The inlet manifold feeds the fuel-air mixture from the carburettors to the cylinder head. The 1275cc and 1500cc use completely different manifolds, cast iron and unheated on the 1275cc, water-heated alloy on the 1500, and even within the 1275cc range there are two variants that affect breather system compatibility.
1275cc Inlet Manifold Variants
Two 1275cc inlet manifold variants were used. The earlier type has a tapped boss for the breather control valve adaptor, used in conjunction with the early non-ventilated carburettors and their separate emission control valve.
The later type has a blanking screw in place of the boss, used with the ventilated carburettors that take their breather feed directly from the carburettor body. Both manifolds are sealed to the cylinder head by a common gasket with locating rings.
1275cc Manifold Mounting
On the 1275cc A-Series, the inlet and exhaust manifolds bolt together at the cylinder head face and share a common manifold gasket, replacing one without the other risks a sealing issue, so a complete manifold gasket renewal is recommended whenever either manifold is removed. The carburettors mount to the inlet manifold via studs and brass nuts, with spacer blocks and gaskets between the carburettor bodies and the manifold face. These spacer blocks provide additional heat insulation, reducing fuel vaporisation in the carburettor. The manifold studs for the carburettor mounting and the head-to-manifold studs are different lengths and must be fitted in the correct positions.
1275cc Heatshield
A heatshield is fitted between the exhaust and inlet manifolds to reduce heat transfer to the fuel-air mixture, which would otherwise cause vapour lock and poor running in hot conditions, particularly noticeable on slow-moving cars in summer traffic. The heatshield is supported by brackets at the front and rear, secured by screws to the manifold studs. A stainless steel replacement heatshield is available as an upgrade, offering better durability than the original mild steel item and a smarter appearance. The brackets and fixings are stocked separately for restoration where the originals are corroded or missing.
1500 Heated Inlet Manifold
The 1500 uses a water-heated aluminium inlet manifold, a significant departure from the unheated cast iron 1275cc item. Hot coolant circulates through passages in the manifold to warm the fuel-air mixture, improving cold-start performance, reducing carburettor icing in cold or damp conditions, and helping fuel vaporisation across a wider operating range. A pipe connects the manifold water jacket to the water pump housing, with an adaptor and fibre washer sealing the connection. Coolant hoses link the manifold to the cooling system circuit.
1500 Manifold Mounting and Heatshield
The 1500 carburettors mount to the manifold via screws through a heatshield and gaskets. A stainless-steel heatshield is available as an alternative to the standard item. The manifold gasket is specific to the 1500 and uses dowels for accurate location during fitting. As with the 1275cc, attention to gasket condition and correct screw torque is essential to prevent induction air leaks, which cause weak running and unstable idle.