The heat riser is the flapper valve in the exhaust manifold, the one with the weight on it that has a habit of locking up due to rust. It won't change no matter which carb you run, but it needs to be in working condition to allow for quick warm-up and good vaporization of the fuel at all times.
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!