As I do sometimes, I read all of a lot of these threads to learn and get ideas. I have a 1989 FXSTC, stock, except I have updated the Carb to a CV carb. Now I've seen a few threads that discuss the VOES switch, but none directly. So what does it do? I think if I understand it a little, it helps control the ignition advance to help save gas mileage at cruising speeds. Then again I could be way off. I've also seen threads where people just bypass them. Is there any benefit to this? Any cons besides gas mileage (providing I'm right about that)?

 

When intake vacuum reaches a preset level (varies by model of bike) it grounds, sending that signal to the ignition module which fully advances ignition timing.
As to why people disconnect or permanently ground that lead varies depending on aftermarket modules or racing applications. For 99.99% of people who ride, it needs to be connected and working correctly. Otherwise, there's a recipe for major disappointment and/or engine damage.

 

Specifically...when under load, ie open the throttle to accelerate... the vacuum in the intake tract drops... when this happens the risk of Pre-ignition [spark knock] is highest... to prevent this the VOES signals the ignition that a Drop has occurred, and the ignition retards the timing just a bit, to prevent pre ignition...which can be damaging to the engine.
When at constant cruising throttle openings... vacuum is higher, and more advance can be applied...actually increasing efficiency..

Edit; On my EVO FXR, I use the VOES, mostly because I also utilize a Sportster ignition module, as it gives a more aggressive advance curve, as well as a higher rev limit.. I may even advance the timing beyond recommended... depends on the gasoline available.. Anyway...without a VOES, I have experienced pre-ignition under load... Not what I want....

 

http://www.wildwestcycle.com/f_voes.html

 

VOES = vacuum operated electric switch, or pretty close to that. It does as described above, retarding ignition timing under load. Having had my vacuum hose split, through old age, it can seriously affect the way the engine runs and not in a nice or user friendly way! There are aftermarket ignition systems that don't use it.