Avid flyer, ok here's what I would do,
Close the throttle completely with engine stopped!
Now back of both throttle stop screws off the stop, check the stop bracket isn't bent by excessive pulling on the throttle this is your idle setting screw so if both look the same screw the stop screw in until it just touches the stop bracket, now when both the same screw in both screws EVEN AMOUNT ie one and a half turns. This may not give you the correct idle speed however the can be adjusted later too achieve rpm for idle, but must both be turned even amounts start with half then quarter to set idle!
If you have a balancing kit great if not purchase a (twin max) electronic carburettor balancer, they are quite cheep run off a nine volt battery and will indicate accurately which carb is out of sync .
1 disconnect the balance pipe of the carbs ,I remove one end,
2 attatch the small pipes from the balance kit I use a cut down bullet shell which pushes nicely into pipe connector
Now ensure prop clear set the balancer too zero and low sensortivity, watch the indicator when the engine starts and see which side is lagging stop the engine now Slacken the throttle cable and adjust forward or aft accordingly NOT THE SCREWS YOU SET EARLIER,!
When you have the needle remaining centred increase the sensortivity, and check the balance through the range there may be a slight difference so when happy, remove the kit ,re attatch the balance pipe,
3 start the engine ,check the idle, too low screw. The stop screws even amounts until your happy the idle is correct!
4 Please do not be tempted too adjust the carbs with the engine running the prop is too close always shut down and restart from inside the cockpit,
Hope that helps good luck these engines I think should be balanced more often,
Cheers