Choke terminology:
On = Closed = Up position (required when engine cold)
Off = Open = Down position (Engine operating temp position)
Big idle knob adjusts idle speed (1000 RPM-ish) when engine is at full operating temperature.
When cold, the operator holds the throttle open during warm up and modulates choke as required. Or, adjusts friction knob under throttle to keep RPM/throttle higher.
With this knowledge, restate the problem.
Do all the head pipes get equally hot, generally, with throttle in idle position?
Cheers,