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Backfiring is usually an exhaust leak; bad muffler or pipes. Lower compression on one side could be from between the cylinder and head. Best troubleshooting method is a leakdown test. Otherwise, let it idle and lay down beside the engine and listen for an exhaust noise in that area. Compare noise on both sides, if louder on the low compression side, you probably have a head leak. Broken head stud on a 912 or 356 engine is EXTREMELY rare.
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