Is it 650 in P or N? I was thinking it was supposed to be 650 while in gear, which would put it around 800-850 while in park or neutral.
My bad, I misstated. The tiny iPhone screen caused me to overlook the error. 650 in Drive, yes. With A/C off in our case. I have found 750 in Neutral or Park lets our engine run at nearly 650 RPM in Drive, so depending on if Lynda is available to help keep the car in Drive and the brake on, r if I am working alone,I use either idle speed (650D, or 750 N/P) Some later vehicles 74 (and later) had an idle spec with A/C on, sometimes two idle speeds specified, one for A/C off, and the other for A/C on when a Throttle Position Solenoid (TPS) was used to bump idle speed a little when the A/C is on.
An aside, be sure to set your dwell and ignition timing to spec before adjusting the carburetor. While you are at it test the distributor vacuum advance diaphragm to make sure it is not leaking or ruptured. It won't impact your idle speed or quality (unless it is incorrectly connected), but a ruptured or leaking diaphragm will adversely impact your off idle performance and your MPG.
For our 71-73 Mustangs those Throttle Position Solenoids were used to adjust the idle in Drive
when the TPS actuating rod/pin is extended, as opposed to using them top increase idle speed with the A/C turned on. And, of course, I have a YouTube videos showing how the TPS works, and how the base and TPS On & Off idle speeds are adjusted on our 73 Mustang Convertible (302 2v):
Here is another video where video where closer to the end I show how I adjust the idle air fuel mixture screws. There are a few ways to set the A/F idle screws, using a tachometer, using a vacuum gauge, using both tach anf gauge, and of course setting it "by ear." Later on in more concisely controll emission control environments we get into setting by "Lean Roll," using "Propane Enrichment," and/or using a CO/HC exhaust gas analyzer. For me, on these older Mustangs, I tend to "go by ear," although at times I will use a vacuum gauge and/or tachometer - depending on how I feel at the moment, heh heh... In the video description I provide the mm:ss into the video you can find idle or solenoid related info: