Throttle Valve Shaft Seals

[Rikugun]

Being just a little crazy, I'll stick my two cents into it. My experience as a dealership motorcycle mechanic in the '70s and attending Honda technical training school ("factory trained") in those days is this: the multis were synced with a four vacuum gage set (the factory tool) at idle.

Thanks for you input Neil. This was my experience as well and no worries about being called crazy for voicing your opinion. I've been assured by the administrator of the site we promote an environment where all can feel safe voicing an opinion with no fear of reprisal.   :)

I also just checked three factory manuals from my bookshelf, a Honda CB/CL 450, a Honda CL 72, and a Kawasaki KZ 400. All specify carb sync at idle, the Kawi running with vacuum gages, the older Hondas not, running with throttle movement synced through feel and cable adjustment. The Brit bikes I worked on ... were synced like the older Honda twins - engine off, air cleaners off, fingers in the carbs to feel the throttle slides as they moved slightly up from their fully closed positions. With a carefull ear, that could also be done by listening to the slides click as they hit their stops when the throttle grip was opened and closed and adjusting for one simultaneous click.

Thank you for looking up and sharing the info from the Honda and Kawasaki manuals which plays into this. There are key differences in the carbs on these bikes - split cables vs. integrated linkage - which dictate available sync options. The KZ manual outlines vacuum gages at idle because it is faster, more accurate and more importantly - it can be done.  Those carbs have mechanically linked throttle plates and idle speed adjustment as well as ports for connecting gages - just like the Vision. The Honda and RD twins referenced have split cables and separate idle speed screws. This is true of Yamaha TX carbs as well as most old British, Italian, German etc. makes as well. A very few of these (usually later versions) may have vacuum ports available making the sync procedure less dreadful as gages rather than less accurate visual/tactile methods can be employed.

There are as many ways to sync split cable systems as there are opinions on the matter. I won't address mixtures here but relative to sync, there are two concerns - the individual cable adjusters and individual idle speed screws. The first step is to tension both cables with the twist grip and adjust the slides to be the same. The Mikuni carbs with the dot on the slide used on some RD's and other models simplified this step. This ensures both slides lift simultaneously with throttle application and that they are open the same amount with tensioned cables but does not alter there resting (idle) position.  The next step is to sync the carbs at idle using the idle speed screws. Without this step, idle/off idle performance can suffer dramatically. This also plays into the ability to effectively make mixture adjustments but I promised not to go there...  :D  Again, there are numerous effective and no so effective methods. Most of these methods aren't practical or won't work on the Vision but are interesting from a historical reference point.

Relative to mechanically linked carbs in general and Vision carbs specifically, I've looked for evidence the 2500 RPM sync method offers some advantage and could not find anything other than anecdotal. Moreover, I can see potential problems with this method which make it less desirable to me.  This is  just my opinion and I will continue to use sync at idle as a given. Naturally anyone can disagree with me and I will support their right to do so 110%.   ;)

[VisionMeister]

While I was chasing carb demons I put long hoses on my synch gauges and set it on top of my tank bag so I could watch the gauges under acceleration and deceleration. Other than being a very dangerous distraction, it did provide an amplified view of vacuum differences.
In general I found it to display and possibly clarify the variances I could see when setting the synch at idle. Slight differences, that were hard to distinguish with needles bobbleing at idle, became more exaggerated under load.
It helped answer questions like did I screw the damping adjustment on the gauges more on one than the other? and is the variance I am seeing within the tolerance of accuracy of the individual gauges?(the cheapo ones from Mike's XS).

[Re-Vision]

I too used extra long hoses without any damping at all, even when they were imbalanced I didn't have to worry about sucking red transmission fluid into the cylinders. Once they were balanced I didn't see a lot of difference from one idle adjustment to another. I've only done my bike and the help available here made it go smoothly. Should be a simple matter to try both methods and see where your bike performs best.     BDC

[Rick G]

The Honda's you mention have Constant velocity carbs , The Vision does not . There is really no comparison . Its apples and oranges.