Specifications

Post 5: Cleaning and Adjustment, Pilot Circuit
1. Cleaning
The most important thing to know about carb cleaning is how to avoid it in future once
you have it done. Sintered or paper in-line fuel filters will keep debris out of the
carburetors. If the bike will not be run for awhile, draining the carbs will prevent varnish
from accumulating. For extended storage, drain the carbs, lift the slides, stick a few
silica gel cylinders from an aspirin container under each slide and in the air intake
throat, and seal, to prevent condensation and corrosion.
The pilot circuit fuels the engine in low-speed operation, when the slide is lowered.
On the back of the BS38 (70-79) carburetor, you will see two small holes in the sides of
the intake bell (see Appendix, item 6). One is marked with a cast "M" on most
carburetors This is the main circuit air jet, which feeds air to the needle jet of the main
circuit. The unmarked hole is the pilot circuit air jet, which feeds air to the pilot circuit. In
BS 38 carburetors, both of these jets are fixed; nothing is removable.
In the back of the BS34 (80-83) you will find four round holes (see Appendix, item 6).
The upper pair (below curved diaphragm chamber vent) vent the carb body above the
floats. Below those are the air jets. The pilot air jet is removable, and the main air jet is
fixed.
The BS38 pilot jet seats inside the float bowl. It is the small brass fitting with a slotted
head. Above the pilot jet in the BS38 there is a recessed area in the float bowl casting,
and above that, a small hole in the carburetor body. This is where the pilot air passage
emerges after running through a tubular casting. On the other side of center, the main
air jet feeds into a similar hole and casting. In BS34 carbs the pilot jet is under a rubber
plug at the end of the smaller vertical tubular casting in the carburetor body; the plug
must seal correctly.
Use a screwdriver or bit that completely fills the slot of the jet (see Section I). If the jet
area is fairly clean and uncorroded, one sharp push-and-twist should free the jet. If jets
are fuel varnished, prep them for removal with solvent. The BS38 float bowls can be
immersed in Chem-Dip or similar solvent. In the BS34, spray-in solvent can be applied
to the jet area and followed by compressed air until varnish breaks up--never immerse
carb bodies. Apply a bit of penetrating oil and allow it to soak in. Then seat the
workpiece firmly in a padded vise or clamp, and remove the jet. Use a sharp push-and
twist motion to break the jet loose. Light taps on the screwdriver will often break a
stubborn jet free. If the slot deforms, use a screwdriver with a thicker blade or regrind
the one you're using.
Once the jets are removed, check to see that the correct series is in place. From 1970-
1975 (XS1 through XS650B), the BS30/96 pilot jet was used. In this jet, the tightest
restriction (metering orifice) is located toward the barbed end of the jet, opposite the