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correctly. 8. WORN VALVE GUIDES: When the needle oscillates over about a 13 kPa (4 in-Hg) range at idle speed, the valve guides could be worn. As engine speed increases, the needle will become steady if guides are responsible. 8. 9. WEAK VALVE SPRINGS: W hen the needle oscillation becomes more violent as engine RPM is increased, weak valve springs are indicated. The reading at idle could be relatively steady. 9. 10. LATE VALVE TIMING: A steady but low reading could be caused by late valve timing. 10. 11. IGNITION TIMING RETARDED: Retarded ignition timing will produce a steady but somewhat low reading. 11. 12. INSUFFICIENT SPARK PLUG GAP: When spark plugs are gapped too close, a regular, small pulsation of the needle can occur. 12. 13. INTAKE LEAK: A low, steady reading can be caused by an intake manifold or throttle body gasket leak. 13. 14. BLOW N HEAD GASKET: A regular drop of fair magnitude can be caused by a blown head gasket or warped cylinder head to cylinder block surface. 14. 15. RESTRICTED EXHAUST SYSTEM: When the engine is first started and is idled, the reading may be normal, but as the engine RPM is increased, the back pressure caused by a clogged muffler, kinked tail pipe or other concerns 15. When vacuum leaks are indicated, search out and correct the cause. Excess air leaking into the system will upset the fuel |