What does bench syncing mean?

I see it mentioned often, but I don't recall seeing it explained, so please teach me something.

 

I'm guessing it means visually inspecting how the four slides move in sync with each other when you manually operate the throttle mechanism.

Someone will chime in eventually to confirm or correct and probably say it's not a good method....

 

Close! It is the spacing of the butterflies with the throttle closed. The butterflies have oval holes for the mounting screws so that they can be moved slightly until all have the same clearance at the bottom of the venturi. This will keep you from having them so far out that the sync with carb sticks will be almost unbearable.

 

I've never checked this, might be something for me to take a look at though, I probally shouldn't try to fix what's not broken either.

 

It's a Piece O' Cake.

Cut across the width of a business card ... making four "Strips" about 1/4" wide. Discard the rest.

Slide the first quarter-inch strip beneath the Bottom of the Number Three Butterfly and Carb Body.
Use the Idle Adjustment Rod to Close the Butterfly of Three so that the Business Card Strip slides beneath the Butterfly creating a little bit of drag on the Business Card as it "Feels" both the Carb Body and Bottom of the Buterfly.

The Business Card Section will curve to follow the contour.
Feell the Bottom of the Butterfly and the Carb Body with light drag on the Business Card as you slide the strip in and out getting the "Feel."

Leave the strip in Three.
Do Number four.
Get the drag on the Strip to match the drag on Three by Adjusting the No.-4 Sync Screw until you have "Matching Feel" on No.-3 & No.-4

Leave the strips in 3 & 4

Do No.-1 & No.-2 to MATCH 3 & 4.

Do micro adjusting of 1, 2, & 4 to Perfectly MATCH 3.

Carbs are "Bench Synched" ...

Now you do it for real!

 

Here's a How-To tutorial.

 

Also covers setting the mixtures to match each other (best seems just about 3 turns out from bottom) and setting the floats to spec.

 

Thanks for the info, I am gooing to pull mine off and see what they look like. I think I read somewhere that you could use popsicle sticks also under the butterfly.

 

Popsicle sticks.

No!

You're Sync-ing the Carbs.
Not putting the Butterfly's up on blocks!

 

Good point Rick! Actully we have discussed two different things so I need to clarify. Rick's post mentions adjusting the sync screws which is the correct thing for Sbmaxim to do.
I mentioned the oval holes and the screws that attach the butterflies to the throttle shaft and aligning them using those. Not what Sbmaxim should do. Why? Because this is done when the "butterflies have been removed" to either replace the throttle shaft seals or seperate the carbs from the rack. In which case I also failed to mention that the #3 butterfly should be installed first and all others synced to it since it is the static carb. Since Sbmaxim hasn't removed the butterflies the bench syncing should be done as Rick suggested using the sync screws. Those are the ones with the springs mounted on the outside of the carbs to the throttle shaft.
And as Kiwi posted, yes the setting of the mixture screws is also part of bench syncing. Factory calls for 2.5 turns out on each for a starting point. This is later refined by use of the colortune plug.

 

I find this post to be very informative, but I do have one question.

What is the reason behind this if you have a set of sticks to do your synch with? or did I just answer my own question?? (aka, this is for some without synch sticks??)

good article nonetheless

 

When fiddling with the carbs on the bench, it is possible to get them so far out of sync that the bike will not start when you put them back on. Therefore, you must bench sync them to make sure the bike will run so you can finish syncing them with carb sticks and colortune plug.

 

The Bench Sync is a Mechanical Sync-ing of the Butterfly's all to the same measured opening.

Depending on other factors ... the ACTUAL Sync depends on the Vacuum each of the Cylinder is able to pull.

Rings, valves, cam lobes and general health of the engine will cause the Actual Vacuum to vary some ... which you correct for when you hook-up the Sticks and measure how much the Cylinders each pulls a column of Mercury.