Checking Vacuum Dependent Timing Advance

Check the vacuum advance only after determining that the rpm dependent timing advance is correct.
1. Start the engine and increase the speed to approx. 3400 rpm.
2. Record the timing advance and let the engine return to idle.
3. Remove the vacuum advance hose from the knock sensor control unit.
4. Recheck the timing advance at 3400 rpm.
5. The timing should advance approx. 6° more with the vacuum hose disconnected.
6. If the timing advance is not within specification, check the vacuum hose for kinks, blockages, or leaks, and repair as necessary.
7. If the vacuum hose is not at fault, the knock sensor control unit may be faulty.