The Carbtune II is a mechanical manometer.
It gives four simultaneous readings so you can
balance the throttle valves on carburettor or fuel injection bodies. It
uses stainless steel rods in close fitting guides to indicate vacuum on
The indicator tubes are made from high impact
plastic. Damping is by a small airflow restricter in the rubber
The manometer must be used vertically for a scale
starting at 8cmHg (centimetres of mercury. 1 inch=2.5cm 1cm=0.4inches)
and going to 42cmHg. Most bikes have readings higher than 8cmHg, but
some bikes such as R-series BMWs and two strokes can have readings
lower than this.
To get a reading below 8cmHg remove the plastic
cable clip from the top cap and insert it in the bottom cap. Hang the
Carbtune II upside-down. Ignore the scale figures. Zero will now be
around 17cmHg on the scale but the full width red graduations will
still be 2cmHg.
Absolute values are not important. Comparisons are
skip to setup the gauges
skip to use the guages
skip to maintenance of the gauges
skip to problems
skip to guarantee
In these instructions the four column model
The 2-column model has half the contents of
The term “carb” refers to both fuel injection bodies and carburettors.
The Carbtune, rubber tubes, plastic cable clip to
hang gauges from the handlebars, inside the small plastic bag there are
4 x 5mm adapters, 4 x 6mm adapters and one fine bore thick wall clear
plastic tube about 8cm (3”) long by 5mm (0.2”) diameter to make the
The ultra-high temperature 30% glass filled
engineering Nylon 66 adapters have several advantages over brass
adapters. They’re easier to use. They screw in and out easier and don't
need as much tightening. They won't damage the inlets if over
tightened. They do not transmit the heat as much so it's kinder to the
rubber hoses and you don't burn your fingers as much removing them.
There is a slot on the spigot so they can be removed with a small
screwdriver. Do not tighten with a screwdriver.
Damping. The restricters
must always be used.
Without restricters the rods will fluctuate
wildly. There are two components to the damping.
1. The small amount of friction between the rods
and guides which is overcome as soon as the pulsating vacuum of the
engine is connected. This friction can vary from tube to tube in the
Carbtune and is not a fault.
2. The air-flow restricters that fit inside the
rubber tubes and damp the fluctuations. There will always be a small
amount of fluctuation and this is necessary for the gauges to work
properly. Do not oil the rods.
How to make the dampers
Refer to drawing opposite. In the bag with the
adapters you will have received a fine bore thick wall clear plastic
tube (No 1). It is about 8cm long x 5mm diameter. With a craft knife
cut the thick wall tube into four roughly equal pieces. These are the
four restricters (No 2). (For the 2-column the tube is about 4cm long
and is cut into 2 pieces.)
Cut about 10cm (4”) off the end of each 1 metre
black rubber hose. Push a restricter into the 10cm rubber hose. Now
push the remaining 90cm (3ft) rubber hose onto the other side of the
restricter (No 3). Do this for each hose.
The length of the rubber hose is not critical. You
can make them longer by adding more hose. The restricters need to be
about 90cm from the Carbtune. The longer the tube between the
restricters and the Carbtune, the more the damping. The shorter, the
less the damping.
The restricted part of
rubber tube must go towards the engine, not the Carbtune. If the
restricters are beside the Carbtune it will not be properly damped. The
restricters will reduce fluctuations to acceptable levels for most
bikes but some bikes may still give readings that pulsate too much even
with the dampers in place. Moving the gauges very slightly off vertical
will add some extra friction and damping but make sure the rods are
still pulsating slightly or the readings may be affected.
The rods need to pulsate to some degree for the
gauge to work properly.
see detailed photos
Using the Gauges
The gauges are calibrated in CmHg (centimetres of
Hang the gauges vertically from the left handlebar
grip or other convenient position (lifting handle on BMW twins) using
releasable cable clip.
The clip fits into the slot at top of gauge and
you push it into the slot from the back of the gauge. (Release clip by
pushing the two tabs to open the jaws).
If your bike needs the adapters remove the
blanking screws from the inlets, then connect the adapters to your
bike. Remove and blank off any balance pipes.
For removal only there is a slot in the end of
the adapter to use a screwdriver to remove it. If the adapters feels
tight let the engine cool first before removing. Do
not under any circumstances use a screwdriver to tighten the adapters.
Make sure the adapters are seated but don't
overtighten. Finger tight using fingers not tools.
