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Carburetor Synchronization
using a Carbtune II

Please note: this page is in rough draft from as of July 15, 2001, and placed online because of the recent interest in the Carbtune II and the manual directions for synchronizing the carbs

A Modern  Muscle Bike right out of the box? Yes.
One bike that can do it all for me? Yes


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Yamaha FZ1
Carburetor Synchronization



Synchronizing the Carburetors, Manual 3-10

.Prior to synchronizing the carbs, the valve clearance and idling speed should be properly adjusted and the ignition timing checked. Basically synching the carbs is the last step of a good tune up.

.Place the bike on center stand.

.Remove the vacuum cap from each carb.

.Install the carburetor synchronizer (1). I like the Carbtune II made in the UK. Information on its use and acquisition can be found on a page lined off the FZ or Vmax sitemaps.

.Install the inductive self-powered tachometer (2) to the spark plug lead of cyl. #1, with is the first cylinder on the left side of the bike Inductive tachs are available at any auto store in the tune up sections. Often a multipurpose instrument and a timing light are sold as a tune-up package.

.Start the engine and let it warm up several minutes. Do not run you engine in a garage without good ventilation.

.Check engine idling speed and if out of spec (1,050-1,150 rpm) adjust the idling.

.Sync carb #1 to carb #2 by turning the synchronizing screw (1) in either direction until both gauges read the same.
NOTE: The carbs are number 1-4 in order from the left side to the right side of the bike. After each step, rev or blurb the engine two or three times, each time less than a second, and check the synchronization again.

.Now synchronize carb #4 to carb #3 by tuning the synchronizing screw (2) in either direction until both gauges read the same.

.Now synchronize carb #2 to carb #3 by turning the synchronizing screw (3) in either direction until both gauges reach the same.

NOTE: Vacuum pressure at engine idling speed is 8.86 in of Hg, or 30 kPa. The difference in vacuum pressure between two carbs should not exceed 0.39 in Hg (mercury). Or 1.33 kPa.

.Measure engine idling speed again, and if it is out of spec, adjust.

.Stop the engine and remove the measuring equipment.

.Adjust the throttle free play to 3--5 mm or 0.20 inches.

.Install vacuum caps.

Carb Sync Directions post from the FZ List------(Do a search on the list for synchronizing, or a derivation of carb/sync).

....You want the bike to be very warmed up before doing this. Having long pliers not only lets you reach in and remove the cap clips, you also don't get burned. .... Where are the vacuum plugs.   If you look in the space between the carbs and the engine's heads from the left (clutch) side, you can see three rubber plugs poking straight up, one in front of each carb, with those spring clamps on them. The fourth one, seen from the other side, is actually a vacuum hose (at least on the CA model)...... All you are doing is attaching the four hoses of the manometer to those fittings. I tried to get a picture of that part, but the camera didn't cooperate.

....I raised the tank and held it up with a rubber mallet. I put a tiny screw in the end of the handle to poke through the bolt hole in the front of the tank so that it cannot slip.

....Then I hooked up the manometer and idled the bike while I took to looking for the danged adjustment screws. There are three of them, sitting between the carbs. The one furthest left adjusts #2 to #1, the next one #3 to #2 (actually 3 & 4 to 1 & 2) and the furthest right adjusts #4 to #3.

....You will need a long thin Phillips head screw driver to get at the screws. .

....I adjusted them in the order 1/2 then 3/2 then 4/3.

....I recommend at least looking at the service manual before doing this if you have never done a sync. A test ride after the sync showed a bit smoother engine, but nothing significant. .

....Another thing - folks will tell you to adjust the carbs at idle. This is very true. But, I also run the engine up to 2000, then 3000 RPMS and watch how the carbs follow each other. I  then make tweaks at idle according to what I see.

....All adjustments made here are very small. Also give the engine a little time between adjustments to settle down before you assume you are seeing the results of the change you made.

....I have a tip that I hope will be helpful for all the do-it-yourselfer's.I found that adjusting the middle carb screw was a bit difficult because the #3 diaphragm cover partially obscures the screw head. My simple fix was to modify a long #2 Phillips screwdriver by grinding the shaft down to 3/16`s diameter at point of contact which is about 1.5 inches from the tip. To grind the screwdriver I chucked it in to an electric drill and rotated it against a grinding wheel. It turned out uniform. Now I only use this screwdriver for carb-sync`ing,  because its a little weaker but I feel Its a small price to pay considering how much easier it is to access thee middle carb screw.

