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the manual

How to set up and use your Carbtune Pro        Instructions for previous Carbtune II


The Carbtune Pro is a mechanical manometer. It is an upgraded model of the previous Carbtune II(In the instructions the four column model is described. The 2-column model has half the contents of the 4-column.)

It gives four simultaneous readings so you canbalance the throttle valves on carburettor or fuel injection bodies. (In the instructions the term carbĚ refers to both fuel injection throttle bodies and carburettors.)  Stainless steel rods indicate the vacuum on the scale. The clear tubes are made from high impact plastic. Damping is by a small airflow restricter in the rubber connecting tubes.

The manometer must be used vertically for a scalestarting at 8cmHg (centimetres of mercury. 1 inch=2.5cm 1cm=0.4inches)and going to 42cmHg. Most bikes have readings higher than 8cmHg, butsome bikes such as R-series BMWs and two strokes can have readingslower than this.

To get a reading below 8cmHg remove the plastic cable clip from the slot at top of gauge and insert it into the slot at the bottom. Push cable clip into slot from front of gauge. Hang the Carbtune Pro upside-down.

Ignore the scale figures. Zero will now be around 16cmHg on the scale but the full width scale graduations will still be 2cmHg. Absolute values are not important. Comparisons are important.

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In these instructions the four column modelis described.

The 2-column model has half the contents ofthe 4-column.


The term “carb” refers to both fuel injection bodies and carburettors.



The Carbtune Pro, rubber tubes, plastic cable clip tohang gauges from the handlebars, inside the small plastic bag there are4 x 5mm adapters, 4 x 6mm adapters and one fine bore thick wall clearplastic tube about 8cm (3”) long by 5mm (0.2”) diameter to make therestricters.

There is a slot on the spigot of the adapter so they can be removed with a smallscrewdriver. Do not tighten with a screwdriver.

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Damping. The restrictersmust always be used.

Without restricters the rods will fluctuatewildly. There are two components to the damping.

1. The small amount of friction between the rodsand guides which is overcome as soon as the pulsating vacuum of theengine is connected. This friction can vary from tube to tube in theCarbtune and is not a fault.

2. The air-flow restricters that fit inside therubber tubes and damp the fluctuations. There will always be a smallamount of fluctuation and this is necessary for the gauges to workproperly. Do not oil the rods.


How to make the dampers

Refer to drawing opposite. In the bag with theadapters you will have received a fine bore thick wall clear plastictube (No 1). It is about 8cm long x 5mm diameter. With a craft knifecut the thick wall tube into four roughly equal pieces. These are thefour restricters (No 2). (For the 2-column the tube is about 4cm longand is cut into 2 pieces.)

Cut about 10cm (4”) off the end of each 1 metreblack rubber hose. Push a restricter into the 10cm rubber hose. Nowpush the remaining 90cm (3ft) rubber hose onto the other side of therestricter (No 3). Do this for each hose.

The length of the rubber hose is not critical. Youcan make them longer by adding more hose. The restricters need to beabout 90cm from the Carbtune. The longer the tube between therestricters and the Carbtune, the more the damping. The shorter, theless the damping.

The restricted part ofrubber tube must go towards the engine, not the Carbtune. If therestricters are beside the Carbtune it will not be properly damped. Therestricters will reduce fluctuations to acceptable levels for mostbikes but some bikes may still give readings that pulsate too much evenwith the dampers in place. Moving the gauges very slightly off verticalwill add some extra friction and damping but make sure the rods arestill pulsating slightly or the readings may be affected.

The rods need to pulsate to some degree for thegauge to work properly.

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Make and connect the dampers

see detailed photos

Using the Gauges

The gauges are calibrated in CmHg (centimetres ofmercury).

Hang the gauges vertically from the left handlebargrip or other convenient position (lifting handle on BMW twins) usingreleasable cable clip.

The clip fits into the slot at top of gauge and you push it into the slot from the front of the gauge. (Release clip by pushing the two tabs to open the jaws).

If your bike needs the adapters remove theblanking screws from the inlets, then connect the adapters to yourbike. Remove and blank off any balance pipes.

For removal only there is a slot in the end ofthe adapter to use a screwdriver to remove it. If the adapters feelstight let the engine cool first before removing. Donot under any circumstances use a screwdriver to tighten the adapters.

Make sure the adapters are seated but don'tovertighten. Finger tight using fingers not tools.

Other makes have stubs on the inlets covered byrubber caps or tiny screws which you remove. (On a very few bikes thestubs are of a very small diameter and need a sleeve-down tube toconnect.)

Push the rubber tubes onto the spigots on top ofthe gauge. Make sure you have the correct tubeconnected to the correct inlet. Keep rubber tubes and the gauge itselfaway from hot exhausts and make sure the rubber tubes are not kinked orflattened. Route rubber tubes over handlebar grip as shown inphoto above.

