitar to pitch, and 2) loosening/tightening the wheel, so that when the guitar is in perfect tune, the step-stop is just slightly touching the three-ridge spring plate. That’s it! A little patience and you’ll be all set. If you have an original Fly, you don't need any tools to do this; if you have a refined Fly, you just need the 1/8" round bar that came with your Fly or something similarly-sized (like the T-shaped 3/32" hex wrench) that can fit in the holes of the step-stop and tension wheel.
It’s worth noting that you’ll likely only have to get this set up correctly one time unless you change the string gauge on your Flys, so it’s worth taking the 30 minutes or so to get this right. It’s also worth noting that trying this with brand new strings will also introduce the variable of “new string stretch”, which, of course, is fine, but just something to be aware of since string age/use is always a factor when tuning.
1. Regardless of whether your step-stop is touching the three-ridge plate or note, tune your strings close to pitch; it does not need to be exact as it will change once you start moving the wheel
2. Remove the backplate (optional, but recommended; especially for Refined Flys) so you can see what’s going on better; on a Refined Fly, don’t rip out the battery connector when you do this
3. Tighten the wheel enough so that the step-stop is touching the three-ridge spring plate; again: if it’s not touching, it’s as if you’re tuning in floating bridge mode, which you don’t want to do. The threaded rod should probably show about 10 threads, and no more than 15.
4. Put a screw in the center of the backplate holding it in place (optional) and tune; the tuning will probably be a little bit sharp. Tune, and, after you tune each string, check each string again: if the tuning holds, the step-stop is probably still touching the three-ridge spring plate. The likelihood is that the two points are touching too much, so we’ll need to loosen the wheel in small increments while checking the tuning along the way
5. Remove the backplate (optional) and loosen the wheel three quarter-turns up
6. Put a screw in the backplate (optional) and tune; the pitch most likely dropped. As you tune up, the three-ridge spring plate will pull away from the step-stop, so you’ll be tuning in a floating mode, which, as noted, will not be very stable. Get the tuning close and then...
7. Tighten the wheel a quarter turn; watch the step-stop come to meet the three-ridge spring plate. Again: you want the two points to be “just barely touching”, so that you can tune with stability when the step-stop is down, but then not have the pitch change (drop or go sharp) when you move the step-stop up to floating mode
8. At this stage, getting the balance perfect is just a matter of 1) tightening/loosening the wheel with small a quarter-turn or eighth-turn and then 2) re-tuning after each turn. You can see where the step-stop meets the three-ridge string. Before you tune each time, use the vibrato bar arm to quickly dive-bomb (push down all the way on the bridge) to ensure that there is no slack anywhere in the strings
9. You will know you’re done when you can tune stably with the step-stop down, and retain that same tuning when the step-stop is up
Yes and no—not really! The floating mode of the bridge will not work as intended; the spring will not be able to compensate for the tension of the strings, so the bridge will never float properly. The spring can be "extra-tightened" inside the body and still facilitate "bend down only" operation; that is, you bend the bridge forward/down and when you release the whammy bar, the spring returns the bridge to pitch. This is possible because the extra-tightening on the spring is pushing the bridge against the step-stop, which keeps the bridge from moving in any direction except down/forward.Can I use a 9 spring with a set of 10 springs? Or a 10 spring with 9 springs?
However, the bridge can only bend backward/up if it is set to "float"; that is, there is no step-stop or other mechanism to keep the bridge anchored: the tension placed on the spring must be adjusted to counterbalance the tension placed on the bridge (when tuned to pitch). If the spring cannot produce the right amount of resistance (in lbs) to counterbalance the tension placed on the bridge, the bridge will not be able to go from fixed mode to floating mode without the guitar going out of tune.
Switching between the two modes is simple and easy: you just flip the step-stop up and you remove the constraint touching against the bridge; the bridge floats because of the tension you're placing on the spring! When you're tuning, however, you want to have the step-stop down adding the constraint against the bridge, otherwise, and this is strange and important: the tension you place on the spring by tuning one string to pitch will cause other strings to go out of tune.
If you think about it, it makes sense: unless you stabilize the bridge in some way (literally holding it in place with the step-stop) the tension on the spring will constantly change in response to the tension placed on the bridge by the strings being tuned to pitch; then it's like whack-a-mole: you tune one string to the appropriate pitch, and the one you just tuned is now out of tune, so you tune it again, and it throws something else out of tune. Eventually, the guitar will reach an equilibrium where it is "in tune with itself", and most musicians with floating bridges are used to dealing with this annoying balancing act. The Fly bridge is exceptional because of this unique functionality where you can anchor the bridge in a fixed position with the step-step and then quickly flip a switch to let it float. The trick, of course, is that you need to set the amount of tension placed on the spring in proper counterbalance to the strings so that when you switch the step-stop up or down, the tuning doesn't change; this the reason why the resistance of the springs matters and why the string gauge and the spring must be matched.
If putting the step-stop down against the bridge causes the bridge to move, it will make the guitar go out of tune. The same is true if the tension on the spring is too tight: you can bend down properly as long as the step-stop is still touching the bridge, but if you flip the step-stop up and try to "float" the bridge, the guitar will go out of tune. With a properly "tensioned/adjusted" spring, when there are no physical constraints keeping the bridge in place other than the tension of the spring (e.g. when its floating), the bridge can be bend down/forward or backward/up and, when you let go of the whammy bar, the bridge will always return to the point of origin, in tune.
Other thoughts:
-all of the springs will eventually fail; it’s just a piece of metal. If you experience weird slipping scenarios, swap out your spring for another with the same string gauge rating.
-unless you are trying to block the bridge, only use a spring that’s not rated for the strings you’re using; if you’re trying to block the bridge, use a piece of wood or another inflexible material
-it’s possible, though not likely, that the screw that holds the step-stop in place can loosen or that the step-stop can start to have a little space between itself and the back of the cavity, so use a shim if that’s the case. In order for this all to work, the step-stop should only be able to move “up or down”; it should not be able to rock forward or backward
-there are many different factors that go into finding a balancing point, so knowing the number of threads that are showing on the tension screw of one guitar is not really useful for setting the bridge on another guitar beyond getting you in the ballpark
This is a lot of information, but, in reality: it’s not really hard to get this set right! Once you get a sense of how things work, you can quickly figure out what needs to happen. The Fly bridge is really something special!
In conclusion: As a rule of thumb, try to get the threaded rod to show about 10 threads, and if you see more than 15 or so, then something is wrong. If the strings have too much tension than the spring can resist, it will not float properly and you may have issues getting the three-ridge spring plate to sit up against the step-step.
#VibratoBridge
Yeah, as I mentioned in the last sentence, this long description/explanation might be over-talking it; it's actually a beautifully-designed system, and, straight-forward in terms of how it works, and, once you get it right, you can really go nuts with the vibrato arm and it will hold tune and return to zero without a problem. The locking tuners replace the need for a locking nut, which is nice. 
Patience is the key for me every time 
A vibrato bridge is a tricky system, but a system like ours on the Fly does help compensate for some of the peculiarities by providing a hardtail-like option with the step-stop.