Carbtune II Synchronizer



What is Carbtune II?
The Carbtune II is a four way mechanical manometer (reads the vacuum, an indication of the fuel/air mixture flow from four carbs at a time).  The indicator tubes are made from high impact plastic.  Damping is by a small airflow restrictor in the rubber connecting tubes.  The Manometer must be used vertically.  The scales read in centimeters (cm) of Mercury (Hg).  Absolute values of scale readings are not important, it is the comparison of the readings that is important, meaning the flow in each carb is the same as its partners.  Carbtune II like other manometers, works on the Vmax by reading  the vacuum created at the capped air inlet below each carb.  Fluctuations in all four carbs can be seen at once.   The manometer is relatively easy to set-up and use,  therefore there may be a tendency to sync the carbs more often.  Both methods however are no major time consumers or difficult to do.

What is the need for having the carbs synchronized?
1.  Carb balance will affect the response, smoothness, mileage, performance and running temperature of the engine.
2.  Carb synchronization consists of adjusting each carb butterfly so that it passes as much fuel-air mixture as all the others, thus balancing the load carried by each piston.
3.  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.
4.  The actual mechanics of carb balancing involves adjustment screws that adjusts the relative position of the throttle butterflies of each carb with its partner.


Where can I get information regarding Carbtune II?

Contact John Morgan, builder, Unit 33 Townsend Net. Pk., Townsend Street, Belfast BT13 2ES, UK; Tel & Fax +44 (0) 02890 247294, 


Contact John by email if you have any questions regarding needed supplies.  John is the real deal and most helpful.  If you are using the instrument on another bike or vehicle there may be some accessory fittings needed.

Arrival is generally within a week.   Once assembled, a storage box is useful to keep the instrument, directions and log book in good condition and handy to use.



What is Dampening?
Without dampening, the rods in the tubes would fluctuate wildly.  There are two things that dampen or control this movement:  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.  2.  The air-flow restrictors that fit inside the rubber tubes and dampen the fluctuations.  There will always be a small amount of fluctuation that is necessary for the gauges to work properly.  The dampeners must always be used, and the rods must not be oiled if taken out and cleaned. 

NOTE:  It appears the very thin capillary tube is no longer used in the dampener for the Carbtune.  The newer units (2001) apparently only use the thin inside diameter of the clear thick walled clear tube sent with the unit.  I suggest you just follow the directions sent with the Carbtune.

How are the Dampeners made?  Older units
1.  In the kit there are two small short pieces of clear plastic tubing.  One is much larger in diameter and has a very fine bore.  
2.  The other tube is extremely thin.  With a sharp blade, like a Stanley knife, on a piece of wood, cut the thick wall tube into four roughly equal pieces.  
3. Then cut the fine tube into four equal pieces, but at an acute angle, to make the end "pointy", thus easier to insert into the fine bore of the larger tubing.  
4. Firmly push each fine tube a few millimeters into the bore of the thicker tube.  Do this for all four pieces of clear tubing.  These are the four air flow restrictors which dampen the erratic movement of the Carbtune II rods.  
5. Next cut about 10 cm (4 inches) of the end of each 1 meter (1+ yard) black rubber tube.  
6. Push each clear tubing restrictor you just made firmly about 1/2+cm into the 10cm black rubber tube with the fine tube tail point out.
7. Now push the remaining 90 cm black rubber tube firmly about 1/2+cm onto the tail side of the restrictors.  The fine clear tube (the restrictor part) points away from the carb or down the long part of the black rubber tubing toward the Carbtune II instrument.
8.  The dampeners will reduce fluctuations to acceptable levels for most bikes, however some bikes may still give readings that pulsate too much even with the dampeners in place.  Moving the gauge very slightly off vertical will add some extra friction and dampen the rods.  But make sure the rods are still pulsating slightly or the readings may be affects. 
9.  Note:  The rods need to pulsate to some degree for the gauge to work properly.


Some of the information may apply to the FZ so I have placed it on the FZ Carbtune page until I sync my FZ carbs and complete the sync procedure and info for the FZ.

How do I set the Carbtune up for use the first time?    
1.  The dampeners are now made and the long ends of the black rubber tubes can be pushed onto the spigots of the Carbtune II gauge.  Loop the red tie wrap so the instrument can be hung from the bar grips.
2.  The short end of the black rubber tubes will be pushed over the spigots on the air intake. 
3.  Before that is done the rubber covers over the set screws and spigots will be removed.

