Two 1275cc inlet manifold variants were produced, and the difference between them affects the crankcase breather system compatibility. The earlier manifold (part number 12G583) has a tapped boss on one of its runners for the breather control valve adaptor, this boss provides the mounting point for the emission control valve assembly that regulates crankcase gas flow into the inlet tract.
The later manifold (12G1450) has a blanking screw in place of this boss, as the later breather system routes gases through the carburettor bodies directly rather than through a manifold-mounted valve.
1275cc Inlet Manifold Specification
Both manifolds are sealed to the cylinder head by a common gasket with locating rings that ensure correct positioning. The gasket also seals the exhaust manifold to the head on the opposite face. A heatshield is fitted between the exhaust and inlet manifolds to reduce radiant heat transfer to the fuel-air mixture, without the heatshield, the inlet manifold and carburettor float chambers absorb exhaust heat, which can cause vapour lock (fuel vaporising before reaching the combustion chamber) and poor running in hot conditions, particularly in traffic or after a hot restart.
Carburettor Mounting
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. The spacer blocks provide additional thermal insulation. The manifold studs for the carburettor mounting and the head-to-manifold studs are different lengths, care must be taken during reassembly to ensure each stud is returned to its correct position.