The crankcase breather system routes combustion gases that pass the piston rings (blowby) back into the inlet tract to be burned, rather than venting them to atmosphere. The system evolved significantly during Midget production, driven by progressively stricter emissions requirements, and the components differ between each engine variant.
Early Breather System
The earliest 1275cc configuration used non-ventilated carburettors with no breather hose union on the carburettor body, and the crankcase gases were vented through the oil filler cap and a road draught tube. An emission control valve fitted to the inlet manifold (via a tapped boss casting) regulated the flow in this system.
Cars with this arrangement can be identified by the carburettor tag, the early non-ventilated type, and by the absence of breather hose connections on the carburettor bodies.
Later Breather System
From the introduction of ventilated carburettors, the system was revised to route breather gases from a separator canister mounted on the timing cover, through a Y-piece, and via individual hoses to each carburettor body, where the gases are drawn into the inlet air stream by manifold vacuum. The emission control valve on the inlet manifold was retained with an adaptor and connecting hoses. This system uses a specific set of hoses, clips, and connectors that must be matched to the engine variant.
Hose Sizing and Ordering
The separator-to-Y-piece hose, the Y-piece itself, the Y-piece-to-carburettor hoses, and the hose clips are all individual components. The hose sizes and routing changed with each carburettor specification, so the carburettor tag number stamped on the float chamber lid screw should be quoted when ordering breather system parts to ensure the correct hose dimensions are supplied.
1500 Sealed Breather System
The 1500 uses a sealed breather system that is fundamentally different from the 1275cc arrangement. A T-piece connects a hose from the rocker cover to hoses running to adaptor fittings on each carburettor body. The system relies on manifold vacuum to draw gases from the rocker cover and crankcase into the carburettor air stream.
1500 Sealed System Notes
This sealed system requires the oil filler cap to be of the non-vented type to maintain the closed circuit. If a vented aftermarket filler cap or chrome rocker cover cap is fitted without the appropriate blanking screw to seal the vent hole, the breather system will not function correctly. The symptoms of a broken breather circuit include oil mist in the engine bay, the smell of burnt oil, and potentially high crankcase pressure that can push oil past seals and gaskets. This is one of the most commonly overlooked details when fitting aftermarket rocker covers to the 1500.