Joe,
"The heat riser only job is to warn up the carb faster then the engine can warm it up. Once the exhaust manifold gets hot and the spring relaxes, the flapper valve diverts heat away from the carb and out the pipe. The manifold will be just as hot with a non working heat riser as a manifold with a working heat riser. The only difference will be the amount of time it takes to heat the carb."
Given your statement above, specifically "warm up the carb", how can that occur with the insulator in place? My view of how it works is the diversion of heat from the heat riser is to quickly warm up the the intake manifold. The area of the intake manifold that needs heat the most to prevent loss of atomization is directly above heat riser valve, as this is where the flow of atomized fuel strikes before it moves 90 degrees left and right through the runners toward the intake valves. The rest of your post is spot on.
