Tuning with a vacuum gauge

Vacuum gauges are a cheap but effective method of ensuring that your classics are well tuned and problem free. The Vacuum gauge is a useful instrument to have on any classic as it can be used to tune for power, diagnose engine faults and for maintaining fuel economy.
This article will cover tuning, with articles still to come about how to diagnose common engine problems using your vacuum gauge.

Whichever gauge you choose whether its a test gauge such as the Gunsen Lo Gauge or a permanent instrument it will need to be connected to the car's inlet manifold. Often there is a connection or pipe already but if not it may be necessary to drill and tap the manifold.


One your gauge is connected and seen to be registering a vacuum you can begin the tuning process

(normal vacuum at idle should be about 18-22In (4 Cyl), 19-21In (6 Cyl) or 15-18In on low compression engines (In is an abbreviation which stands for inches of mercury))

  • Start by first warming the engine and note the idle vacuum reading
  • Adjust the mixture on your carburetor to obtain the highest steady vacuum reading.
  • Richen the mixture setting until the needle begins to fall. This will give you the correct mixture setting for maximum power.
  • Slacken the distributor clamp bolt, and with the engine still at idle advance or retard the ignition until the highest steady vacuum reading is obtained.
  • Retard the timing until the vacuum gauge reading drops slightly (½In).

Your engine should now be tuned to perfection. Tuning with the engine running compensates for wear in the timing gear and therefore will provide better results than the manufacturers ignition settings.

If you are having problems then click here on how to use the vacuum gauge to do fault diagnosis




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| Updated 26-Dec-2007 |