What does it do, and how? Briefly, the VOES is a device to control the ignition advance - this is necessary to avoid detonation. Real detonation can destroy your engine - you can read about it here. Knowing about deto will help you understand the VOES function. Now, low vacuum (the term "low vacuum" describes a state in which pressure in the intake tract is relatively closer to normal atmospheric pressure) is not what causes detonation, but low vacuum is an indicator of conditions that lead to detonation. We can use it to tell us to change the situation, by increasing our RPMs for instance (with a downshift, which increases the vacuum), or rolling off the throttle (which reduces our speed), or an electronic ignition can use the low vacuum to operate a switch to tell the black box to change the ignition parameters. And that is what the VOES does - it detects high vacuum and tells the black box to advance the timing. The VOES is connected by a hose to a fitting on the intake manifold or carburetor. Inside the VOES is a diaphragm, a spring, and a set of electrical contacts. The switch has only two positions: on, and off, and they are selected by vacuum through the hose pulling the diaphragm up and turning the switch ON, or the spring pushing it back to OFF. The switch selects one of two ignition maps in the black box - there is no in-between; it's either OFF, and the timing is retarded, or it is ON with the timing advanced. The normal (default) position is OFF/RETARDED. That is the position you want for starting, acceleration, and high-load operations like going up a hill in a gear that's too high. It is also the position you want it to go to if the switch fails or the vacuum hose breaks or leaks. When a measured amount (more on that later) of vacuum is applied to the switch the diaphragm lifts, turning the switch ON, which switches the black box to the advance curve. When the vacuum drops again, the spring pushes the diaphragm back down, and the switch returns to OFF, its default position. There are only two positions, ON or OFF, and there are only two ignition maps (in the Harley box), retarded or advanced. (There. I've said that twice now, and I might say it again just to make sure everybody understands, since I just know you're not taking notes.) Are there different VOES settings for different bikes? Yes. Think about it. The heavier the bike and the smaller the engine in relation to the bike's weight, the wider you will have to open the throttle to develop or hold the speed you want without downshifting. That's the formula for detonation. The vacuum level will be dropping waaaay down and the ignition needs to be retarded. Make the bike lighter or increase the engine size or downshift and you won't have to open the throttle so far to get the power you want - the vacuum will stay higher, and the ignition can stay advanced. What that means is that the VOES for a bagger will be set to advance the ignition at a stronger vacuum level (that's a higher number of inches of mercury) than the VOES on a Softtail that weighs over a hundred pounds less, even before you load the bagger up with the old lady and a week's worth of camping gear. Take that bagger and gear; they're at a traffic light on the way out of town. Sitting there idling the throttle is nearly closed, so the vacuum is strong - a high number, so the spark is advanced at idle - about 30 degrees if you use a Crane Hi4 ignition with the advance curve selector set in the middle. Now roll the throttle open to pull away and the vacuum drops. The RPMs are still low and they build slowly because of the weight, so vacuum stays low. The VOES, responding to the low vacuum, switches OFF, retarding the timing to about 10 degrees (again, Crane Hi4) while still at idle speed. As RPMs build and enough vacuum develops, the ignition throws in more advance - by 3,000 RPM with the throttle rolled off enough to hold that speed, you'll see about 35 degrees of advance. The Crane box will allow even more advance - with a steady throttle (high vac) at 4,800 RPM you'll see nearly 40 degrees of advance. You don't want the bagger's ignition to advance too soon, though, and induce detonation. Now let's have some 130 pound chick on an FXR pull away from that light. Her engine's not going to require full throttle to pull away smartly, and even if she gives it full throttle the RPMs will build fast, developing vacuum quickly. For that bike we can set the VOES to go to full advance at a pretty low level of suck, and the VOES will quickly advance the timing.	Keep this in mind: The VOES is a vacuum advance. The default position for the ignition is RETARDED. Some people think that means the curve is inferior somehow, but not so. The engine runs just fine on the RETARDED advance curve; think of it as the normal operating parameter. It is designed to avoid detonation and still give good performance. And when operating conditions are right to avoid deto (like cruising along on level ground with steady throttle) then the VOES tells the ignition to advance, which makes for a more efficient fuel burn. That gives you better mileage, cleaner exhaust, better overall driveability at part-throttle operation, sweeter farts, more muscles, more hair and all-around irresistability to women. There is no significant downside to the VOES unless you are searching for the maximum grunt out of your engine, and are willing to play tag with detonation to get it. Understand this: Vacuum is a state of air pressure below normal atmospheric pressure, which is 14.7 pounds per square inch. 14.7PSI will push a column of mercury 29.9 inches up a tube. Vacuum is measured in inches of mercury below that figure. That is, when you see a figure like 7 inches