Other makes have stubs on the inlets covered by
rubber caps or tiny screws which you remove. (On a very few bikes the
stubs are of a very small diameter and need a sleeve-down tube to
Push the rubber tubes onto the spigots on top of
the gauge. Make sure you have the correct tube
connected to the correct inlet. Keep rubber tubes and the gauge itself
away from hot exhausts and make sure the rubber tubes are not kinked or
flattened. Route rubber tubes over handlebar grip as shown in
After use remove black rubber tubes from gauge
first then from the inlets. Hang rubber tubes to drain any fuel from
them. When using if there is any blowback of fuel into the gauges hold
them high, upside-down and connected to a running engine until all fuel
has disappeared from gauge scale tubes.
Hang the guages from the handlebar
using the clip supplied
For difficult to get at inlets cut 1 inch of
rubber from the end of one of the rubber tubes and push it onto a
ball-point pen. Push the adapters into the other end of the cut off
rubber and use the pen as a flexible extension to screw the adapters in
and out of the inlets.
Read your workshop manual.
The engine should be warm and running at just enough above idle speed
so it doesn’t falter and stop during adjustments. Don’t try to balance
at high engine speeds. Adjust to makers specification or average of
initial readings, but the readings should be balanced to within 2 cmHg
(some models up to 4 cmHg) of each other.
Adjust in small amounts, waiting a few seconds
between each part adjustment. Write each adjustment down so you can go
back to the original settings if necessary.
If lending gauges to a friend please ensure your
friend has a copy of the instructions and is familiar with the gauges.
Ideally balance the throttle valves after
you have serviced the rest of your bike.
A Guide to Carburettor Synchronizing
Carb balance will affect the response,
smoothness, mileage, performance and running temperature of your engine.
Carb synchronisation consists of adjusting each
throttle valve for carburettors or fuel injection throats so that they
pass as much fuel-air mixture as all the others. This balances the load
carried by each piston.
If one carb is opened further than the others
that cylinder will run hotter than the others. The other carbs will
also supply a richer mixture and mileage will suffer.
The actual mechanics of carb balancing vary a
lot. Some bikes are balanced by adjusting the relative length of the
throttle cables (R series BMWs) while others have a system of levers
with adjustment screws but they all adjust the relative position of the
throttle slides or throttle butterflies of each carb to each other or
to a master preset carb.
A workshop manual for your model is very useful
as the balancing procedure for different models can vary in detail.
Familiarise yourself with the section on throttle valve synchronizing.
We assume that anybody
using the Carbtune will have some knowledge of the mechanics of their
vehicle particularly in regard to safely removing the fuel tank. If you
feel unable to do this or the other tasks safely it will be better to
leave them to an expert.
Carb adjustments should always come last
when servicing or rebuilding your bike. The rest of the bike should be
in good tune before balancing, and that means the points, plugs,
ignition, tappets, air filter and oil, should all be in proper service.
Air leaks between the airbox and carbs,
carbs and cylinder head inlets, and in the exhaust system or any
blockages in these systems will affect the carb balance and should be
rectified before starting.
Fit the gauges to the carbs. Warm the engine
until it will idle reliably without choke. The engine should not be at
full operating temperature at this point as it may lead to overheating
later. If you have a fan place it so that it blows cold air over the
cylinder head or radiator. Don't remove the fuel tank at this stage
unless it is necessary to do so to connect the gauges. Work in a well
ventilated area so that the exhaust fumes will dissipate.
Set the engine to slightly above idle by
adjusting the master idle screw, then read the gauges.
Some workshop manuals specify a vacuum reading,
usually at a high idle. The full red scale lines on the gauge are 2cmHg
apart and most factories specify the total difference between the
lowest and highest readings should be 2cmHg.
If the carbs need to be balanced then switch off
engine and remove fuel tank (most bikes) so that you can get at the
adjusters. On some bikes you can turn the tank back to front and set it
on the seat or rear frame and still be able to reconnect the fuel
lines. Secure tank with plenty of luggage straps.
If the fuel taps are vacuum operated set them on
prime. Remember fuel is dangerous both as a fire
risk and fume inhalation. Be very careful.
If you use an extension fuel line with a remote
tank (something like a lawnmower tank) keep it roughly at the same
height that your fuel tank would normally be. If you are very
experienced you may be able to balance your carbs with the fuel left in
the float bowls.
Make sure that the vacuum gauge lines are
correctly routed so that you don't find yourself adjusting the wrong
Yes, this bike needs adjusting
Start the engine and determine which throttle
valves have to be adjusted. If there is a master carb all the others
will have to be adjusted to match it. If all the carbs are adjustable
find one that is nearest to the middle reading and use it as the
Adjust the carbs until all the columns are within
2cmHg. It doesn’t really matter whereabouts on the scale the readings
are as long as you get them all to within 2cmHg of each other.
The carbs are very sensitive to adjustment and
even doing up the locknuts can affect the readings.
