It ain't from that (low comp in 1cyl). There is something else going on that's affecting multiple cylinders. Get down there & do a quick visual to eliminate the mtr mounts. check rotor phasing, blast out the 2 carb mixture screw holes with brake kleen, check the plugs & wires. Spray for vac leaks. EDIT Is that a solid lifter 273? Not sure if the lash was way off if that would do it otherwise if so I'd think it would of showed up on the compression test. Maybe someone can chime in if that is a potential