Either kerosene, or mineral turpentine (which they call something else in the US, I don't recall what), is in effect extremely low volatility, low octane petrol. Neither substance will harm a carburetor intended for petrol, even with quite a long soaking. I don't know if you mean mineral turpentine when you say paint thinner. For example, traditional automotive lacquer ("Duco") was thinned by acetone, and you shouldn't put that anywhere near a carburetor that has any plastic or elastomers in it - and just about all of them do. There are other exotic solvents used for thinning some types of paint, none of which should be used on carburetors.
So, if you mean mineral turpentine, go ahead and soak the carburetor in it. It won't do much of a job of cleaning the internal passages, though, since it will not dissolve gum. And for Pete's sake don't try to run an engine on it - Joe Carroll has described what happened when he tried it once. You can't expect even a low compression engine to run for long on fuel of well under 60 octane. Joe's blew spectactularly and irreversibly.
