How to adjust the idle speed

There is a plate near the pivot for the thottle where the throttle cable is attached. There should be a screw that strikes the playe near the throttle pivot which is the idle adjustment. The farther the screw is in, the higher the idle. Some things that can cause a high idle besides this screw being incorrectly adjusted;
Check the idle screw and make sure it is triking the throttle plat. If it is not, you might be missing the throttle return spring.
You either have a cble or electrical choke to actuate the choke plate which is a plate that closes over the primariy intake side of the carb intake and has a linkage that actuates a choke cicuit in the carb to make it raise the idle. If the cable does bot fully open the plate it is going to make the idle higher. If it is electric, the plate should be closed and the linkage as well wheb operating temperature is reached. If not it may need replacing.
One of the most difficult things with a dual carb setup is sinking the carbs with the linkage that attaches the thottle plate pivot arms together. One carb affects the others performance. So, if the linkage is to tight and is opening the second carb more than the first, things linke the idle adjustment will be off as this will hold the second throttle open. The other idle issues can affect the second carb as well depending upon the setup. So, you have to make sure that the linkahe is adjusted so that at closed thottle both carbs ar at the same plate abgle. The easiest way to do this, unless the carvs are in bad shape, is to adjust them at wide open throttle and make sure they hit the wide oppen thottle ponit at the same time working off the carb that the throttle cable or linkage is attached to as the primary and adjusting the other carb by the connecting linkage to open at the same time the primary does.
The other thing that can affect idle is the idle or air screw for the gas. They are generally founf on the base plate and are small screws coming out. The best way to adjust this is tp get the best vacuum at idle by changing this screw with the throttle plate at the lowest bagle possible, meaning having the idle screw is far in as possible droping the idle as much as possiblr. You can play with this while changing the low end or idle jet as aforementioned for vacuum with the lowest throttle plate abgle possible. Again this is a challenge with dual carbs as you have two setups to adjust to achieve this at the same time.
So, it can get very complicated even if you know where the adjustments are. If you are not sure, go to the carb manufacturer website as they usually post owners manuals or setup guides for every carb they make. They might have dual carb setup tips as well.