Most cases of backfiring on a 440 are due to old carb boots not sealing. Buy new ones. The oil out the breather would be your piston rings are not sealing due to excessive clearance between the pistons/bores OR they are just shot OR the cylinder walls are oil fouled and glazed and need to be honed and new rings installed. You could also have a cracked ring or a chunk of the material between the rings missing.
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wiredgeorge
Texas Hill Country
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