Heat riser

The heat riser's job cold,, was to direct hot exhaust gas up to the carb base,, to help prevent a conditions called "carb icing", where the throat of the carb would get covered in ice (snow) to the point it would plug the idle circuits.. causing stalling , (usually at the second and third stop, after a cold morning start)... this would persist until there was enuff heat around the carb to melt the snow..(this happened in hi humidity mornings)

When hot, a thermal spring or vacuum pod would open the valve to direct exhaust straight out the pipe..