toss a carb kit in them. The kits come with bowl gaskets. The bowl-to-body joint is sensitive to seeping gaskets, and by now all the original gaskets are long flattened out/shrunken and they rarely seal. This will also cause "flooding". Makes no sense, right? On a lot of Yamaha's the fuel inlet to the carb is in the bowl, there is pressurized fuel that runs up through the bowl and into the main body, where it's then fed to the float valve assembly. If the gasket between the bowl and body leaks, there is nothing to shut the fuel flow off and it will overfill the bowl causing excess fuel usage hard starting and occasionally external leakage.
I don't remember if the old C40 twin cylinder used that style, but if you are updating the entire fuel system, there is no reason not to do the carbs too. Make sure to synchronize them after reinstalling them and check/adjust the timing, idle, pickup, and maximum advance.
One of the carb mounting nuts isn't much fun but not impossible either.