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A little known fact --- at the compression pressure you have ( around 200 lbs ) not knowing the valve spring pressure but my guess is 120 lbs on the seat 275 / 300 open you could have a slightly waffled valve head caused buy a vacume leak or not enough timing or too much -- or just carbon build up the gas today is different -- ( you could have a bent or not seating valve and the compression test will not pick it up ) even a leak down might not as well we just finished a 69 350 HP 396 and that was the case with it -- compression pressure and spring pressure can seal off an other wise incompatible valve seat combo -- i would exhaust all other possibilities before taking it apart -- dist advance - sticking 1/2 way out . vacume booster if any check valve -- poor coil out put , plug wires having to much resistance , incorrect plug heat range for the gas we have now ( full of alky ) and bubbles -- maybe this will help -- jz
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