Whats the best and easiest way to set the carbs on a 1990 yamaha radian yx600

Your problem is probably the sycronizing as long as you havent messed with the fuel screw or jetting. If you dont have a set of vacuum guages you can eyeball the slides in the carbs to at least get you by and you can usually get them pretty close. Find the carb that is directly linked to the cable. the other carbs are adjustable to that carb so you need to make all the other carbs identical to that one. Look at the gap from the bottom of the slide to the bottom of the bore of the carb on that base carb with the throttle at idle bike not running. With the adjustments on the linkage to the other carbs adjust the other slides to the same height as that base carb as close as you can. Measure if possible to get all the carbs as close as possible to each other. you may have to adjust the idle after. If the adjustments on the carbs are all cables, for example there is a cable to each carb directly connected to the slide just adjust each cable accordingly and achieve the same as above. make sure you have a little freeplay at the throttle when you are done. you can adjust that by the cable adjustment by the throttle which will keep all the carbs in sync