I wanted a fast idle for my on board air to fill tires faster. So not wanting to spend much money on it, I just used some old Jeep parts. On the early XJs, there is a vacuum actuated high idle. It consists of an electrically operated valve and vacuum motor on the carb. The valve is found on the inner driver side fender right behind the washer bottle. It will have two wires that disappear into the harness and go to a pressure switch on the high side power steering hose. The purpose of the system is to give a 200 rpm increase when the power steering pump is pumping pressure.



The valve is a special valve and is directional. The hose that points the same direction as the wires in the photo goes to the vacuum motor. It is normally open, allowing atmospheric air pressure to the vacuum motor and thus allow it to be in a normal state of rest. When closed, the atmospheric pressure is sealed off and vacuum goes to the motor and the plunger is extended. The plunger in the photo has been extended almost all the way allowing the full range of the motor to be utilized, about 1/2"





Here is the vacuum motor attached to the throttle cable bracket on my Chevy engine. I used JB weld to attach the back of the motor. However there is a nut that goes on the large threaded portion that can be used if a bracket is fabricated. I welded a small tab on my throttle linkage to give the motor something to push against.

So how well does it work? It is wired to my compressor clutch circuit. I get an engine speed of about 3000 rpms from it which allows my Sanden compressor to fill four 31" tires from flat to 35 psi in about 3.5 minutes

 

good idea