Sudco Vol 38

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

Single Carbs

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.

Synchronizing Mikuni

Smoothbore Carburetors

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.

Mechanically Synchronizing

Smoothbore Carburetor

Remove the carburetor from the engine and drain loat

bowl.

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

carefully.

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.

Carburetor Synchronization

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