Sounds like raw fuel. By any chance you have the wrong metering block gasket? Like the one that has an extra notch above the tombstone pv cutout gasket for an accelerator pump crossover tube? My description is complicated, but the powervalve cutout in the gasket is tombstone shaped, some have an extra cutout on the top of the tombstone because some early cargs have a brass tube with 'o' rings for the accelerator pump shot, it joined the metering block to the main body. If you used this gasket then raw fuel will enter the cavity behind the power valve and into the engine via the power valve vacuum port. You can't see it run into the engine. When you have had the carb apart is the cavity behind the power valve wet?