The carburettor heatshield sits between the exhaust manifold side of the engine and the carburettors, reducing the transfer of radiant exhaust heat to the fuel system. On the MGB, heat soak to the carburettors is a well-known cause of poor hot starting, rough idle after a heat soak period, and fuel percolation, where fuel boils in the float chambers and overflows from the carburettor vents. These issues are most pronounced in traffic, on warm days, or immediately after the engine has been switched off and the under-bonnet temperature rises.
The original factory heatshields were a functional pressed steel item, and several types were used across production depending on the carburettor fitted. Cars with the larger 1¾-inch HS6 carburettor conversion require a different heatshield from the standard HS4 or HIF4 types, owing to the wider carburettor body and different mounting geometry. Polished stainless steel heatshields are available as an upgrade for cars fitted with the HS6 carburettor conversion. Beyond the obvious cosmetic improvement, adding a bright, mirror-finish element to the engine bay, stainless steel offers excellent durability and will not corrode or deteriorate in the way that the original mild steel shields can over time.
They are a direct replacement, fitting in the same position using the existing mounting points. A missing, damaged, or poorly fitted heatshield should always be replaced rather than left off. The proximity of the exhaust manifold to the carburettors on the B-series engine means that even a modest increase in radiant heat transfer can affect fuel temperature enough to cause running problems, particularly on engines that have been modified with a performance exhaust manifold, which may run hotter than the original cast iron item.