TOPIC:

Carburetors are 100% after ultrasonic cleaning. Pilot screws 3 1/2 turns. Fuel level 7,5 mm. All parts original Z1300 A1 1979.
Spark plugs (all) white - too lean mixture, I think. Engine running very good at idle and at all modes except if throttle is little open during road. In this case engine running not smoothly around 3500-4000 rpm - typically during gear change without load when driving on a flat surface. Where to find problem?

 The arbitrary 3.5 turns should be considered a starting point, only.  Try setting the pilot needles slightly richer @ 3 3/4 turns. 
If you don't have a "Gunson ColorTune".....consider getting one.
 

I have to ask, is this a new problem you discovered after cleaning the carbs or did you buy this bike, pull the plugs and find white plugs? The answer to this question may or may not lead you in a completely different direction as to what to look for.

We often get drivability questions with no history on the bike and then struggle to figure out what your problem is.

Spark plugs- Are they BPR6ES or something else?
Is it running white hot on all 6 cylinders or ????
When you reassembled the carbs, did you install the needle jets with the circlips in the middle slot?
Did you remove the emulsion tubes from the carb bodies prior to ultrasonic cleaning?

Question is where to buy Gunson ColorTune. UK schop will send to UK only. Amazon etc. have not any for sale.

I studied all discussions about carb issue before cleaning. Before were the same symptoms as after cleaning. No progress.
The same problem was before cleaning. Removed and separate cleaned were all parts.
Spark plugs NGK BP6ES.
Needle jets are in the middle slot.
Engine running very good, only when is little open throttle around 3500-4000 rpm.
I know, that i must recleaned three holes again... Or?

Bartus wrote:

I studied all discussions about carb issue before cleaning. Before were the same symptoms as after cleaning. No progress.
The same problem was before cleaning. Removed and separate cleaned were all parts.
Spark plugs NGK BP6ES.
Needle jets are in the middle slot.
Engine running very good, only when is little open throttle around 3500-4000 rpm.
I know, that i must recleaned three holes again... Or?

The 3 holes- You're talking about the transition ports at the butterflies. Yes, Likely the third hole maybe blocked on one or all of the carbs. I suspect on one or two of the carbs. Cleaning of the transition ports can only be done by disassembling the carbs and back flushing the transition ports. They are .020" in diameter and the smallest point that can choke up in the carburetors when debris gets in there. Scotch's cleaning tool was specifically designed for back flushing those ports and to do it properly, the pilot jets have to be out so that any debris that got stuck at the outlet of the transition port can be blown backwards through the passages and cleared out at the point where the pilot jets screw in. Engines that stumble at 3000-4000 rpm and light load  are suffering from a problem at the transition between the low speed circuit and the main circuit.
If the transition ports are clear, then enrichening the idle circuit can help with the transition so opening up the air bleed screws to 3 3/4 turns may help.

It still doesn't answer the question why you have white plugs. Running carbureted engines with white hot plugs is running too lean. That's because carburetors are not designed for changes in barometric pressure so on days when the barometric pressure is low carbs will run rich and when the barometric pressure is high, carbs tend to run leaner. Jetting for a happy median is the goal. That's why on race day, factory teams may rejet carbs for that race day.
If it was a fuel injection engine with a closed loop system monitoring the O2 of the exhaust, running white hot plugs or really light brown plugs would be the goal.

If the throttle needles are in the middle slot, I wonder if a previous owner changed out the main jets for a leaner set of jets. The stock jets and throttle needles in the middle slot have been the standard. Some have tried rejetting and ended up going back to stock jets.