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68 mustang: 302 SB, Edlebrock RPM performer Manifold, 34 degrees timing, 1406 Edlebrock carb. electric choke is "keyed" on and adjusted for current weather. I've performed the lean best idle mixture with vacuum, dialing in idle mixture screws. Choke High idle is adjusted to run at about 1200-1400rpm until I blip the throttle. Idle is dialed to run about 800-900 rpm.. any lower(750rpm) with the 260 cam and it gets too lopey.

So here's the prob. Cold start, choke on and RPM at 1400 until I hit throttle. Idles at 800 rpm warmed up.. I then drive the car for 10 - 20 minutes and when at stop lights car in neutral and clutch pedal out, RPM is now at 1000/1100. it's not terrible but it goes up.

I can't seem to grasp why this happens... If I dial the idle screw down to meet my preferences to 800, the next time I cold start car, and let it warm up to point of taking off choke, it idles too low...

*So either my car just takes that long to warm up and I have the idle set screw wrong, or am I missing something? no vacuum leaks that I can tell, all hoses connected. Maybe after engine warms up for some reason it leans out? weird.

I'm going to install an Air/Fuel Mixture gauge to help me dial in car but doing so may help me solve this too.
 

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Jtingle: If your car has the original vacuum plumbing on it, there's a fitting on the manifold that messes with your idle speed a bit. It switches from ported vacuum to manifold vacuum when your coolant gets to a certain temperature. Basically, on ported vacuum, there's no vacuum going to the distributor's advance when the throttle is closed. It only kicks in when you start to open the butterflies. Manifold vacuum just sucks all the time - more when you close the throttle plates, of course.

The idea was, if you were at a light, and the car was running a bit warm, the added vacuum advance would speed up your engine and make the fan blow more air to help keep your engine cooler. The fitting (if you have one) should be screwed into the back of the manifold. It has three vacuum line nipples. One line goes to the dizzy, one to the side of your carb, and the other probably to the base of the carb at the back, or possibly another vacuum nipple on the manifold.

Maybe that's what's going on?

Also - you should generally tune your engine with the vacuum advance on the dizzy disconnected anyway. It only matters during part throttle, and helps with economy; it shouldn't affect idle or WOT performance significantly. (Assuming you're using ported vacuum) - but I'm guessing you already know that, and just meant you were tuning to get max manifold vacuum using a gauge. =)
 
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