If you currently have a vacuum operated petcock and switch to a manual type, be sure to cap the vacuum port of the carb (where the vacuum line from the petcock attaches) otherwise that carb will suck air in through the vacuum port and may cause a lean condition on that cylinder.
Regarding the leaky carb bowls, you probably need new gaskets. Be very careful not to over tighten and strip the screws trying to stop the leak with old gaskets. I never pull my carbs just to remove the bowls. I use a screwdriver bit in a small ratchet (see pic below). Using a mirror helps too. Ed
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