I have the same thing happening at idle at the red light. I also went from about 3 - 3.5 to 2 + turns but with regular plugs. I followed the colortune but is not a precise tool. I'm thinking about backing off to enrich a bit. I haven't found any other reason so far.
An accurate sync is critical to ensure the carbs are working in harmony and achieving a smooth/consistent idle. If gauges are being used to sync. it's important to know they're accurate at the sync. range. Never mind if they don't read "Zero" exactly. What's important is that the 3 gauges read exactly the same at an arbitrary 8" hg. To determine this the 3 gauges must be manifold'd together to a common vacuum source and have 8" of vacuum pulled. Don't be surprised if the three read slightly different at the intended 8"hg! If there's any error in how they read at the intended range - you must take this into account when sync'ing! If you don't then the carbs aren't sync'd and if they're not sync'd...... well ...........?
The easiest means to compensate for error is to start by choosing ONE gauge to be a "master". For example: Mark the gauge with the highest reading. Then mark the other two appropriately.
Example: Gauge 1 (master) : reads 8"
Gauge 2 reads 7.5" mark it +.5
Gauge 3 reads 7.25" mark it +.75
or mark them however you choose as long as you can remember to compensate.
From the manual:
This should be stricken from the manual.
#1 - The gauge needles can not bounce around !!!!! Period ! And certainly not "less then 3 cm". Pack cotton into your intake manifold adapters and keep cramming it in until the gauge needle reads without "bouncing".
2) Forget "Averaging". With a correctly dampened gauge and understanding your compensation factors you can achieve sync. to within the width of the needle points across all three. This of course requires patience and a fine touch on the adjusting screw and lock nut.
This will deal with the issue, if the issue originates at the sync procedure..
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !
I'm on the phone here so just a quick remark. First clean unobstructed fuel and air supply. Second good spark plugs set properly. Third correct valve clearances. Fourth correctly serviced and bench set carbs. Next proper synch. Then fine tune the A/F screws. Then, pray. Haha But not joking too much, proper valve clearances and carb synch are two very important steps that need to be dealt with before digging any deeper.