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.
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Joel Adams
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