Just purchased 1966 Ford Fairlane with a mid-80s Mustang 302 engine with a carburetor, Ford part number D4JL-9510-J part number. Holley list: 7128, date code: 1658.

When car is 1st started, idle is set at ~700 RPM. I'll take it for a drive & floor it. The car will not return to idle. Engine will running 2000-3000 RPM. Idles at ~45 MPH. I check the idle set screw and it's still sitting against the idle plate as before. What's causing this issue and how do I fix it? If you turn the car off and start it later, it's fine. Seems as long as you don't floor it, there's no issue. Please help.