Since the intake manifold on normally aspirated 4 strokes is directly connected to the cylinder head, heat transfers readily to the intake pipe at low airflows when the throttle is at idle. This significantly enriches the idle mixture and this enriching becomes much more pronounced at higher ambient temperatures. Try setting the idle mixture as lean as possible consistent with good throttle transition.

Try putting your finger on the intake pipe with the engine at idle, then open the throttle about 1/3 and notice the immediate decrease in temperature. The warming of mixture temperature when throttling back is partially responsible for the slowing of idle rpm after about 5-10 seconds.

Since the carb on 2 strokes is mounted on the comparatively cool running crankcase, the 2 strokes are not similarly affected.