I'm with you, Willib. Same setup on my 216, same result. Installed a new spring (mine from Jim Carter) following the photos and steps in the shop manual. Tested it with a heat gun... worked great, but when the engine is run, it doesn't gather enough heat to rotate the flapper. I don't know if I should be concerned or not. Engine runs fine but I don't take it out in cold weather...no heater.
