Your carbs may need balancing if :
You have put on new throttle cables,
You have any flat spots, or the motor does
not "sing" like a good 2-stroke when ridden hard,
General power loss, roughness or lethargy,
Poor starting
You have changed the exhaust system or
As part of the general routine service
Before attempting to balance the carbs, check the following
:
The
ignition timing is correct, use a strobe light, Plug leads and
plugs are in good condition.
The
intake tubes are not leaking and any rubber parts are not cracked,
if they are, replace them.
Air filters are clean and correctly secured
The
tappets are adjusted correctly (All V-twins and singles based
on the V twins,1.0mm inlet and exhaust)
Petrol
can flow easily through the taps, swill out the tank if needed.
Carb
internals are clean and no jets or holes are blocked
Chokes
are firmly closed
You will need the following equipment :
A couple of 8mm open ended spanners, a flat blade screwdriver
that will fit the mixture screws and a set of mercury vacuum
gauges with adaptors to fit your bike. Mercury U-tube gauges
are preferable to the dial type gauges (cheaper and will give
more accurate results).
Some WD40 or rubber lubricant spray will also come in handy
If this is your first time allow a couple of hours, as with
everything else it gets easier with practice.
What to adjust
The throttle stop screw is the larger one
with the knurled edge with the spring visible behind it. This
controls the height of the throttle slide and so the tick over
speed.
The mixture screw is the smaller one in the body of the carb,
this adjusts the idle mixture.
On top of the carb is the hex nut cable adjuster which is
covered by a rubber boot. This alters the length of the cable
and so controls the movement of the slide. If possible, try to
obtain throttle cables with in-line adjusters, these are usually
positioned a few inches away from the handlebar end and will
make balancing a lot easier, no more fumbling under the tank.
Balancing
Firstly, go for a ride. Use the motor hard, full throttle
if possible. There is no point balancing the carbs on a cold
or cool motor. On returning home, switch the manual fuel tap
to reserve and start the balancing procedure at once. Do all
adjustments outside or in a well ventilated garage or shed.
During balancing the motor may start to overheat, if it does,
stop at once and allow it to cool to normal.
1. Take off or lift seat and prop up the rear of the
tank so that you can reach the carb top adjusters with the spanners.
Don't lift it too high or the petrol pipes will be stretched
and may disconnect. Connect the mercury gauges. Mine have a strap
so they can be hung from the handlebars. There should be a rubber
plug in each manifold between the carb and the cylinder head,
remove these and attach the tubes from the mercury gauges to
the holes using the adaptors provided with the gauges. On rubber
manifolds, ensure that there is no rubber flap on the inside
of the hole when the plug is removed. Connect right hand carb
to right hand mercury column to make life easier. The carbs on
my 500 are joined by a balance pipe, this is disconnected from
both carbs and the mercury gauges are connected to the balance
pipe stubs on each carb.
2. Start the motor, try not to open the throttle when
doing so, if the motor revs too high there is a slim chance that
some of the mercury may be sucked into the motor. If this happens,
stop immediately and drain the motor oil. Drain the clean oil
again after 250 miles. If mercury is picked up by the oil pump
it will destroy the bearings. Adjust the tick over to around
2500rpm by screwing in the throttle stop screws (clockwise) a
little at a time and by the same amount on each carb.
3. Look at the mercury levels in the U tube, one side
will likely be lower than the other, if this is the case, lower
that sides carb slide by unscrewing (anti-clockwise) the throttle
stop screw, so that both levels are equal. Make any adjustments
in small steps and allow the motor and the gauges a few seconds
to even out after each change. The tickover will drop, do not
allow the motor to stall. If the tick over gets too low, raise
the slide on the other carb by screwing in its throttle stop
screw and start at 3. again. When the motor is idling
easily and the mercury columns are level go on to the next step.
4. Adjust the mixture screws one carb at a time so
that the motor runs fastest. Turn the screw one way 1/8
to 1/4 of a turn at a time and wait a few seconds to see the
effect. A point will be reached where the motor starts to "stumble".
