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A couple of suggestions:
1. You may have a bad or dirty carb needle and seat. In that case, gas may leak past the needle and seat after the car has been turned off. The gas will flow into the cylinders and then seap past the piston rings.
2. Your fuel pressure is too high causing problems similar to no. 1.
3. Are you running a fairly radical cam? If so, the idle vacuum may be less than the rating of the power valve in your carb. That will cause the power valve to be open at idle. Most people compensate by opening the carb butterflies so the engine gets more air. That prevents the engine from dying at idle but it will cause it to run very rich and it will allow fuel to work it's way past the piston rings. This is easy to fix. Hook a vacuum gauge up to figure out how much vacuum you have at idle. Install a power valve that is rated 1.5-2.0 inches greater than your measured vacuum.
Good luck.
Chris
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