The only time a heat riser activated by a bimetal spring will release fully will be at highway cruise speed with a pretty good flow of exhaust gas coming through the manifold. Within a couple of minutes after startup there will be enough heat in the manifold to unwind most of the spring tension, so the flapper valve will open partway. You should be able to fully open a manual choke within the first mile or two of driving and the engine will idle at its normal speed without stumbling or stalling.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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