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Bad synch results in noticeably rough running at small throttle openings. This refers to airflow through the carbs, though, not mixture imbalance.
Remove air filters, put your head down in the engine bay and listen to the carbs at idle. You can hear quite accurately if one is drawing more air than the other. Balance them out with the idle speed screws at the outboard top on each carb, maintaining a civil idle speed, until they both sound the same.
Then work the throttle gently by hand. You want to see the pistons on both carbs rise at the same instant. Adjust the cranks on the shaft that goes between the carbs so they do that (8mm nut driver is handy here).
Your best indicator of mixture is to examine the spark plugs. Look at the center procelain insulator -- electrode color has zero meaning, contrary to eons-old popular belief. If there's any soot on the insulator, you're running too rich for both best power and best economy. Richen until you get a little ring of soot at the base of the insulator, then lean until it just goes away.
If it doesn't look like that in all situations -- gets sooty at idle but look right after a freeway drive, for instance, or vice versa -- you have the wrong needles.
A stock B18 does well with ZH needles, and a stock B20 SM needles. KD and DX are more for high-performance motors, and will run rich on the highway in a mild motor if adjusted properly at idle.
Of course it does help to have the fuel level the same in both float bowls. Not saying you don't, but it's hard to have confidence in it if you have two styles of float that adjust differently.
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