There's no substitute for running the engine the way it was designed. A heat riser routes exhaust gas against the bottom of the intake during warmup to keep the richer mixture provided by the choke vaporized. Once the manifolds heat-soak from a few minutes' run time, the extra heat isn't required so the bimetal spring and the counterweight moves the flap valve and routes the gases straight down the exhaust pipe. Backpressure during warmup shouldn't be a concern, unless somebody is in the habit of running a cold engine wide open. Not smart!
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
Love your enemies and drive 'em nuts!