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The fuel metering block may be warped. Not an uncommon problem, generally the result of over-torqued fuel bowl screws. New gaskets often hold for a few thermal cycles but then begin to bypass and leak. If metering block warpage is the case, the block can be straightened in a press with careful application of pressure. The blocks are soft and distort easily. Fortunately, they also can be repaired fairly easily.
Convert to fuel injection instead of dumping your money into a crappy rebuilt Q-jet.
300 PPM of what?