There are special tools available from the
importer or tool specialists to make this easier but you can do the
same thing by using a small box spanner to hold the locknuts and with a
long screwdriver down the centre to turn the adjustment screw. Or buy
The individual adjustment screws can be found on
the side of the carbs or on the operating linkages. On some carbs they
are under the carb tops which have to be removed. Some models have
spring loaded adjusters which are easier to adjust.
Where each carb has an individual adjuster check
to see that each carb has enough adjustment left. If one is almost out
of adjuster threads then turn all of them back a couple of turns and
set the cable adjuster and master idle screw to match.
Adjust one carb and then check the reading. Give the gauge a few
seconds to settle after each part adjustment. It is often necessary to
gently blip the throttle to seat the carb that has just been adjusted.
It is a good idea to write down all the
adjustments you make. This way you can go back to the original settings
if you make a mistake.
On older Suzuki models with CV carbs (except
GSX400F) the two outside carbs are set about 2.5 - 3.0cmHg higher than
the inside pair. For specific model details consult your workshop
manual or the Suzuki importer. (If the airbox has been removed set all
the carbs the same).
Adjust the carburettors to get the
readings as even as possible
When all the carbs are within 2cmHg, tighten the
locknuts, making sure you don't upset the readings.
Switch off the engine, remove the gauge fittings
and replace the blanking screws or caps on the bike. It is sometimes
easier to remove the adapters if you let the engine cool down. Replace
the fuel tank and ride the bike for a few minutes to get it to its
normal temperature, then set the idle speed with the master idle screw.
Modern bikes hold their carb settings very well and you shouldn't have
to adjust them more often than once every 5,000 miles. Older machines
can be more difficult to balance to within 2cmHg and need more frequent
adjustment. Bikes like BMW twins with separate cables for each carb
also need more frequent adjustment.
As long as the rest of the bike is in good tune
and you have some patience you will find it fairly easy and very
rewarding to balance the carbs.
To between two red lines, and
What do the readings mean?
The absolute readings aren't important as long as
they are within the manufacturers specifications. It's the comparative
reading that's important not the absolute values.
If one carb has a significantly different reading
to the others there can be several reasons for it.
It may be at the end of it's adjustment so you'll
have to back off all the adjusters and start again.
It may be that a rubber tube has been wrongly
routed so that you have been adjusting the wrong carb or that the
rubber tube is kinked or flattened on its route to the carb or is loose.
Possibly there is a leak in the carb to airbox or
carb to head joints.
It may be an engine fault and a compression test
can help to pinpoint the problem.
Information on the Gauge
Gauges do not need to be zeroed or calibrated.
All parts have been matched at assembly.
Damping of the gauges and friction of the rods
may be slightly different in each column. This does not affect final
readings, only the time it takes to get there.
Maintenance of the Carbtune
If dirt or grease gets into the gauges and makes
rod movement sticky then they can be cleaned.
Lay gauge flat on its back and remove the two
screws from the bottom cap. Slip off cap and front of case and the rods
will be exposed. (Do not lose soft sponge pad in cap). Be very careful
doing this. Do not lift or move the Carbtune with the bottom cap
removed. All the parts will fall out and will be mixed up.
Important - Each rod and
spring must go back in its original position and the right way up. Do
not mix. Do not put loose springs together as they will tangle.
For simplicity only remove one rod at a time for
cleaning. Carefully remove spring from rod and polish rod and inside of
rod guide with a cloth. Do not scratch.
For severe staining use a brillo pad or the
finest steel wool to polish rods. Wash and dry thoroughly afterwards.
Only assemble when dry.
Do not remove top cap. Do not remove clear tube
assembly from casing as it may affect rod alignment. Do not disassemble
tubes from the black seals as they may not reseal.
But remember "if it ain't
broke don't fix it."
For permanent adapter fixture in the engine
do not use an O-ring but use high temperature locking thread sealant
with brass adapters available here.
We also sell a special extension kit.
Spare parts are
available from Morgan Carbtune at the address below. Ring, fax, write
or email for price details.
Check the FAQ page
for common question and answers. Any instruction updates or amendments
will also feature on the website.
The Carbtune II and previous Carbtunes have been
tested and used on many different bike, car, marine, and aviation
engines. Obviously we cannot test every bike, car, boat or plane so if
you encounter any problems with the operation of the gauge or find the
supplied range of fittings don't fit then please contact us with the
details. Quite often problems can be quickly sorted out over the phone
or by email.
Carbtune is guaranteed against defects in
materials and workmanship for a period of 12 months from the date of
purchase. Should your Carbtune be damaged through such a defect, please
contact the manufacturer, details overleaf.
We will repair or replace the equipment (at our
option) provided you have followed the usage instructions. This does
not affect your statutory rights as a consumer.
Specification may change without notice.