Carb sync advice needed

Alright, my bike's got 104 000 kms but no clue when the last time was that the carbs were synchronized.
The service manual's got comprehensive step-by-step instructions with pictures, but it's no good...
I can't locate the vacuum ports or sync screws.

Where exactly are they and furthermore, what is the size of the port so I know what size of tube I need.
The purpose is to build a sync tool and get the job done. Reason for syncing is unstable idle.

The bike is a vfr 750 f -91 and the manual says the ports and screws are accessible after removing the fuel tank
though the picture in the manual clearly shows the carbs removed altogether.

 

the port are mostly like 1cm long (0.4 in) with a rubber(condom) over it. Can't tell you the place. (searched a picture but no succes sorry)

 

carbs wouldn't need to be synched unless the throttle screws had been messed with or the carbs dismantled or rebuilt. All they do is adjust the butterfly valves to open the same amount at the same time.

More likely is that the tiny mixture port screws have gotten dirty and the port is clogged somewhat. Idle speed is set by adjusting the mixture screws more so than using the big knob thumb screw on the throttle stop. If the bike has set up for any length of time the tiny pilot jets could be clogged also.

But nevertheless, the synch ports are on the intake manifold below the carburetors, and are either nipple tubes with rubber caps or threaded ports covered with a plug screw.

 

AS K-B suggests, it might be a good plan to clean the carbs first, paying attention to idle jets and idle mixture screw passages.

Note that the synch operation should be done at 1100-1200 idle speed after verifying that valve clearances are correct first. Synch must be done in an EXACT
(somewhat complex) sequence, starting with the carb with the idle adjustment screw. (Read and reread service manual !!) No screw would need more than about 1/8- 1/4 turn usually, and the job can be messed up badly by turning too much. Helps to blip the throttle a few times after each screw adjustment so throttle butterflys and linkages can settle into their new positions.

 

Thanks for the replies!
I still can't quite understand though how I am supposed to get access to the ports and screws without removing the carbs...
Also, how accurate is the mechanical synching by which I mean measuring the butterfly valve openings by hand when the engine isnt running?

 

i think the mechanical synching works as good as or better than pressure gauges. i use ground drill rod stock and can measure it to 0.0005". Most of the typical pressure gauges are not that accurate unless you are willing to spend huge sums.

You would have to remove the fairings to get to the side of the carbs/intakes to install the pressure gauges. The adjustment screws face down and have to be accessed from the sides also.

But it is quite easy to measure or check the mechanical synch positions from the top side without removing the side fairings. So you could check it as is to determine if they are way off that might be causing the unstable idle. In any case if the carbs need to be cleaned or adjusted the fairings come off.

 

Its a bear to do the carb synch on a gen three bike, dont ask how I know. Its really nice to have the bike on air table and raise it up as high as it goes and this way, you can look up with your head light on your head (cause-one can not discount being able to see what the fuck your working on.) I happen to have about 4 dedicated carburetor tools and use a Triumph one that got mis-appropriated at some time :evil: it just has a flat blade on the end of its helical cut little gear thing with a cup to guide you into the hex-screw adjuster.

If you have the carbs off the bike, its nice to bench synchronize them, then; when you install them back on the machine, they are damn close. I have a Morgan Carb synch tool that I have not broken out as of yet, but I got fed up with liquid manometers...Good luck and let us know how you make out fer-gawds-sake!