There are four mechanisms involved which interact.
The main throttle shaft translates the cable pull into rotation, the only adjustments are the idle screw and the full throttle stop that limits the rod rotation so at full throttle the shaft rotation is stopped before the slide lift mechanism reaches full travel (a bit past where the slide is fully out of the carb bore)... I suppose you could break some part of the mechanisms if you forced them too far.
The main shaft lift arms translate rotation to linear motion, lifting the carb slides through a series of connections and links.
Each carb's external lift link is adjustable, the adjuster screw moves the slide up and down with a few mm total travel.
The idle screw adjusts the main shaft rest position with the pull cable slackened.
The choke throttle cam is also adjustable and adds throttle when choke is on.
At the Keihin factory these were all adjusted to work together properly. I do not know what procedure they used... but the slides would close fully with idle screw most of the way out, choke cam would lift the slides a bit, and the slides would be synced. From what I've read, vacuum sync was not done with running engines on the assembly line.
When we take the rack apart we have to be careful to get these things working together properly on reassembly.
The easiest thing to mess up if you don't understand the interactions is the slide lift link adjustments. You can get the slides bench synced really nicely but unless you're lucky you will get a high idle with the idle screw all the way out because with the main shaft all the way "down" the slides are still open (you would notice if the slides were set so low the idle screw can't raise them).
So on a rack where everything has been apart (except the main shaft and frame: as I said, that is a disassembly to avoid - taking the lift arms off is OK but leave the bellcrank and the shaft itself alone) the factory settings are then lost: you have to proceed systematically to get it together to work properly.
- assemble and install the throttle slides (cutouts to choke) and connect the internal lift links (there is no need to take apart the slide lift balljoint, the ball will wiggle in between the black parts), install and tighten the internal rod screw and bend its locking tabs up to prevent loosening.
- install the external lift links onto the carb lift levers. This is much easier with a loose body than it is after the body is mounted. You can fully loosen the sync screw to make it easier to get the ball in place. The rubber shield goes on first, I don't think it matters which way around these are but the V faces away from the lift link.
- mount bodies #3 & #4 to the frame with the fuel tee and the return spring attachment piece in place. Install the return spring, it's a chore to put it in later. Make sure the chokes connect properly: the tab from #3 goes between the screw and tab from #2. The other two pairs connect the same way. The vent nipples should not have been removed but need to point straight up if you did.
- Get the link arms attached to the main lift arms. You can remove the hex cap, spring, and upper joint piece to make that easy (easier?). Put those parts back in of course.
- install #1 & #4, rather similar but they have a fuel rail piece and a small vent hose to the adjacent body.
- Set the idle screw so the entire tip is protruding (but no threads). With the sync screws still all the way out, the slides should be open a bit. Screw them in until each slide just closes completely. Put their bottom washers, crosslink plates, top washers, and nuts on. Confirm that all four slides are (just) fully closed, the idle screw tip is not touching the 3/4 lift arm tab, and the choke advance adjustment screw in the 1/2 lift arm is not touching the choke cam mechanism lever (choke fully off/open). At this point I tighten the #3 locknut and dab it with bright paint or nail polish to mark it as "master".
- Get a bench sync gauge tool: a 1/16" drill bit is typically used. Use the idle screw to lift the throttle slides until it just slides in between the carb body and throttle slide (from the engine side) on #3. Adjust the other three sync screws until you feel the same drag on your gauge for all four.
- Tighten the rest of the sync screw locknuts. Check the gaps again, tightening those nuts will pull up the screws a tiny bit - readjust as required.
- turn the bellcrank by hand and adjust the bellcrank stop screw so the slides all just clear their carb bore when the screw stops further rotation.
That completes assembly, just install the body caps - and check all screws are tight, choke linkage connections are good, and the fuel rails, fuel tee, and vent hoses are in place.
Adjust the choke bellcrank link for #1 then the intercarb linkages (in order 2-3-4). I do that so that the plates are flat to the carb throat at full off - they sit where they sit at full choke. I think it's more important to minimize restriction of the throats than to have all four totally sealed closed at full choke. They're all almost fully closed anyway.
Once you get the engine running, adjust your idle after it's warmed up. Go for 1200RPM or so. It may not keep running that slow until the ignition timing and tappet clearance are set up. It should be stable at 1200RPM if all is well.
Finally, adjust the choke throttle advance. I try for 4000RPM at full choke with a warm engine but that's just what feels pretty good to me, I don't know a spec. Don't run with full choke longer than is absolutely necessary as that carbons up the plugs and washes the lubricating oil film off the cylinders.
Vacuum syncing is a good idea. You need a remote fuel tank and a decently strong fan pointed at the cylinders as the engine will be running too long to go without some cooling airflow. Again do not adjust the "master" carb. Adjust the other three to get idle vacuums close to all four matching.
Now if you need to do carb tuning and fiddle with the needles, you just free the internal lift arm from its shaft, pull the ball out of the arm, remove the slide, and reverse when done. The play between the rod screw and the arm makes exactly identical positioning impossible but it's close enough to run plug chops or a dyno run. You can complete the tuning before needing another vacuum sync.
This carb rack has an insane number of parts, and too many adjustments. The original CB750 4-way throttle cable splitter had problems, true... but I think it must have been friendlier for maintenance and adjustment.
