Odd idle speeds, need help

[Primer66]

Hey everyone,
I have a '66 with a 289 V8. When I'm in park it idles at around 600rpm, but when I give the pedal a couple taps it'll bump up and stay at 900rpm. This also happens when I'm in gear and slowing coasting a parking lot. Any ideas on what causes this?

 

[wicked93gs]

My first guess would be a sticking spring or throttle plate. ATX or MTX?

 

[KRUISR]

Carb or EFI?

 

[7991LXnSHO]

If it’s the carb, check that the choke is opening fully and the high idle cam is not getting engaged after warm up.