Hi Bill. Don't mean to state the obvious but I'll mention it anyway: With the carbs on the bench (and separated), confirm the throttle plates are closing tight by opening each set a
few degrees and letting them snap shut. Repeat several times to confirm. Each carb should do this and will confirm a perfect closing fit of the plates to venturi. If they do, then the issue is relative to something further into the assembly. This is the only time the throttle-plates will close tight ! After assembly and adjustment the throttle-plates will always be "cracked"......or open, ever so slightly. You haven't mentioned replacing the throttle-shaft seals which would involve the removal of the throttle-plates and shaft......so won't elaborate or question further on this point.
Clearly: if you can manually force the throttle closed enough to get the idle to lower, there is something causing this resistance. The two most obvious causes are the linkage rod(s) being adjusted too tight or a conflict between the two throttle cables.
Do your due diligence in re-assembling and when refitted to the engine - leave the 2nd (CLOSE throttle) cable off. Do your sync but try this procedure: Start with the 3 adjusters backed off 1 turn, then start your adjustment by lowering the Highest reading. (Turning the adjusting screw out) I Know this sounds counter-intuitive. This should increase the RPM by opening the throttle-plates slightly, on that carb. This should have the affect of increasing the vacuum readings slightly on the other 2 carbs. because of the RPM increase. If the idle RPM becomes too high, ( more then 1500 RPM) lower it with the idle adjustment knob. If your "Static-Sync" and preliminary set-up is correct you should be able to lower the RPM, in this manner.
An adjustment to one adjuster should have a noticeable affect on the vacuum readings of the other 2 carbs. Avoid the mistake of trying to achieve the highest vacuum reading on one carb and then trying to match the other 2. Start with the goal of getting all 3 carbs to read 7" Hg. (18cm/Hg rounded up). As little as 1/4 turn on any adjsuter will (should) have a significant affect on the readings. I don't subscribe to the manuals "less than 2 cm/Hg." If your gauges are "dampened" - that's to say, NOT bouncing, there is no reason why all 3 readings can't be identical. This should apply to "sticks" and dial-gauges.
I'll mention again that the ".55mm" the manual states and you made reference to is not likely going to be acheived in the end. In fact it could be as much as 3 to 4 mm as is typical with mine and every other set of carbs I've rebuilt. Age and Wear on all the bits and pieces including the condition of the engine!
Don't know what you're using to adjsut the sync. but seriously consider this:
www.kz1300.com/index.php/forum/carburato...arb-sync-tools#20256
Easy to use and far more precise then using "Piers and a Wrench" !
IF you can acheive a base vacuum reading of 7"Hg then you can attempt to get a slightly higher reading by sequentially turning IN each adjuster. This is the point where you will truely see how sensitive this adjsutment is. 1/8 turn at a time !
7.5" to 8"Hg is a realistic goal BUT keep in mind "my suggested" 1 initial turn out on the adjusting screws. If you don't pay attention to your adjustments, you'll end up with one that is too tight (Again?) !
Hope this makes sense and wasn't bouncing all over the place.
