Here is my take.
1) A cold engine runs better with a warm intake. Fuel won't puddle up on the bottom of a warm intake runner.
2) A warm engine performs better with a cold intake. Do not remove the flapper.
3) A heat riser stuck in the warmed-up-engine position will not warm up the intake at start up, and cause the fuel to fall out of suspension and puddle up on the bottom of the intake runner. This liquid fuel will not burn and will end up leaking past the rings and end up in the crankcase, diluting the oil resulting in excessive engine wear.
4) A heat riser stuck in the cold-engine position will cause excessive heat at the intake, resulting in poor performance once the engine warms up (see #2) above).
5) You live in Michigan (a great place to be from BTW). You need a fully functional heat riser. Sure, you can get by with one stuck in the warmed up position (many people do), but your engine will suffer premature wear.
Carl
