Replace the carb to manifold gaskets, do not use old gaskets. Double check you have ALL the remnants of the old gaskets removed from both surfaces. Carefully clean with a razor blade, very fine sandpaper, or your finger nail. It is crucial that there is no gap for air to enter.
When reinstalling the carbs hold the new gaskets in place with a small amount of grease, so they don't shift while installing the carbs.
Make sure you have the throttle cable end in the first, not second hole of the linkage. Double check with a mirror to make sure the butterflies are closed when the cable is in the idle position when the engine is off.
Also check the o-ring on the main fuel filter bowl and replace if necessary.