What needs to be done before setting up the gauge for use?
1.  Have good tools available.  Use the right size of screwdrivers, pliers, wrenches, etc.
2.  Cardboard beverage trays make good holders for small parts and tools.

Where do I place the gauge and tubes?

Where do I hook up the gauge tubes and make the adjustments to each carb?
1.  Under the plastic cover you removed, you will find rubber caps covering the air inlet spigot for each carb, and also the set screws for each carb.

2.  Remove the rubber cap from each spigot.  There is a clip putting tension on the cap.  Use your fingers or a pair of pliers to remove the tension on the cap as you pull the cap off the spigot.  

3.  After the caps are removed, place the end of a rubber tube on each spigot.
4.  Again, you can connect any tube to any carb.
After doing the synchronizing a couple of times you will know what you like best.

Is it time to start the engine?
1.  After the hoses and gauge have been fitted to the carb/bike, start and warm the engine until it will idle reliably without a choke.  the engine should not be run at full operating temp at this point as it may lead to overheating later.

2.  You have to run your engine at idle speeds and depending on the conditions it might be advisable to have floor fans available to move air over the engine, or to have a manual fan switch installed on your bike.  Better yet both.  Do not let your engine overheat!!

3. Check and set the idle speed at this time if necessary (1200rpm is a nice idle to work with).

How do I set the idle?
1.  On the left side of the engine,  there is a thumb set screw that can be reached and turned to adjust the idle speed. 

2.  Reach in and adjust the idle of somewhere over 1000-1500 rpms.

3.  You can use this idle set screw to adjust your idle speed at other times.

4.  An idle speed of about 1200 rpms works well.

5.  After synchronizing  the carbs you can run the bike a little and then reset the idle to your preference.

The tubes are connected to the spigots, engine running and idle set, what does the gauge reading mean?

1.  Use a light blipping of the throttle to see where the four SS rods settle.  

2.  If you have not synchronized for awhile they will probably be slightly uneven.  The bigger the variation the more out of sync the carbs are with each other.

3.  To sync the carbs, start with
------------ making adjustments to the set screw to the point where the rods are the most even.  At that point ------------- and then-----------.  Slightly blip the throttle after each adjustment to see the affect.  

4. The carbs are very sensitive to adjustment.  Adjust ------------- at a time and then check the reading.  Give the gauge a few seconds to settle after each  adjustment.  It is generally necessary to gently blip the throttle to seat the carb that has just been adjusted.

What does the gauge reading mean?  The actual readings are not important.  It is the comparative reading between carbs that is relative.  If one carb has a significantly different reading to the others there can be several reasons for it.  Immediately check to see if the gauge is properly installed.  There may be a leak in the carb to air box route, or cart to head joints.  It may be an engine problem that a compression test may help pinpoint. 

Where is the adjustment screw for each carb?
1.  -----------

2.  By turning the set screw in and out, you can make adjustments to that carb in an attempt to bring the SS rod in sync or more even with its partners.  

3.  Adjust in small amounts, waiting a few seconds between each adjustment, blipping the throttle.

4.  NOTE:  the rods should be relatively even, but will not be perfect when the carbs are in sync. 


What is a review of the procedure?

1.  ----------
2.  Remove caps on inlet spigots.
3. Hang gauge on left grip.
4. Place tubes over spigots in order.
5. Start engine and adjust idle to 1200rpm.
6. Blip throttle and check rod alignment.
7. Start by adjusting -----------
8. -----------
-------- It may take several times. Be patient.
10. Blip the throttle after each adjustment attempt.
11.  It will take a number of times----------
12.  ---------------
13. Be careful of engine temp.  Use fans or your manual fan switch.

14.  After a number of attempts and experience, you will be able to bring the SS rods even, an indication that the carbs are synchronized in the level of fuel/air mixture they are using.  
15. When all the carbs are within 2cmHg or closer they are sufficiently synchronized.
16. Switch the engine off, remove the gauge and close up the engine.
17.  Then ride the bike a few minutes and reset the idle speed with the master idle screw if necessary.
18.  Generally the Vmax holds its carb settings very well and should not need checked for about 3-5k miles.

Synchronized carbs follow the other steps you make to have a smooth, responsive engine.  Synchronizing ties it all together for Max Power:)

Carbtune II website

John Morgan, builder and supplies of Carbtune II


Use the FZ1 Sitemap  to navigate all of the FZ pages.





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