How to "bench sync" mikuni vm28 before installing

I have a 75 Z1B and two sets of Mikuni vm28sc carburetor sets (pictures provided) On both sets I have cleaned the internal parts and passage ways, adjusted and checked fuel level in the float bowls. All jets, needles appear to be original and correct numbered size for this bike. But before I install them, everything I read indicates doing an initial "bench sync" on the slide assembly. I need a step by step list of how to do this if anyone has ever....as I have not!
The next question is which set of carbs would work best on this bike or would they both be similar in performance and reliability? The only info I could find or see different was that one set has a single spring, all 4 slides are adjustable, float bowl has side drains and number stamped on each carburetor is 2170.
The second set has two springs, the #4 carburetor slide has a cap on it (non adjustable?),17mm single drain plug in bottom of float bowl and stamped 1473 on each carburetor.
Any "how to" information on bench syncing and or best carburetor set to install will be appreciated!

Your photos suggest the sync is pretty close. Hold the carbs up to the light and look at the gap under the slides and then adjust them all the same. To adjust remove the top cap, unlock the lock nut, and then adjust with a flat blade screwdriver. The process is shown in the factory manual for your bike. This site has manuals if you are in need.

While doing the bench sync, be sure to leave sufficient unused thread on the idle knob adjust screw to allow further lowering of slides (reduce idle rpm speed) which is often needed to accomplish the later "running" sync .

Good Fortune!

The carbs with the large center drain plugs are correct for a Z1 and early-production Z1-A's. The set with the small drain screws are correct for late-production Z1-A's and all Z1-B's. The cap on #4 on these is intended to be a "home" reference relative to the other three.

Sync on either rack does not require removal of the top caps; sync is accomplished by adjusting the slotted, threaded adjusters external to the carb bodies at the top of the linkage.

Bench sync and final sync is described in detail in the factory service manual, which can be found here:
kzrider.com/service-manuals

We'd like to see a pic of the other side of the early carbs. It's unlikely, but they might be much more rare "ribless" carbs.

Good Ridin'
slmjim & Z1BEBE

The bottom carbs are correct for a Z1B.

slmjim & Z1BEBE has done a great job of describing the differences in carbs. I would only add that the Z1B carbs don't have a "home" synch position on #4 carb. All four carbs have the same synch mechanism, consisting of a barrel screw topped with a nut. For easy synch work I use this tool. It allows the nut to be tightened while holding the barrel screw in a fixed position. This is critical when finishing up. I also use only JIS screwdrivers on JIS screws. Phillips screwdrivers will damage the screws.

For synching I use the following method.

1) Adjust the slides so that when the throttle is fully open the slides are above the carb throats. This is done with the spring loaded screw under the throttle cable actuator in the photo below.

2) Adjust the idle screw so that it does not touch the idle arm.
3) Completely unscrew all four synch barrel screws.
4) Check to ensure the idle screw is not touching the idle arm
5) Screw all four barrel screws in until the barrel just barely touches the chrome arm attached to the side of the adjusting mechanism.
At this point there should be a large gap below the throttle slides in the carb throats.
6) Begin tightening all four screws by a half turn and check the throttle slide position. If there is still a large gap then tighten all four another half turn. Continue this process until the slides are almost closed. Final tightening of the barrels should result in a gap of about a half mm on each slide.
7) Open the throttle fully and allow it to snap shut. Do this several times and check to see if the slides have moved. If so then re-adjust the barrels. Once the throttle slides close to within 0.5 mm properly you can tighten the nuts on the barrel screws but do not overtighten. This are very small and do not need a lot of force.
Re-adjust the idle adjust screw to ensure it does not touch the idle arm throughout the procedure. It can be adjusted back to the adjusting arm once the synch nuts are tightened.

The manual says 0.7 mm gap but I target 0.5 mm. I "measure" this by inserting a small wire under the throttle body. If it doesn't fit then it's too tight. If it just slides under then I know I'm close.





fortnine.ca/en/motion-pro-carburetor-adjusting-tool

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