Specifications

Post 6: Main Circuit, Enricher (Choke), and Synchronizing
Main Circuit: Cleaning
The main circuit has three components. The jet needle is secured in the slide by a clip
and a plastic retainer held down by the slide spring (70-77), a snap ring (78-79), or a
screwed down plate (BS34, 80-83). It hangs into the needle jet; as the needle rises with
the slide, more fuel is drawn up through the needle jet. The main jet meters fuel flow
into the needle jet. In the BS38, the main jet threads into the float bowl. Remove it by
removing the float bowl plug and inserting a tight-fitting screwdriver or screwdriver bit. If
it is badly varnished, apply solvent to free it. In the BS34, the main jet threads into the
base of the needle jet. Remove, soak in solvent, and blow out with compressed air.
The BS38 needle jet is the large perforated brass tube that hangs down from the middle
of the carburetor body. It is secured by an O-ring. Using solvent to free it if necessary,
pull it free of the carb, soak, and follow with compressed air. Avoid marring or deforming
the needle jet with tools. Replace using a new O-ring. The BS34 needle jet is enclosed
by the carb body and is installed and removed through the venturi. To remove it, install
a main jet and push the needle jet up; if necessary, tap it gently with a plastic drift until
the needle jet comes free, using solvent and penetrating oil as needed. In reassembly,
the BS34 needle jet is aligned by a pin in the carb body which matches a groove in the
jet. Be sure they are lined up correctly before trying to seat the jet.
Needles are cleaned by wiping with solvent. Do not sand or remove material.
Inspect the needle jets and needles for wear or corrosion damage. If worn or corroded,
replace them.
Enrichers (Chokes): Cleaning
The BS-series carburetor uses enricher valves to richen the fuel mixture for starting. In
the BS38, the valve is mounted vertically and is opened by a fork on a rotating shaft
which lifts the enricher plunger from its seat, admitting extra fuel through one of the
large nozzles in front of the throttle plate (see pilot section, above). The enricher
chamber is covered by a removable plate, secured by three screws. In the BS34 the
plunger is mounted horizontally in a fixed casting and the fork is mounted on a push-pull
rod. In both, plungers are secured by a nut under a seal. With the plunger lifted, back
the nut off. The plunger can then be compressed against its spring, giving enough play
to slip the plunger top away from the fork and remove it. Some models have viton tips at
the foot of the plunger. Be sure it is intact; if it shows nicks or cracks, replace the
plunger.
In both carburetors, fuel is fed to the enricher through a brass pipe in the forward edge
of the carb body, which hangs down into a tubular casting in the float bowl (see