The 1500cc inlet manifold is a water-heated aluminium casting, a significant departure from the unheated cast iron 1275cc manifold. Hot engine coolant circulates through passages within the manifold body, warming the incoming fuel-air mixture as it passes from the carburettors to the cylinder head ports. This manifold heating serves two important purposes: it improves cold-start running by preventing the fuel from condensing on the cold manifold walls (which would cause a lean mixture and poor running until the engine reaches temperature), and it reduces the risk of carburettor icing in cold, humid conditions.
Coolant Feed Pipe
A pipe connects the manifold water jacket to the water pump housing, with an adaptor and fibre washer sealing the connection. This water connection is part of the engine’s cooling circuit, so the cooling system must be drained before the inlet manifold can be removed. The fibre washer at the adaptor joint should be renewed at every removal, as a reused washer may not seal reliably and will cause a coolant drip that is difficult to trace once the engine bay is reassembled.
Carburettor Mounting
The carburettors mount to the 1500 manifold via screws through a heatshield and gaskets. Despite the manifold’s own water heating, a heatshield is still fitted between the exhaust and inlet manifolds to protect the carburettor float chambers from direct radiant heat. The manifold gasket is specific to the 1500 and uses dowels for accurate location, the dowels must be in place before the manifold is offered up to the head, as attempting to align the manifold without them risks damaging the gasket.