Specifications

Behind the throttle plate are more small holes, 2 in BS38's, a cluster of 3 in BS34's (see
Appendix, item 11). In early BS38 carbs these are fed by the bypass pipe, a brass tube
between the slide and the throttle plate in the middle of the bore. Find its source in the
carburetor body and shoot with carb cleaner and air until clear.
In later BS38 carbs and all BS34's, the bypass nozzles are fed from the fuel screw
passage, and no tube is present. Seal the tube on a can of carb cleaner in the fuel
screw hole with a rag or drilled plug, cover the small nozzle in front of the throttle plate
with another rag and hold it in place with your thumb, hold the throttle plate open, and
spray until cleaner emerges from the nozzles. Follow up with compressed air.
In badly corroded carbs it may be necessary to probe passages with a wire to clear
them. This should be resorted to only as a last-ditch measure and avoided if at all
possible; it's easy to block, enlarge or deform the passages, and if this occurs, grief will
follow and require a new carb body for correction.
2. Adjustment
Before attempting to adjust fuel screw settings, ensure that you are not compensating
for incorrect settings elsewhere. Inspect and if necessary adjust cam chain tension,
valve lash, and ignition timing, in that order.
Turning the fuel screw clockwise (in) leans the mixture, turning counterclockwise (out)
richens. Adjustment of fuel screws is best performed using the "dead cylinder" method.
Turn the fuel screws in until they bottom lightly (overtightening will damage the screw
and/or seat), and turn them out to one of the following settings.
XS1, XS1B: 1 turn
XS2, TX650, TX650A: 0.75 turns, XS650B: 0.75 turns
XS650 C, D: 1.5 turns
XS650E, F: 2.25 turns
BS34, all: 3 turns
Warm the engine to operating temperature, then raise the idle to around 1500 rpm. On
bikes with breaker point ignitions, pull off a sparkplug cap. Electronic ignitions can be
damaged by operation with an ungrounded ignition wire. To prevent that, shut down the
engine, attach a spare sparkplug to a cap, ground the plug solidly on the engine, and
restart. Adjust the throttle stop to hold lowest steady idle, then move the fuel screw 1/4
turn each way, seeking highest idle. When you find the direction of improvement, set the
screw 1/8 turn in that direction from your starting point and again move it 1/4 turn each
way. As idle rises, lower it with the throttle stop, as changes are easiest to detect at low
engine speeds. Set the screw at the inmost (leanest) position that yields highest idle
speed. (And yes, I know; some owners advocate finding the point where idle drops due
to leanness, then the point where it drops due to richness, and setting the screw at the
center point. I do not concur.) Repeat for the other cylinder.