It is closed somewhat on startup (cold) to send hot exhaust up, under the carb, and out the other exhaust to warm up the carb and choke. It has a heat coil on it, and as it heats up, the coil opens the valve.
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
The heat riser valve is there to redirect some of the exhaust up thru the intake manifold to heat it quicker, so the engine will run better when cold. The valve is either controled by a thermal spring, as on your '74, or by a vacuum valve, as on the later cars. When cold, the valve shuts and exhaust gas runs thru small ports in the heads, up thru the intake, warming it. Once the spring warms to a specific temp, it starts opening the valve back up.
Your old valve must have been stuck open, if the engine is now running a bit warmer. However...once the valve opens, it shouldn't cause any increase in engine temp than before. If the engine continues to run hotter than before, you need to check and make sure the valve is opening up, and staying open when hot.
____________________________________ Joel Adams C3VR Lifetime Member #56 My Link
Vette(s): 1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.
While the heat riser does cause the engine to warm up faster, that was primarily to reduce emissions. You can actually wire the heat riser open and the car will run perfectly fine without the added exhaust restriction.
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