Manifold Heat Control Valve
The manifold heat control valve assembly, (heat riser) is often overlooked as the source of low speed engine performance issues, which is a serious mistake. This simple device is essential for smooth engine operation, even in warm summer weather. The reason is simple physics; converting a liquid (gasoline) into a vapor (air/fuel mixture) requires the application of heat. Start your car on a cool day and watch the base of the carburetor; it will become frost-covered for a short period of time as the engine warms to operating temperature. The ambient temperature of air entering the engine will drop as latent heat within that air is consumed in the process of vaporizing droplets of liquid gasoline into the combustible air/fuel mixture. The manifold heat valve forces hot exhaust gases up around the base of the carburetor and the intake manifold plenum, (the central area directly under the carburetor) to warm the mixture. If additional heat is not supplied, the gasoline can easily condense back into a liquid as the air/fuel mixture comes in contact with the inner surfaces of the intake manifold. This “leans” the mixture causing rough idle, hesitation on acceleration and stalling. As the engine warms to operating temperature, less heat is required and the manifold heat valve swings open, allowing the exhaust gases to flow directly out into the exhaust pipe.
Starting a cold engine requires a very rich mixture for the very same reason. The manifold passages are so cold, the only way to get enough air/fuel mixture to each cylinder is by use of the carburetor choke. The majority of gasoline drawn into a cold engine condenses on the walls of the manifold and never reaches the cylinders to fuel the engine.
The automatic choke and the manifold heat control both rely on the same control mechanism, a bi-metallic spring. This type of spring is made of 2 metals with different rates of expansion, which are sandwiched together into a flat strip and then coiled. The differing rates of expansion cause the spring to curl in response to its temperature being raised or lowered. In both of our automotive applications, the bi-metallic spring closes its respective valve when cold. Once the engine is running and heat is being generated, these springs begin to relax their tension causing the choke and heat control valves swing open. These valves are a team!
The choke spring is calibrated to relax in proportion to the level of heat being supplied to warm the manifold plenum area. The choke spring tension is adjustable by rotating the Bakelite cover it is attached to. The cover is marked with notches and arrows indicating “rich” and “lean” settings. Increasing the tension richens the mixture by keeping the choke closed for a longer period of time.
It is essential that the heat tube, running from the exhaust manifold “choke stove” to the choke spring housing on the top of the carburetor, is securely attached and not plugged. An internal vacuum passage in the carburetor draws air past the sheet metal plate attached to the side of the exhaust manifold, where the air is heated, and up the tube to warm the choke bi-metallic spring, which causes it to relax its tension.
The heat control valve spring is attached directly to the side of the exhaust manifold, so it is heated by the flow of hot gases exhausting from the engine. In many Pontiac applications, this spring is also adjustable, your model year service or owner’s manual will advise you of the specific details.
While airflow into the carburetor will open the choke, the heat control valve relies on a heavy counterweight. This weight is attached to the valve shaft opposite the bi-metallic spring. There may also be a small extension spring at the counterweight end; this is an anti-rattle spring. Once the engine reaches operating temperature and the tension controlling the heat valve is fully released, it is possible for the flow of exhaust to cause the heat valve to rattle within the manifold, hence the anti-rattle spring.
Beyond setting the heat control spring to the proper seasonal adjustment, if any; you need to be sure that it moves freely. Since the exhaust manifold is subject to extreme heat, lubricating the heat control valve shaft is problematic. The factory recommended mixing powdered graphite with alcohol; this mixture flows easily into the shaft / bushing area, the alcohol evaporates but the graphite remains. “Heat-riser” valve lubricants are still available, and they are liquid/graphite solutions.
The heat control valve is most likely to be stuck in the closed, (full heat) position, since that is its normal position on a cold engine. If it is stuck closed, the excessive heat causes abnormally high operating temperature, lack of engine power, spark knock, fuel percolation in the carburetor and vapor lock.
If the heat control remains open at all times, the lack of heat causes problems during the warm-up period, as I have already mentioned.
The parts illustration on the next page appears to be a 1939 eight cylinder engine; but is representative of what you should see on any 1935-’39 eight. The six is very similar, and of course, the group numbers are valid for any Pontiac parts catalog.
The “cover” shown just below the manifold gaskets faces the cylinder head, so unless you remove the manifolds, it is not accessible. The small tube with the compression fitting is the automatic choke heat tube. You should be able to blow air down this tube without obstruction.