The heat riser applies heat to the intake when the heat riser spring is cold. As the spring heats up, it gradually reduces the amount of heat to the intake until the plate closes.
The heat riser is designed to allow the engine to run smoothly throughout this slow opening process.
While it may remain running smoothly, there are some significant changes in air/fuel conditions during this transition.
It is plausible to me that the final idle speed would be different after the the engine is fully warmed up/normalized.
I would not be surprised if by adjusting the idle mixture screw setting either richer or leaner, that the final idle speed would change (maybe higher than when the engine was cold).
Carl
