The heat riser valve is attached to the R.H. exhaust manifold outlet, sandwiched inbetween the manifold and the exhaust pipe flange. It's cast iron, and has a valve plate inside that's controlled by an external coiled bi-metallic spring. When the engine is cold, the spring holds the valve closed, which re-routes the hot exhaust gases from the R.H. manifold through the passage in the center of the intake manifold, out the left side, through a passage in the L.H. head and out through the L.H. exhaust manifold.
The hot gases going through the intake crossover passage do two things - they heat the floor of the intake plenum, which helps the fuel/air mixture vaporize during warmup until the intake manifold heats up (which improves cold driveability), and they heat the little "well" the remote choke thermostat mounts in so its bi-metallic spring can unwind, which gradually pulls off the choke to the full open position.
You can tell if it's working by pushing on the weight (cold engine so you don't burn your fingers) - it should move a full 90 degrees, and spring back when you let it go. Generally, the weight should be in the "up" position when cold, and in the "down" position with the engine warmed up and running. If it's hard to move or stuck, GM makes a "Heat Riser Lubricant" in a spray can that will usually free it up. If it's stuck closed, it kills performance due to the exhaust restriction, and will pass hot gases through the crossover passage all the time, which will heat the intake constantly, causing fuel to percolate in the float bowls, especially after shutdown.
If you don't drive in cold weather, just wire it open; it may take a little longer for the choke to pull off, but it will also make the R.H. side of the exhaust system last longer. The right side always rusts out first because it doesn't get as hot as the left side, because of the heat riser valve (especially if it doesn't open all the way). If yours is shot, replacements are about $35.00.
