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Reving up and down at idle

Started by jackbof, 25 May 2024, 04:30 PM

jackbof

Hi all,

I have a 1975 350SE and it will idle nice and smooth when I start it but as soon as I touch the fan AC fan speed control ever so slightly it will start to rev up and down continuously. If I turn the fan off and fiddle around with the controls a bit it will return to a smooth idle. Sometimes while driving it will just do it randomly at idle but its smooth when throttle is open.
Any ideas on what might be causing this?
Thanks

Randys01

For a start have a look at the a/con vacuum advance  compensator which is on the firewall engine side . It is an electric solenoid connected to the distributor.