Balancing the carbs is a very important part of making the bike run as it should and smoothly. Unbalanced the bike will get poor gas mileage, be hard to start, be ruff and vibrate and even make unnatural noises. This is the key to making an Airhead run. It is some what an Art, maybe more Art than science. That can't really be true because these are machines but it's a fact that some tuners are better at it than others and it's hard to say why.
This is one method of balancing carbs. It involves shorting out the spark plug leads so the bike tries to run or does run on one side. You need this tool. You can buy it or you can make it yourself;
Sorry the picture is so small. These are for sale at Northwoods Airheads;
To make a pair of these tools use a long 4mm screw from the neighborhood hardware store. The screw is 4 x .7 and should be 60 or 70 mm long. The nipples that came with a new pair of spark plugs go on one end and you will also need a 4 x .7 nut to lock the nipple on the end. Leave enough thread of the nipple open on the end so the rod can be attached to a spark plug. The head of the screw is cut off so the plug wire can be attached to it. Tighten it down a little with a wrench, it will vibrate loose if not tightened. It is important that the plug wires do not become unconnected when the bike is running. They should be firing the plugs or be shorted to ground to not fire the plugs but they should not be just unconnected, that can cause ignition problems.
Once the bike is warm and the studs are connected with the bike at idle you short out one plug with a screw driver so the bike will run momentarily on one cylinder. Note the sound and speed of the bike on one cylinder. Take the screwdriver away and rev the throttle a little to clear pooled gas in the intake and then short out the plug on the other side. Note the sound and the speed of the bike on the other cylinder. If one cylinder is running faster than the other adjust the speed screws a little, either one side up or the other side down depending on whether you want the bike to run faster or slower over all, repeat. Do this several times back and forth till you have the idle speed you think will work and the carbs are balanced. These adjustments are made with the idle speed screws only.
After the idle is set the throttle cables are adjusted so the throttles both start pulling at the same time. This is sometimes done with other machines at 1400 rpm or 3000 rpm. Some riders try to set the higher rpm using the shorting rods. I just try to get both butterflys moving at exactly the same moment. Note the amount of slack at the throttle cable barrel on top of the carb where the adjustment is made. In theory both cables should have the same amount of free play. If you pull on the cable sheath at the top of the carb you can see it is free till it pulls the actual cable inside the sheath. You can see the free play by tipping the sheath ends to the side until they start to pull the throttle. Anyway the free play should be even and it should be as small as possible.
Go for a longer ride. You may have to repeat this process several times. Once the speed is stable and the bike idles you may now adjust the mixture screws a little but they should probably not be far off from the setting in the book. For your bike the book calls for 1 turn out on the mixture screw.
Do not ride the bike for any distances with the shorting rods attached. They will vibrate loose and cause ignition problems. I think your bike has an after market ignition of some kind? This is a very important warning for these ignition systems.