The 1500cc breather system is fundamentally different from all 1275cc configurations and uses a sealed closed-circuit design. A T-piece fitting connects a hose from the rocker cover oil filler area to individual hoses running to adaptor fittings on each HS4 carburettor body. The system relies on manifold vacuum to draw blowby gases from the crankcase, through the rocker cover, and into the carburettor air stream where they are burned during normal combustion.
Sealed System Function
The sealed nature of this system is critical to its function. The oil filler cap must be a non-vented type to maintain the closed circuit, if a vented cap is fitted (or if an aftermarket chrome or alloy rocker cover cap has an open vent hole), the system loses its vacuum and ceases to function. The result is crankcase pressure build-up, oil mist expelled from the open vent into the engine bay, increased oil consumption, and potential for oil to be pushed past gaskets and seals. Blanking screw kits are available to seal the vent hole when fitting an aftermarket filler cap or rocker cover.
1500cc Breather System Fitting
The T-piece, adaptor fittings, and hoses are specific to the 1500. The hoses should be inspected for cracking or deterioration at each service, as a split breather hose allows unfiltered air to enter the carburettors (bypassing the air filter) and disrupts the crankcase ventilation, potentially causing rough running and increased emissions.