of vacuum, that means the atmospheric pressure in the source of the measurement has been reduced enough that the column of mercury will only be 22.9 inches high . When the engine is off there is 14.7 psi (normal atmospheric pressure) pressure in the intake tract - that is NO vacuum at all. Now, let's keep the throttle plate completely closed and pull a piston to the bottom of its stroke. That'll develop low pressure (that is, vacuum) in the intake tract; right? The VOES is hooked up to that tract and it senses the vacuum and clicks ON. Until some air can get in the vacuum will stay there. So now let's open the throttle a little bit - air rushes in, fills the vacuum and the VOES switches to OFF. Now let's start the motor. That pulls the piston down real quick, which develops a momentary vacuum in the tract before air coming in past the throttle plate fills the vacuum. Have two pistons doing it at a high rate of speed and you wind up with a pretty constant state of vacuum there - it actually does fluctuate, but it averages out for our purposes. The faster they go the more air they are pumping out of the intake tract, which leads to a stronger (higher number of inches) vacuum. Your throttle controls how fast air gets in to fill it. When the engine has a large throttle opening and low RPMs the intake vacuum is low. That's because the pistons aren't moving up and down real quick sucking the air out of the intake tract, and the atmospheric air has a big hole to flow through to fill the intake tract. Think about it - it does make sense. So imagine what happens when you maintain a constant engine speed (which means a constant state of vacuum), but open the throttle wider, like when you're going up a hill or accelerating away from that traffic light. The hole through the carburetor just got bigger when you opened the throttle, which lets more air in, which lowers the vacuum. So low RPMs and a large throttle opening produce low vacuum.
Are big engines different? Yes. Let's take Biker Bill, for example. Last month he dropped a brand-new S&S 113" Super Sidewinder into his bagger, and he switched the VOES off of his old 80" motor when he did. Now he's headed up Raton Pass with the OL on the back, and man, he's rollin'; that motor is sooo strong . . . About halfway up, BLAM! The top of a piston vaporizes and all the smoke comes out of his motor, 'cause detonation blew it up. Here's what happened. On his old 80" motor he'd have had to open the throttle big-time to haul up that hill. The VOES would have responded to the low vacuum caused by the wide open throttle and gone to the RETARD map. But his big, strong new motor didn't require him to open the throttle wide to hold 70mph, so the ignition stayed at full advance and detonation merrily ate the top out of his piston. It ain't pretty. What he should have done was recalibrate the VOES to switch to the advance curve at a higher vacuum level (that's more inches of Hg, for those who still don't quite get the idea that the higher the number the stronger the vacuum is sucking, and the longer the ignition stays retarded.)
Can I recalibrate my VOES? Yes, and it's easy to do, but none of the manuals tell you how. I will. What's more difficult is figuring out what amount of suck ought to operate the switch. I'm not going to tell you that because sure as hell if you screw it up (or if I'm wrong; like that could happen . . .) and you grenade your motor you're gonna be pissed at me. And bikers, being like they are and everything, you wouldn't be happy calling a lawyer to sue me. Oh, no, you'd be on my porch with a ball bat in your hand or something. So you figure out that part. What I will do is tell you that the stock FXR switch works at about 3.5 to 4.5 inches as I recall, and baggers at around 5 to 6. Now that I have an S&S 107" motor in my FXRT I reset the VOES to 7" and it seems to work for me. Here's a tip: If you get intermittent pinging on acceleration, and if switching to better gas doesn't help, try recalibrating your VOES to retard the timing at a higher vacuum number before you start going crazy doing other, expensive, stuff. Here's what you're gonna do. You'll to need a vacuum pump, like a Mighty-Vac and an electrical meter.

There are people out there who tell you that you cannot recalibrate the VOES by adjusting the screw. They say that you have to buy a heavy-duty spring if you want to change the way the switch responds. I don't know why they say that, maybe that's the results they've gotten, or maybe they sell springs; I don't know. But I'll tell you that I have adjusted and readjusted several of them, from low to high and back, with nothing but a screwdriver. FYI, Harley is silent on the subject of adjusting the VOES. Instead, they sell two parts, one for FLs and one for FXs. They don't sell one for big-inch motors, but you need one anyway.	Locate your VOES and take it off - you've gotta do this operation on the bench or sitting at the dining room table or somewhere besides squatted by the bike fumbling under the tank. Usually, the VOES is on a bracket between the cylinders somewhere behind the horn or coil. It may not look just exactly like the picture above, but it'll be close. You'll see a hose connected to it, and two wires. Usually, once you've found it it's easier to unbolt the bracket than it is to take the VOES out of the bracket. Carefully disconnect the vacuum hose from the VOES, then disconnect the ground wire from where it's secured close by somewhere, and finally, unplug the other lead (if it's a Crane ignition it's a green wire that goes to the black box - other makers use different colors.) Note: In some years, Harley used a two-wire plug for the VOES, with the ground wire in it, too.
