CARBURETTOR JETS

Our carburettor is really two carbs in one. At high speeds the airflow through the carburettor’s venturi (choke) is sufficient to draw fuel through the main and compensating jets.  The fuel is entrained in the airflow and finally atomised—which is what the engine requires—but at low engine speeds the airflow is insufficient to either entrain or atomise any petrol.  Accordingly, an extra supply of fuel/air is introduced via an orifice just downstream of the nearly-closed butterfly.  This orifice is subject to the vacuum of the inlet manifold which has the effect of both pulling in the fuel/air mixture and atomising the fuel.  Just upstream of the nearly-closed butterfly is another orifice called the progression jet.  At idle speeds, air is drawn into the progression jet from the main airflow and helps atomise the fuel leaving the idle orifice.  But as the butterfly is opened further, the progression jet is also subject to the inlet manifold vacuum and delivers extra air/fuel.  As the butterfly opens still further, the depression in the manifold fills and both the progression and idle orifices provide less fuel with the main system coming into operation.

If there is any blockage in the channels supplying the idle and progression orifices or in the orifices themselves then the slow-speed running is obviously compromised.  Both jets, which sit just below their orifice mouths, can be gently rodded using a very fine copper wire such as a single strand of electrical wire.  The progression jet is particularly liable to blockage.  This produces a flat spot when accelerating from idle to moderate speed.  The progression jet can also be accessed by removing the screw in the carburettor body which is often assumed to be merely a channel-blanking plug.  Make sure this screw is well-sealed or air will leak into the carb at a very critical point.

MIKE SHARP

 (With many thanks to Mike Sharp for this article from the PWA7C magazine)