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joehev
07-27-2017, 04:28 PM
Can someone explain to me what the spark modifier does, please? I don't think I've ever seen the term.
Thanks very much.

JoeHall
07-27-2017, 04:32 PM
Vacuum and centrifugal advance mechanisms in the distributor?

PackardV8
07-27-2017, 04:56 PM
As Joe suggested, spark modifier is an early automotive term for vacuum and/or centrifugal mechanism to control ignition timing. Up until the early '30s, many cars had a manual spark control lever on the steering column. The driver had to remember to retard for cranking, advance for cruise. An automatic spark modifier was considered an advance in technology.

jack vines

RadioRoy
07-27-2017, 06:27 PM
I have always heard the vacuum advance to be called the spark modifier, not the mechanical advance. No?

JoeHall
07-27-2017, 07:33 PM
In older Harleys, till around 1969, the left hand grip was used for, "spark control"; twist it one way to retard the spark for kick starting, and the other way to advance the spark after it fired up. Some years replaced the grip control with a pull spring on the distributor that would auto-rotate the distributor to advance position, once the motor fired up. Beginning around 1970, the spark was automatically controlled (modified) by fly weights inside the distributor. In Studes, I'd classify both the VA and centrifugal weights as spark modifiers (since they both modify spark timing), but that's just an opinion.

joehev
07-28-2017, 09:36 AM
can someone explain to me what the spark modifier does, please? I don't think i've ever seen the term.
Thanks very much.

thank you everyone. I now know what we're talking about.