Make a mental note of the screws position. Then turn the screw
in the opposite direction until another "stumbling"
point is reached. The optimum and fastest setting will be approximately
mid way between the two. When you have done one carb, adjust
the other side. The settings will be different on each side,
this does not matter, give each cylinder what it wants to run
quickest. Clockwise rotation will lean out the idle mixture,
anti clockwise will richen the mixture.
5. Gently blip the throttle a couple of times to settle
the carbs down.
6. Lower the tickover to a normal level (1250-1750
rpm) by unscrewing the throttle stop screws evenly and a little
at a time, check that the mercury levels are still the same.
7. Re-do step 4.
8. Re-do step 6. The motor should now be idling evenly.
9. If you have not done so already, switch the motor
off and allow it to cool down
10. So far we have adjusted the motor to run correctly
at slow speed. The proceedure so far has allowed for varying
cylinder strengths and demands and allows each cylinder to pull
as well as it can at idling speeds. Now we need to adjust for
higher engine speeds. Apart from the occasional blip, you should
not have touched the twist grip so far.
11. Slip back the rubber sheaths from the top of each
carb, a squirt of the WD40 may help, to reveal the adjusters.
Keep the handlebars pointing in the straight ahead position.
Observe the manometer, listen to the note of the engine and gently
lean on the twist grip so the engine note just changes and no
more. The mercury levels will deflect, one rising, one falling.
The cylinder whose mercury level rises indicates that the vacuum
formed behind the slide is too high and to reduce this we need
to raise the slide by screwing out the hex carb top adjuster
using the 8mm spanner.
Watch out for the free play in the cables. If it gets too
small the other cylinder's hex adjuster should be lowered. This
has the same effect as raising the first one. Switch of the motor
while adjusting the hex nuts so as not to overheat. After each
adjustment open the throttles wide, with the motor off, a couple
of times to seat the cables properly.
Ultimately you will be moving the hex adjusters by a very
small amount and getting near that point where, when the twist
grip is just leant on and the engine note changes, the mercury
columns do not move, but stay level. Slight "warbling"
is acceptable. At this point, both levels will drop equally when
the throttle is opened slightly and will rise and level off when
the throttle is closed.
12. Slip the rubber sheaths back down and check again,
some of the caps may leak air which the sheaths may plug, affecting
the manometer readings. Re-adjust if neccessary to get the levels
correct again.. Turn the steering from lock to lock with the
engine running to check the idle speed does not change due to
incorrect cable free play.
13. Go back and check 4. and 6. Stop engine.
14. Disconnect the gauges, fit back the rubber bungs
or balance pipe and refit the tank and seat. That's it. Now go
for another ride and be amazed at the difference. Acceleration
when quickly opening the throttles should now be smooth with
no stuttering. Check the tickover at the end of the ride, if
it needs adjusting go back to step 6. for one last little adjustment.
Problems
1. If you find that, while reducing the idling speed,
it is not possible to keep the columns level (ie dropping the
slide on one carb does not raise its mercury level), then check
for the following likely causes
a. The inlet system is passing too much air, killing the vacuum.
Check the manifold for air leaks, or perhaps the throttle slide
or carb itself is worn.
b. The cylinder is not drawing enough air to create the vacuum.
Check ignition, plugs and spark. If these are OK it could be
burnt valves or broken piston rings, do a compression check.
2. If you find the optimum position for the idle mixture
screw is less than 1/3 of a turn out, then you may have trouble
setting up a reliable tickover. Consider reducing the pilot jet
by one size.
American market models
These need additional adjustments carried out after the carbs
have been synchronised correctly.
Start the engine and allow to warm until normal operating
temperature is reached. Detach the balance pipe and connect the
vacuum gauge adaptor pipes to the carburettor tubes or (it depends
on the model) to the inlet manifolds by means of the balance
pipe unions. Adjust the throttle stop screws and the pilot mixture
screws until a 6% CO level is obtained at 1300rpm. This CO percentage
should be obtained with the carburettors synchronised (vacuum
readings the same for both cylinders)
Disconnect the vacuum gauge pipes and refit the balance pipe:
the CO content of the exhaust gas should decrease from 6% to
5%.
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