Good convo. At what temp is it heated enough? I dunno. My 216 has a working heat flapper with the bi-metal spring installed correctly per the shop manual. I know that within a minute the intake manifold is getting pretty hot to the touch and after it's run awhile the intake almost feels as hot as the exhaust manifold and the flapper is closed i.e. directing exhaust gases straight out the pipe and not around the intake plenum.
My mpg isn't great, around 13 mpg, but considering the age of the bottom end (est over 120k miles) the 216 runs real good. No hessitation, power to pull hills in high gear and it can hit 65+ in the rare instances when I have to pass a slower vehicle. It runs good in all 4 seasons so the manifold getting hot seems to be a benefit year round. Sometimes with the manifold heated up I can still see some condensation around the base of the carb.....
I believe it was Dads50 that posts a video showing how to use a hair dryer or heat gun to heat up the spring to see if the flapper works. It's a good test.
Dave
