In a compression test, low readings in two cylinders and normal readings in others could indicate what issue?

Low compression readings in two cylinders coupled with normal readings in others can indicate a blown head gasket. This condition can lead to decreased compression in the affected cylinders due to the loss of the sealing ability between the cylinders and the water jacket or combustion chamber. A blown head gasket can cause engine coolant or oil to enter the combustion chamber, which results in lower compression and, subsequently, engine performance issues.

While other factors can contribute to abnormal compression readings, a blown head gasket specifically affects the compression across the cylinders, leading to the scenario described in the question. For example, improper valve timing may affect the timing of the valve openings and closings, causing compression issues but would not typically lead to low readings in just two cylinders without affecting others in a similar way. Similarly, a weak ignition system would not cause low compression by itself, nor would clogged fuel injectors directly result in reduced compression readings; these issues are typically more related to the fuel-air mixture rather than the integrity of the cylinder seal.