That looks like solder rosin fluxThis is why we clean float bowls, run them dry, and exercise equipment regularly. This showed up at a shop I used to work at, "It won't start, can you take a look at it." This is extreme obviously and took a long time but it gets the point across. Anyone who disassembles and cleans there own carbs should buy a cheap welding tip cleaner kit. Very stiff wire in various very small diameters, perfect for getting into carb passages.
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