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Old 01-14-2013, 11:39 AM
MouseFink MouseFink is offline
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The heat riser butterfly valve on the passenger side manifold is equipped with a bi-metal spring. The bi-metal spring keeps the butterfly closed and the LH exhaust is diverted through the RH cylinder head, through the intake manifold, under the carburetor, through the LH cylinder head into the LH exhaust system in order to heat the carburetor and atomize the fuel. As the bi-metal spring heats up it relaxes and allows the exhaust pressure to open the heat riser butterfly valve. As the intake manifold it heated, a bi-metal spring in the carburetor choke control is heated and opens the choke butterfly and drops the engine idle speed. Some systems use a bi-metal spring in the intake manifold with a rod to the choke control or use a 1/4" tube from the manifold to a bi-metal spring in the choke control to open the carburetor choke butterfly. When the exhaust pipe butterfly valve is fully opened, exhaust gasses flow in the usual manner, out the exhaust system on both sides. After the engine reaches operating temperature, the carburetor is heated by exhaust gasses from the cylinder heads. The carburetor choke butterfly is fully open and the idle is stepped down to normal speed. All this equipment must be operational for the automatic choke system to work properly. The automatic choke and heat riser system replaced the manual choke and pull cable. Modern cars with a carburetor have a electric choke system and it is not effected by corrosion.

On most cars after five or ten years, the exhaust heat riser butterfly valve is corroded shut or open and the bi-metal spring is missing or non-functional. The engine is more sluggish due to the exhaust restriction. The later cars with electric choke did not have a exhaust system butter fly valve. If tubular headers are installed, a manual choke with a pull cable or a electric choke is required to operate the carburetor choke in cold weather.

We always cut the exhaust butterfly valve out with a acetylene torch. You can buy a electric choke kit for most carburators. I had one on the center carburetor of my 1963 Pontiac tri-power. I had to install a electric choke system because I was stupid and blocked the heat risers in the heads with aluminum. The Pontiac would not start without a choke when the temperature was below 40 degrees and never would run correctly because the fuel was not fully atomized.

Last edited by MouseFink; 01-14-2013 at 11:44 AM.
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