You may have to do everything more than once anyway.
Some Dellortos have an "idle Bypass" screw (page 40 in Hammill's book) which can be used to balance the two throats of a twin carb.
If you are sure that you want to do this, since you said it had been setup on a Rolling Road this is how to do it, well at least this is how I did it.
Actually Rolling Road was probably more to do with higher speed stuff, jet sizes etc rather than idling.
This is for after your air-flow meter arrives.
Take the front of the airbox off so that you can get at the trumpets. Get pencil & paper to make notes.
Find a piece of plastic pipe you can use to listen to the flow in the trumpet, i.e. one end at your ear, the other end at the trumpet.
Get engine idling as exactly as possible at 1000 rpm or 900 RPM. When you make adjustments the idling speed may change, so you will need frequently to use the idle speed screw on front edge of the rear carb to reset it to your chosen RPM. If it is not at one defined speed your measurements will not be consistent. All airflow measurements for balancing should be at the RPM speed that you chose as exactly as possible.
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First you are going to record the existing state of the carbs as accurately as possible so that you can get back to your starting conditions if you make a right cockup.
Listen to each trumpet in turn and note down a description of the sound. A burbling sound probably means the idle bypass screw is closed and a slight hissing sound is added when the idling screw is partly opened. If one throat of a carb is burbling and the other is hissing slightly, the bypass screws have probably already been set correctly. One bypass screw should be closed (in my opinion) and the other used to balance the throats.
Use the meter to measure the air flow rate in each trumpet and write it down.
Stop the engine then
for each idle bypass screw, gently screw tit down in turn untill it 'bottoms' , while counting the turns accurate to a quarter turn. Write it down, then screw it back up that number of turns, back to where it was.
Repeat this turns-measuring process for each of the 4 idle-MIXTURE screws. WRITE THEM DOWN..
There is no convenient reference point for the screw with opposing springs that sets the relative position of the throttle spindles of front & rear carb, so just leave that alone for the moment. It was, for me, the most difficult thing to get right, but with the flow meter it is dead easy and it is the last adjustment you make.
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OK. Now if the flow rate between two throats of a carb differred by more than one mark on the scale of the meter you probably need to set the bypass screws for that carb.
It's worth a quick try with the existing mixture settings at first.
If the carbs are already self-balanced (both throats showing the same airflow) skip this and go set the two-spring central screw as described below..
Just do one carb at a time, don't worry about balancing front carb to back carb yet.
If one bypass screw was closed, leave it closed and adjust with the other one. If both were partly open close one. At the chosen RPM, measure the flow in the throat with the closed bypass screw, then transfer the meter to the other throat and adjust the idle-bypass screw until the flow is the same. Re-adust RPM if necessary. If you cannot get them the same, swap throats, close that idle-bypass screw and remeasure it and try to set the other throat to match instead.
Then do the same to the other carb.
Now each carb is self balanced and those idle-bypass screws will probably not need to be adjusted ever again.
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Then use the central two-spring screw to get the same air-flow on trumpet 3 and trumpet 2. The spindle stays fixed for the rear carb and moving the screw rotates (slightly) the throttle spindle of the front carb. However you will need to adjust the IDLE RPM screw on the front edge of the rear carb to get back to your set speed .
You will need to keep swapping the air-flow meter between trumpet 3 and trumpet 2 as you make this adjustment but you should fairly quickly get them both the same. Do a final check of trumpet 1 and trumpet 4 they should now be the same figure.
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OK, now use the colortune in each cylinder in turn and use the relevant idle-mixture screw (not the idle-bypass screw) to set the idle mixture at your chosen RPM. You want a nice blue colour with pretty sparks of yellow and a flash of all yellow when you dab the throttle lever on the carbs. You will probably need a cardboard tube from an empty toilet roll surrounding the colortune plug so that you can peer down in to see the colours.
When you have done that put the airbox on and go for a brief run to see if all is improved. Then come back and do the balancing all over again because changing the mixture may have slighty changed the required perfect balance settings.
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Fun ennit ?