Condensed from posts by Chris Troudt and Doug ??
The VOES (Vacuum Operated Electrical Switch) is an electric switch activated by the carb
vacuum. It's specific purpose is to retard the ignition when the motor is running under
high load (low vacuum) conditions. The VOES is a normally open switch. With low vacuum the switch
is open. With higher vacuum, the switch closes, connecting the ignition module lead to ground.
Some aftermarket ignitions such as the DYNA 2000, bypass the VOES altogether. Others allow it's
usage. The real problem though is that the VOES is calibrated to specific levels of
vacuum. It is also calibrated differently for different models. Then, to add to the confusion,
modifying a motor changes it's vacuum values and may cause problems with the switch functioning at the wrong times, or not at all.
If you've done the usual mild cam (durations of under 240), carb and pipe mods, some 'experts' recommend changing the stock big twin ignition module
to a 1200 Sportster module and NOT reconnecting the VOES, to take advantage of it's quicker advance curve. The hotter Sportster curve will benefit a
bike that breathes as well as these mods allow.
But be aware that simply disconnecting the VOES wire, hose, or both does not effectively
bypass this system. It is actually telling the ignition module that the engine is constantly under high load, so it selects the
more retarded timing curve. The proper way to "bypass" the VOES system (and I DON'T recommend it unless you understand exactly
what the ramifications are) is to ground the VOES lead going to the ignition module. Also, if you disconnect the vacuum hose or
remove the VOES altogether, don't forget to plug the vaccum port on the intake manifold or
It all boils down to what you've done to the motor? Slight usual mods, like mild
cam, carb jetting and pipes with a stock big twin module, then leave the VOES be. More serious stuff like porting of heads, hotter cam
(durations of over 240)? Then changing to a 1200 module and properly
bypassing the switch is an easy option and works well. At this stage, you can also
consider an aftermarket ignition but since the VOES is calibrated to a basically stock motor, my advice would be to eliminate it and spend time
tuning to find the advance curve that works best under your specific set of riding conditions, be they solo and honkin' down the road or two-up
cruisin' or whatever.