The air filter arrangement differs fundamentally between the 1275cc and 1500cc, and the housings, elements, and mounting hardware are entirely different between the two. The air filter is a service item that should be renewed at every major service interval, as a restricted filter reduces engine power and increases fuel consumption.
1275cc Air Filter Assemblies
The 1275cc uses a pair of individual air filter assemblies, one mounted on each HS2 carburettor intake. The front filter assembly has a different part number from the rear due to its mounting orientation, but both share the same paper filter element. Each assembly consists of a pressed steel housing, a paper element, and a mounting flange that seals to the carburettor body via a gasket. The assemblies are secured to the carburettors by bolts with plain and spring washers, and a connecting strap with a nyloc nut links the two assemblies together for mutual support.
1275cc Air Filter Casings
The air filter casings on the 1275cc were bevelled on their upper face to clear the underside of the bonnet. The taller 1275cc engine left less clearance than the previous 1098cc unit, and the bevelled casings were a necessary modification to allow the bonnet to close without fouling the filters.
This is a useful identification point when checking whether the correct filter housings are fitted, unbevelled housings from an earlier engine will not clear the bonnet.
1500 Air Filter Assembly
The 1500 uses a single large air filter assembly that serves both HS4 carburettors simultaneously. The assembly contains two paper filter elements seated within a common housing, sealed by a backplate with a gasket. Air reaches each carburettor through flexible air hoses (approximately 23.5 inches long) connecting the filter housing outlets to the carburettor intakes. The assembly is mounted to the inlet manifold area by bolts with spring washers, and a pipe clip secures the air intake ducting to the housing.
1500 Filter Elements
The 1500 filter elements are larger than the 1275cc items and are not interchangeable. The backplate seal should be inspected when changing the elements, as a deteriorated seal allows unfiltered air to enter the carburettors, which accelerates bore and ring wear.
Upgrades
Performance air filters are available in several configurations for both the 1275cc and 1500cc. Replacement high-flow filter elements that fit within the original housings provide a discreet improvement in airflow without altering the external appearance. Individual K&N pancake-style filters mount directly to the carburettor intakes, replacing the original housings entirely and providing substantially increased airflow. Stub stacks, open trumpet intakes without any filtration, are available for competition use only, as they offer maximum airflow at the expense of engine protection from dust and debris.
Fitting Notes
When fitting any performance air filter, a carburettor needle change is strongly recommended. The increased airflow will lean the fuel-air mixture unless a richer needle is fitted to compensate. Needle recommendations for common filter and exhaust combinations are detailed in the Carburettors section. For the 1275cc, the needle type depends on whether the carburettor uses a fixed needle or a biased needle .
For the 1500, all needles are of the biased type.