Marine carburetors are not emission calibrated so very generous for fuel delivery. It doesn't take much for them to pull fuel from the boosters and many big blocks will pull some fuel from there even when the carbs are correctly set up, but you would still have some control with the mixture screws to establish good idle quality.
Marine carbs didn't use idle bypass air either, so will pull fuel easily from the boosters if the throttle angle is too high or there isn't enough fuel at the mixture screws.
If you don't have any control at all with the mixture screws I'd start by making sure the idle system is clean and idle tubes have been removed and correctly cleaned/sized. If they are plugged up any at all the throttle angle required to get them to idle will cause most of or all of the fuel to come from the boosters, not from the idle mixture screw holes under the throttle plates.
A few other things that we've seen cause this issue are retarded ignition timing, stretched timing chains, or engines built at some point with an aftermarket camshaft that doesn't make enough vacuum at idle speed like the original Marine cams.
Using brass floats instead of the correct large Marine floats can be a problem, and too much fuel pressure as well.
Another problem we've seen in recent years is that all of the over the counter kits for them will NOT have the correct Marine high flow N/S assembly, and many don't have the correct Marine pump in them either. This may not be part of the idle problem, but for sure if you haven't used the correct parts to build them there will be other issues as well.......Cliff
