Motorcycle carburetor synchronization is best described as a process
where it becomes necessary to keep tow, three, or four, engines running
at exactly the same speed, where each cylinder on the motorcycle is a
separate engine. In order to match the speed of each cylinder to the
next, the slide lift must be exactly the same on each carburetor. Given
an engine with perfect cylinder compression, ignition timing, and
mixture quality, all cylinders will run at exactly the same speed when
the carb are in exact mechanical synchronization.
Synchronizing Mikuni VM, TM, and TMX
There are two sync procedures to execute when setting up
single type carbs on multis. The irst is idle sync and the second
is cable sync, to be done in that order.
The easiest way to start the idle sync process is to choose an
arbitrary slide lift setting. Set each one at exactly the same lift with
the throttle stop screw (idle speed screw). Generally a 1mm (.040”)
under each slide to match the slide lifts at idle. Check the air
screw/fuel screw settings and match as necessary at this time.
Mount the carburetors and attach the throttle cables. Start the
engine and adjust the idle speed screws in or out, counting each 1/2
turn and settling each carburetor the same amount, bearing in mind that
each slide needs to be kept at equal lift. The engine will idle smoothly
when all the slides are adjusted properly. Turn the engine off.
Now that the idle sync has been established, it is possible to
sync the cables that lift each slide. Using the cable adjuster mounted
on each carburetor top, take out all the cable free play until the slide
starts to lift off idle. Back this adjustment off until the slide just
returns to idle. Do this for each carburetor. The cables are now
roughly synchronized. Put s inger on the slide of the #1 carburetor
and watch the slide on the #2 carburetor. Slowly twist the throttle to
lift the slides, both slides should lift off idle at the same instant. If the
slides are out of sync, adjust the retarded carburetor with the cable
adjuster on the carb top to match the other carb. repeat this process,
matching the #3 carburetor to the #2 carburetor and so on for all of
the cylinders. Check the cable free play at the twist throttle, there
should be about 1mm throttle free play. Double check and tighten all
lock nut and screw as necessary.
Sudco recommends that all smoothbore type carburetors be
synchronized mechanically rather than with vacuum gauge or
mercury manometer (“carb stick”).There are several reasons that
vacuum synchronization is not recommended.
The sync adjustments on smoothbore carburetors are
underneath the top cover of the carburetor, which when removed
exposes a large vacuum leak to the cylinder being adjusted, resulting
in inconsistent vacuum levels from one carb to the next.
The vacuum shown on a gauge is a result of several engine
conditions: cylinder compression, spark quality and timing, mixture
quality, and carburetor slide lift. If any one of these parameters is not
perfect it will become increasingly dificult to get a consistent
vacuum level, regardless of slide lift.
Carburetor circuitry is throttle position sensitive. If the slides
are not in exact mechanical sync, then one carburetor may be
operating on a different circuit than the others.
Remove the carburetor from the engine and drain loat
Remove the top cover from each carburetor and note which
carburetor doe snot have a sync adjuster. Some designs will have
adjusters on all the carbs, some will have a ixed or non-adjustable
carb. Usually #2 or the one closest to the throttle wheel.
With the carburetors on the work bench and the spigot or
engine side of the carbs facing you, adjust the idle speed screw so
that the slides are nearly closed.
Place a piece of ine wire or rod about 0.20” (.5mm) thick
underneath the ixed adjustment carburetor slide. Use the idle speed
screw to adjust the tension on the wire gauge, which should slip
between the slide and carburetor bore with some slight drag. If your
particular carburetor design does not have a ixed adjuster, then use
the carburetor closest to the throttle cable wheel and set the idle
adjuster as described above. This is now the baseline carburetor.
Using the same piece of wire or exact duplicates, check the
slide lift of all the other carburetors against the wire gauge. If the
drag on the wire is not the same as the baseline carburetor,
adjust the slide lift of each carburetor with the sync adjuster located
at the throttle shaft/lift arm joint, so that the drag on the wire is the
same as the baseline carburetor.
Sometimes, the sync adjusters are adjust nuts and lock screws,
where the slide lift adjustment is made by turning the nut and then
is locked in place with the screw in the center. Double check the
adjust method used on your particular carburetor and proceed
Once all the slides have been synchronized, open and close
the throttles several times, checking for binding or sticking. Double
check the synchronization at idle by sight. Turn the idle speed
adjustment put and watch the clearance between the slides and their
carb bores. As the clearance nears zero, any difference in slide lift
will become apparent. correct the differences as necessary.
. Replace the carburetor tops, check the gaskets for leaks.
Double check the fuel screw / air screw setting at this time, re-set
to the standard speciication. Install the carburetor and adjust
the throttle cables for correct free play at the throttle 0.20” to 0.40”
(0.5 to 1.0mm)
Now that the carburetor have been properly synchronized,
it is possible to use a vacuum gauge or carb stick to optimize the
fuel screw/ air screw settings. With the engine idling, experiment with
the mixture screw while looking for an increase in manifold vacuum.
The screw setting which produces the highest vacuum level is
generally the correct position.