Maybe you should ask the owner's boyfriend how to synch the carbs..... on a bike whose carbs doesn't have a vacuum plug. At least not in the K4, I don't know in the model fitted with disc.
Nevertheless, you can make a pretty accurate synching following the instructions of the shop manual. Remove one of the plug leads, start the bike and adjust the butterfly screw of the opposite carburetor until the bike idles at, say, 1200 rpm. Do the same with the other plug lead and carburettor. Now, plug both leads. The bike will idle at a higher rate, so with the cable adjuster you reduce the idle up to the desired level.