Vacuum Switches

Vacuum switch, intention and function

The engine of Harley-Davidson motorcycles reacts differently to changes in ignition timing under different load conditions. Simply put, the engine - e.g. of a Sportster or Softail - needs more advanced ignition under high load than under partial load or when it is idling. When there was still the manual ignition adjustment, the experienced Harley rider could hear that from the engine sound and adjust the ignition timing accordingly with the twist grip.

And then came the automatic speed-dependent ignition timing adjustment. Unfortunately, the speed-dependent adjustment curve of an ignition contact is only a compromise solution. If the engine (with soft return springs for the centrifugal weights) reaches fully advanced ignition too quickly, the engine pings. If this happens too slow (stronger return springs), the engine lacks acceleration and peak power.

So what to do? You make use of the vacuum in the intake manifold: high engine load corresponds to high vacuum, at partial load the vacuum drops again. On older gasoline-powered four-wheelers with an ignition distributor, there is therefore a vacuum adjustment for the ignition timing. On Harleys with a centrifugal timing system, this is not possible for reasons of space. Enriching the mixture also helps against engine pinging. Around 1980, when oil shortages and the first environmental regulations called for leaner mixtures, the introduction of the fully electronic ignition system incorporated the vacuum trick. The vacuum switch enables the black box to switch between two different ignition curves depending on the load, a steep one for high load and a flatter one for partial load.

Vacuum switch, do I need it on my Harley?

As just described, it also works without, namely purely speed-dependent. Harley-Davidson did this from 1965 to 1980 with the FLHs and FXs and the Sportster model series. As a sensitive rider, you have to listen to the engine and try to adjust the throttle. On custom electronic ignition systems there is often no connection for a vacuum switch. On others, like Dynatek's 2000i, there is a selector switch.

For the bikes from the 1980s and 1990s with original or original-compatible electronic ignition, we have the matching vacuum switches, with screw-on tab and connecting cable.