My brand new QuickFuel Slayer does the same thing.
I can let it completely warm up and with the idle speed screw set to around 500 rpm it idles in park all day and seems normal when you whack the throttle. When you drop it in gear, it is a touch too slow add wants to stall in gear. If I do an 1/8th turn on the curb idle screw, it jumps straight to around 1500 RPM. Turn the idle screw back an 1/8th turn and it drops back down right to 500 RPM with nothing in between. It's like an on-off switch. When it went to the fast idle, I tried pressing the fast idle cam down thinking it was just catching and sticking on fast idle and it doesn't drop down. This has me stumped also as it is doing this fully warmed up and off the fast idle cam.
I think I've got to increase the idle air via the secondaries and see if that will solve it as something weird is going on with the throttle tip in off idle also. I have to feather it to keep from stalling and then I get an abrupt "wake up" as it gets past the stumble.
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Triple Black 1971 GTO
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