There are 3 ways.

1, You can remove the two screws just infront of the inlet manifold on the carb bodies (on the outer side of each left and right opposite of the oil feeds) and attach a vacume guage as you would on a normal bike.

2, If you have a late MC21 you can remove the air box and looking down the throat of the carbs you will see two drill marks (one on each of the slides). Open the cabs slowly untill the top of one of the drill marks touches teh top of the carb throat and adjust the other so its the same.

3, Get two drill bits the same size maybe 1mm dia. Open the carbs and slide the drill bits under the slides. release the throttle and just the balance screw untill both drill bits and being held. If one is being held looser than the other twiddle untill they are under equal pressure.
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If I have to take the carbs off once more...