Specifications

Appendix, item 5). The casting has a port opening into the float bowl. Clear it with carb
cleaner and compressed air. To clean the fuel pipe, cover the enricher nozzle in front of
the throttle plate tightly with a rag, insert the tube from a can of carb cleaner into the
enricher valve chamber, sealing with a rag, and spray until cleaner emerges from the
pipe. Spray carb cleaner into the intake nozzle until it flows from the enricher chamber.
Follow with compressed air.
Synchronizing
The throttles of the two carburetors need be adjusted so that they open together. As in
all carb adjustment, make sure that cam chain adjustment, valve lash settings and
ignition timing are correct first. Procedures differ for early and late carburetors.
Early carbs (70-75) have separate throttle cables and separate throttle stop adjustors on
the carb cable perches. They also lack vacuum barbs on the mounting boots and
vacuum ports in the carb bodies. Vacuum barbs can be ordered from 650 Central and
installed in the boots. From the TX650A (74) up, boots for 78-79 models with vacuum
barbs can be installed. Barbs are covered with blind plugs during normal operation or
attached to vacuum petcocks on machines so equipped.
If synch. is badly out of adjustment due to full disassembly of the carbs, random
tweaking, etc., remove the slides and sight through the venturi at a light source,
adjusting the throttle plates for equal openings; this should be close enough to start the
machine and proceed with adjustment. On early machines the throttle stop ("idle")
screws (see Appendix, item 12) are used to adjust the throttle plates. Later carburetors
use a synchronizing screw, located between the carburetors (see Appendix, item 12).
Turning the screw clockwise opens the throttle plate on the right hand carburetor;
turning counterclockwise closes it.
Early machines that lack vacuum barbs and vacuum ports in the carb bodies can be
synched by the "dead cylinder" method. Idle on both cylinders is stepped up until high
enough for the engine to run on one cylinder at a time. A plug cap is removed, and the
throttle stop on the running cylinder is backed off just to the point that the engine dies;
then the same procedure is applied to the other cylinder. Moving both screws the same
amount, idle is then set between 1,000 and 1,200 rpm. For a number of reasons this
method is prone to error, and better results might be achieved by using a tuning
tachometer to equalize lowest stable rpm between the two cylinders. (Note: if an
electronic ignition is fitted, warm the engine and shut down before removing a plug cap,
fit a spare plug in the cap and ground it solidly on the engine, and restart on one
cylinder.)
On 76 and 77 models without vacuum barb boots, vacuum fittings are attached in ports
on the outside of each carburetor directly above the throttle shaft (see Appendix, item
9). These are present on 78 and 79 models as well. The ports are plugged by brass
screws with slotted heads. Remove the plugs and insert correct threaded vacuum
fittings; on the left side carb the fitting must sometimes be ground down to clear the