Now look it over. Note that there are wires that come out the bottom, there's a nipple on the side, and sort of a tower on top. That tower is actually hollow, and it's filled with some sort of goop like putty. You're going to dig it out, all of it, BUT FIRST!!! You're going to check to see what the VOES is set at now. It might even be busted, and you want to know that before you go trying to adjust it. Set your electrical meter to check for continuity, then hook each of the meter leads to one of the wires on the VOES - doesn't matter which goes to which. The switch should be in the OPEN position, that is, there should be no continuity through the thing. And usually that's the case, 'cause they're designed to fail OPEN so that if it breaks the ignition automatically retards. Now hook the Mighty-Vac up to the nipple and apply some suck. Watch to see how you much you have to apply to make the switch work - you want a number, and usually it'll be between four and six and a half inches of suck, depending on whether it's for an FX or an FL. I've said use an electrical meter to tell when the switch opens but you can usually hear the CLICK as it works - I'm just queer for tools and gadgets. If you can't get the thing to switch then it's broke. Toss it. Yes, toss it; it's nonrepairable. Get another one. Since they're about $70 to $80 at the Harley shop you're not going to want to, but do it or you'll be sorry. (I've heard they're available on-line, but I've never bought one there and I have no idea where they're from or how reliable they are.) Let's assume that it's still good - they usually are - and that it operates at 5" of vacuum. Now, go on and dig out the sealant in the hole. When it's cleaned out, take a flashlight and look down the hole. Jeez! Who knew? There's a slot-head screw at the bottom (you can see it as the silver thing in the photo.) That's your adjuster. Screw it in and it preloads the return spring under the diaphragm and makes the switch respond to stronger vacuum, and vice-versa. Screw it in too far and you bust the diagphragm - and then you go buy a new switch. You can make most of the adjustments with no more than two turns total, and usually one or less.
Now the reason you're digging around in here is that you need to adjust it, right? Let's say you're putting it on a bigger motor or that you've been getting an ignition rattle when you accelerate. That calls for adjusting it to switch to advance at a higher level - make it a couple inches, say, 7" of Hg. Turn the adjusting screw clockwise a half turn and then check with the Miti-Vac and meter to see when it switches, then adjust from there. Once it's switching at 7" Hg reinstall it and go for a test ride. Repeat as necessary. Here's a tricky part. It's easy to forget, and unless you do it, the whole process doesn't work and will make you think the switch is broken. With the goop out of the hole the diaphragm does not operate normally - it's affected by atmospheric pressure coming through the threads of the screw. So to get a good reading you have to spit on your thumb and put it tight over the hole. (Or you can lick your thumb, but I don't - I lose track of where my thumb's been.) Once you're satisfied that it's set right, squirt the hole full of silicone seal again to keep it airtight. Let it set up before you apply vacuum again so you don't suck goop into the screw's threads.
Since a VOES is an electromechanical device it can fail (hell, anything more complex than a cannonball can fail - and eventually will). If it does fail it reverts to the retard mode full-time so there's no danger to your engine, but you lose the benefits of an advanced curve. It's my practice to check its function whenever I change my spark plugs. The quick and easy way to do that is to unplug its harness (if that's how yours is wired) or to disconnect the ground wire from the VOES while the engine is idling. If the unit is working the ignition is advanced at idle, but if it's not working it'll be retarded. So disconnect it and see if the idle changes. If it doesn't, it's broke.		Wiring a telltale light into your VOES circuit. NOTE: this is significant revision of a procedure previously posted here. If you used the old one you should change it to reflect this setup. There's another trick to using one of these things that'll not only tell you whether it's working or not, but when it's switching. I use it because big-inch motors are sensitive to retarding properly, and I want to know when mine is running advanced and when it's retarded. What you do is wire an LED between the hot side and the ground of the VOES - doing it has no effect on the VOES function. Go to Radio Shack and buy a 12 volt LED telltale light - they're just a couple of bucks, and they're small enough to mount almost anywhere on your instrument panel by drilling a single hole. I slip the LED into a rubber wiring grommet of the right ID, and then drill a hole the right size for the OD of the grommet. That makes a nice vibration isolated mount. Click HERE for details of how to do it. If you roll the throttle on and get ignition pinging and the light is not coming on, you need to adjust the VOES to switch at a higher vacuum level so that the ignition will retard sooner.
So there you are. Internalize this to the point that you really understand what's going on and you'll know more about it than 99% of the bikers out there, including me. I have to stop and figure it out every damn time somebody asks me, so I thought I'd better just write it down. Pilgrim

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