I thought it was constantly running fast, in that case no it wont be an air leak.

Know it sounds daft did you defently unplug the valve with engine running as when you do this it will drop to base idle, as only way engine speed can be increased except throttle is by the ISCV.

Overfueling wont cause the problem as the only thing to raise engine speed is air.

By the way car looks nice

Martin