Until 1966, GM carburetors had a choke heat tube (not a vacuum tube) plumbed from the bi-metal spring housing on the carburetor to the heat cross-over in the intake manifold or to the exhaust manifold. In order to use that that type choke arrangement, Pontiacs required a OE intake manifold that had a choke heat tube inside the heat crossover. For example: I converted my 1963 Pontiac tri-power center carburetor to an electric coke because I foolishly blocked the heat crossover ports in the heads with aluminum in order to get a cooler fuel mixture...big mistake. I first tried gaskets with blocked heat ports but they would last about 100 miles before they were burned through. After blocking the heat ports with molten aluminum, I had to install a electric choke kit on the center carburetor because the engine would not start without a choke! With a blocked heat crossover, the damned carburetor never got up to operating temperature and the engine ran rough and idled erratic when the weather was below 40 degrees. I learn the hard way that no carburetor heat is for racing only.
The 1966 and later GM intake manifolds were equipped with a heat coil inside a choke stove that was mounted on the heat crossover and linkage from the heat coil that opens and closes the choke plate in the carburetor. It was found that the choke heat tube inside the manifold would burn out. Early Chevrolet used a heat tube from the choke housing bi-metal spring on the carburetor to the exhaust manifold.
Last edited by MouseFink; 11-22-2012 at 12:53 PM.