After use remove black rubber tubes from gaugefirst then from the inlets. Hang rubber tubes to drain any fuel fromthem. When using if there is any blowback of fuel into the gauges holdthem high, upside-down and connected to a running engine until all fuelhas disappeared from gauge scale tubes.

Remember the engine and exhausts are hot so be careful with any spilt fuel as it is a fire risk.

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Guage hanging from handlebar

Hang the guages from the handlebarusing the clip supplied

Attaching the clip

For difficult to get at inlets cut 1 inch ofrubber from the end of one of the rubber tubes and push it onto aball-point pen. Push the adapters into the other end of the cut offrubber and use the pen as a flexible extension to screw the adapters inand out of the inlets.

General Hints

Read your workshop manual.The engine should be warm and running at just enough above idle speedso it doesn’t falter and stop during adjustments. Don’t try to balanceat high engine speeds. Adjust to makers specification or average ofinitial 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 secondsbetween each part adjustment. Write each adjustment down so you can goback to the original settings if necessary.

If lending gauges to a friend please ensure yourfriend has a copy of the instructions and is familiar with the gauges.

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Ideally balance the throttle valves afteryou 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 eachthrottle valve for carburettors or fuel injection throats so that theypass as much fuel-air mixture as all the others. This balances the loadcarried by each piston.

If one carb is opened further than the othersthat cylinder will run hotter than the others. The other carbs willalso supply a richer mixture and mileage will suffer.

The actual mechanics of carb balancing vary alot. Some bikes are balanced by adjusting the relative length of thethrottle cables (R series BMWs) while others have a system of leverswith adjustment screws but they all adjust the relative position of thethrottle slides or throttle butterflies of each carb to each other orto a master preset carb.

A workshop manual for your model is very usefulas the balancing procedure for different models can vary in detail.Familiarise yourself with the section on throttle valve synchronizing.

We assume that anybodyusing the Carbtune will have some knowledge of the mechanics of theirvehicle particularly in regard to safely removing the fuel tank. If youfeel unable to do this or the other tasks safely it will be better toleave them to an expert.

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Carb adjustments should always come lastwhen servicing or rebuilding your bike. The rest of the bike should bein 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 anyblockages in these systems will affect the carb balance and should berectified before starting.


Step 1:

Fit the gauges to the carbs. Warm the engineuntil it will idle reliably without choke. The engine should not be atfull operating temperature at this point as it may lead to overheatinglater. If you have a fan place it so that it blows cold air over thecylinder head or radiator. Don't remove the fuel tank at this stageunless it is necessary to do so to connect the gauges. Work in a wellventilated area so that the exhaust fumes will dissipate.

Fitting the hoses to the carbs

Step 2:

Set the engine to slightly above idle byadjusting the master idle screw, then read the gauges.

Some workshop manuals specify a vacuum reading, usually at a high idle. The full width scale lines on the gauge are 2cmHgapart and most factories specify the total difference between thelowest and highest readings should be 2cmHg.

If the carbs need to be balanced then switch offengine and remove fuel tank (most bikes) so that you can get at theadjusters. On some bikes you can turn the tank back to front and set iton the seat or rear frame and still be able to reconnect the fuellines. Secure tank with plenty of luggage straps.

If the fuel taps are vacuum operated set them onprime. Remember fuel is dangerous both as a firerisk and fume inhalation. Be very careful.

If you use an extension fuel line with a remotetank (something like a lawnmower tank) keep it roughly at the sameheight that your fuel tank would normally be. If you are veryexperienced you may be able to balance your carbs with the fuel left inthe float bowls. (Click here to see a customer's design for a remote fuel tank)

Make sure that the vacuum gauge lines arecorrectly routed so that you don't find yourself adjusting the wrongcarb.

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Yes, this bike needs adjusting

Step 3:

Start the engine and determine which throttlevalves have to be adjusted. If there is a master carb all the otherswill have to be adjusted to match it. If all the carbs are adjustablefind one that is nearest to the middle reading and use it as thereference carb.

Adjust the carbs until all the columns are within2cmHg. It doesn’t really matter whereabouts on the scale the readingsare as long as you get them all to within 2cmHg of each other.

The carbs are very sensitive to adjustment andeven doing up the locknuts can affect the readings.

There are special tools available from theimporter or tool specialists to make this easier but you can do thesame thing by using a small box spanner to hold the locknuts and with along screwdriver down the centre to turn the adjustment screw.

The individual adjustment screws can be found onthe side of the carbs or on the operating linkages. On some carbs theyare under the carb tops which have to be removed. Some models havespring loaded adjusters which are easier to adjust.

Where each carb has an individual adjuster checkto see that each carb has enough adjustment left. If one is almost outof adjuster threads then turn all of them back a couple of turns andset the cable adjuster and master idle screw to match.

Adjust one carb and then check the reading. Give the gauge a fewseconds to settle after each part adjustment. It is often necessary togently blip the throttle to seat the carb that has just been adjusted.

