How To Balance / Sync Mikuni VM / TM Carburetors
Two or more Mikuni round slide and flat slide carbs are simple to balance and syncronize perfectly with no carb sticks or other vacuum guages.
Insert the smooth end of a drill bit from the intake side under the slide throttle valve cutaway and set the idle thumb screw so that it just drags on the smooth end of the drill bit. Use the same drill bit on the other 2, 3, or 4 carbs depending on the number of cylinders and set them the same as the first. If the motor idles too high, use the next smaller bit, or if it idles to low use the next larger drill bit etc. Do not do this with the motor running.
Once all the idle positions are set, you can easily set the slides to all lift together at the exact same time, and be at identical slide position heights at part throttle and cruise etc. for superior performance by using only your fingers. On the twin cylinder XS650 I put my middle finger on the slide on the left carb, and my thumb on the same hand on the slide in the right carb. Since you are pushing in and feeling both slides at the same time use your right hand to simply twist the throttle and feel when each slide is lifting. Now adjust the throttle adjusters on the top of the carbs so that you can feel both slides lifing at exactly the same time.
On multi-cylinder bikes you set all the idle positons as explaned above with the drill bit that idles the motor at the rpm desired. Then use your middle finger and thumb on the same hand to check carbs number 1 and 2, then carbs 2 and 3, then 3 and 4. So you have all of them set the same by using the previous carb. Also, when they are all synched the same, when you open and close the throttle it will sound like one click. If they are out of sync you will hear the slides hitting at different times. click click click etc as they return to idle, instead of one solid thump/click.
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