Carburetor
Synchronization
By Doug
Jenks
The purpose
of carburetor synchronization is to adjust all the carburetors so they have the
same air flow through all of the carburetors. If the carburetors are out of
synchronization, they will receive different amounts of fuel and air. This will
allow each cylinder to have different loads which can overheat the dominate
cylinder(s) causing different colors to appear on the header pipes. Carburetor
synchronization means adjusting all of the slides or butterflies to the same
height at all times.
Most motorcycles have one carburetor that has no adjustment while the others
can be adjusted. The non adjustable carburetor is used as a point where all
others are adjusted too. If you are working on a 4 cylinder engine and
carburetor number 3 does not have an adjustment, you would adjust numbers 1, 2,
and 4 to whatever reading number 3 is. Some motorcycles use three screws (most
Yamahas such as the XJ and FZ
series). The left screw adjusts cylinders 1&2 while the right screw adjusts
3&4. The center screw adjust 1&2 to 3&4. You must decide what type
of adjustment set-up you have.
The special tools that are needed are vacuum gauges, auxiliary fuel tank (I use
a plastic fuel tank from a Tecumseh lawnmower), and an adjustment screwdriver.
There are two types of vacuum gauges that most types of mechanics use. One type
uses four individual dial gauges, fig 1 while the other uses mercury in
a set of glass tubes, fig 2. Most motorcyclist will find the mercury
gauges will work good considering their low price and ease of use. Mercury type
gauges do not need to be synchronized with each other as do the dial gauges.
The disadvantage of mercury is it is very poisonous and there is the
possibility it can be sucked into the engine if care is not taken. Mercury
gauges must be kept upright at all times to stop the mercury from spilling out.
Begin
synchronizing by first adjusting the valves and making sure the engine rings,
valves, etc are sealing properly. Synchronization cannot be done if these parts
are worn or out of adjustment. Start and warm up the engine and turn it off
after it has been warmed up. Remove the plugs and install the adapters, fig
3, in the intake manifold. Put the hoses on the adapter making sure that
the hose on the left goes to the number 1 cylinder (left cylinder) and work
from left to right. Restart the engine and check the settings. On mercury
gauges they are marked in 2.0cm increments and this
is the allowable limit. If all of the cylinders are within one line of each
other the carburetors do not need to be adjusted. Even though the limit is
usually 2.0cm set them all exactly the same for peak
performance.
Caution- Yamahas that have the Yamaha Induction Control System (YICS) cannot be synchronized without a special tool to seal
the intakes tracts from each other. If this tool is not used, the carburetors
cannot be synchronized. The Yamaha part number for the tool is 90890-04068. The
XJ series of motorcycles is one type that has the YICS system.
To make
adjustments remove the fuel tank. If your bike has a vacuum operated petcock,
plug the line so it will not leak. Most constant velocity carburetors are
adjusted by a screw located outside the carburetor body fig 4. Use a
special screwdriver, fig 5, to make adjustments to the carburetors.
Rotating the screw counterclockwise opens the throttle and lowers the vacuum.
Rotating the screw clockwise closes the throttle and raises the vacuum. Adjust
all of the carburetors to the non-adjustable carburetor (if your bike has this
type of carburetor). If your bike has the other type of adjustment system,
adjust cylinders 1&2 equal using the left adjustment screw. Adjust
cylinders 3&4 equal using the right adjusting screw. Finally adjust the
center screw until all the cylinders are equal. After each adjustment, blip the
throttle slightly to seat the linkages. Do not rev the engine past 4000 RPM's.
Once all of
the carburetors are adjusted install all the parts that were removed. Your bike
should run cleaner and smoother after the carburetors have been synchronized.
See your local motorcycle dealer for
any special tools that are needed. A set of mercury type gauges can be bought
for between $41 to $72 depending on the quality. The special screwdriver can be
bought for $19.95.