It is a good idea to write down all theadjustments you make. This way you can go back to the original settingsif you make a mistake.

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On older Suzuki models with CV carbs (exceptGSX400F) the two outside carbs are set about 2.5 - 3.0cmHg higher thanthe inside pair. For specific model details consult your workshopmanual or the Suzuki importer. (If the airbox has been removed set allthe carbs the same).


Adjusting the screws

Adjust the carburettors to get thereadings as even as possible

Checking the guages


Step 4:

When all the carbs are within 2cmHg, tighten thelocknuts if applicable, making sure you don't upset the readings.

Switch off the engine, remove the gauge fittingsand replace the blanking screws or caps on the bike. It is sometimeseasier to remove the adapters if you let the engine cool down. Replacethe fuel tank and ride the bike for a few minutes to get it to itsnormal temperature, then set the idle speed with the master idle screw.

Modern bikes hold their carb settings very well and you shouldn't haveto adjust them more often than once every 5,000 miles. Older machinescan be more difficult to balance to within 2cmHg and need more frequentadjustment. Bikes like BMW twins with separate cables for each carbalso need more frequent adjustment.

As long as the rest of the bike is in good tuneand you have some patience you will find it fairly easy and veryrewarding to balance the carbs.

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To between two red lines, andthat's it!

What do the readings mean?

The absolute readings aren't important as long asthey are within the manufacturers specifications. It's the comparativereading that's important not the absolute values.

If one carb has a significantly different readingto the others there can be several reasons for it.

It may be at the end of it's adjustment so you'llhave to back off all the adjusters and start again.

It may be that a rubber tube has been wronglyrouted so that you have been adjusting the wrong carb or that therubber tube is kinked or flattened on its route to the carb or is loose.

Possibly there is a leak in the carb to airbox orcarb to head joints.

It may be an engine fault and a compression testcan help to pinpoint the problem.  

If you suspect there is a leak in the intake manifold let the engine idle and spray a fine mist of water around the inlet area. A drop in engine speed can indicate a leak in the inlet system.

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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 rodsmay be slightly different in each column. This does not affect finalreadings, only the time it takes to get there.



Maintenance of the Carbtune

If dirt or grease gets into the gauges and makesrod movement sticky then they can be cleaned.

Lay gauge flat on its front and remove the four screws holding the back plate in position. Remove the backplate. You will see the rod and tube assemblies. At the middle of the rod tube assembly you will see the brass rod guides. Each tube clips into the gauge at the brass guide. You will need to release the guide from the push fit clip. Just pull it up (away from clip) slightly (or push from the front) and it will spring out of the clip and be at an angle. The top part of the tube with the black spigot will remain where it is. Slide the rod and spring out of the brass guide. Do not remove the brass guide from the clear plastic tube. There is no need to remove the tube assembly from the casing. On reassembly slide rod into guide. It is too long to go back into the casing unless you slide it into the guide a few centimetres. Align the lugs on the clear plastic tube and push the brass guide until it locates into its clip. 


Important - Each rod, spring and washer must go back in its original position and the right way up. (The flat side of the washer goes towards the spring) Do not mix. Do not put loose springs together as they will tangle.

For simplicity only remove one rod at a time forcleaning. Carefully remove spring from rod and polish rod and inside ofrod guide with a cloth. Do not scratch.

For severe staining use a brillo pad or thefinest steel wool to polish rods. Wash and dry thoroughly afterwards.Only assemble when dry.

Do not remove brass guides or black spigots from the clear plastic tubes. Contact us for advice first

But remember "if it ain'tbroke don't fix it."

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For permanent adapter fixture in the enginedo not use an O-ring but use high temperature locking thread sealantwith brass adapters available here.

Or make an  extension kit.


Spare parts areavailable from Morgan Carbtune at the address below. Ring, fax, writeor email for price details.



Check the FAQ pagefor common question and answers. Any instruction updates or amendmentswill also feature on the website.

The Carbtune Pro and previous Carbtunes have beentested and used on many different bike, car, marine, and aviationengines. Obviously we cannot test every bike, car, boat or plane so ifyou encounter any problems with the operation of the gauge or find thesupplied range of fittings don't fit then please contact us with thedetails. Quite often problems can be quickly sorted out over the phoneor by email.




Carbtune is guaranteed against defects inmaterials and workmanship for a period of 12 months from the date ofpurchase. Should your Carbtune be damaged through such a defect, pleasecontact the manufacturer, details overleaf.

We will repair or replace the equipment (at ouroption) provided you have followed the usage instructions. This doesnot affect your statutory rights as a consumer.

Specification may change without notice.

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Morgan Carbtune Ltd
Unit 33W
Townsend Enterprise Park

Townsend Street

Belfast BT13 2ES

United Kingdom


Tel 028 9023 9007

InternationalTel +44 2890 239007

From USA/CanadaDirect dial
Tel 011 44  2890 239007