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The Gentleman's Express: Tech-Tips from the JIOC

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Carburetor Heat Shield

Author: Steve Brenneman

A common problem with Jensens in particular and American V-8’s in general is that of percolation in the fuel bowls of the carburetor after shutting off the engine. This problem manifests itself by making your car hard to start after relatively short (5-20 minutes) off periods. Because of engine heat coming up to the carburetor from the intake manifold (which is heated by exhaust gases on the Chrysler 383/ 440’s when the engine is running) the fuel in the carburetor bowls often boils over and down into the intake manifold passage. This excess fuel tends to flood the engine, which is then best started by flooring the accelerator once - and holding it there until it catches - and cranking the engine to clear the excess gasoline.

When an engine starts, that has been flooded, there usually is a significant cloud of black smoke from the exhausts. What to do? Tell your friendly Chevrolet dealer that you need a GM part #3969835 (carburetor heat shield for 427-454 Corvettes). This is a large flat aluminum plate with proper mounting holes and carburetor baseplate holes to fit your Chrysler manifold and fourbarrel Holley or Carter carburetor.

If you run a Holley, install the plate as is; the gaskets installed on the plate are intended for a Holley four-barrel.

If you are using a Carter AFB, AVS or TO discard the GM gaskets and obtain two new gaskets from your local autoparts store.

I recommend a thin gasket (app. 1/32") between the intake manifold and the heat shield, with a thicker (app. 3/16-1/4") gasket between the heat shield and the carb. In any case you may have to perform some minor trimming on the GM shield to clear obstructions such as ignition coil, vacuum lines, etc. I’ve been using a Corvette shield on a V-8 Ford for five years with good results, and have just installed one on my ’71 Jensen. The Jensen was prone to percolate on hot days.

Note: Only a Corvette wizard will stock this heat shield, so better plan on ordering it from your parts man. Cost is about $15.00. If for some reason you have a spread-bore carburetor - all four barrels not the same size - there is a different shield number required. Check with your parts man.

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