Heat doesn't dissapear when the throttle is released. It takes a much LONGER time to dissipate heat energy than a second or two. I believe you have a tuning issue. Richen the needle until you hear stuttering/sputtering/gurgling noises.

If you have excellent water flow through the head I believe it would be impossible to overheat the engine no matter what you do to it.

If an engine is way too rich it will stutter, load up and almost always stall out.

I have my tunnel set up so that it is quite rich for the first minute. I can tell it is too rich from lack of acceleration and some sputtering noises. Once the engine warms up it leans the mixture to an appropriate level.

Ryan