Intake valve failure

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Have you done a cylinder cutout test?

To be sure it's a valve issue, I would remove the exhaust manifold and inspect the ports. A cylider with a failed valve or seat should have signs of fuel as there is very low compression. While the manifold is off - Rotate the engine so that the suspect cylinder has exhaust open and intake closed, then pressurise the intake and see if it leaks out the exhaust. Do the opposite if you suspect an exhaust valve failure.