Trouble-shooting with a vacuum gauge

Idle (Normal Vacuum)

An engine in good condition should have a gauge reading of 17 to 21 and hand should be steady. 

Idle (consistent but very low reading)

A steady reading below 5 indicates a leaky manifold, manifold gasket or carburetor gasket. Also check heat riser.

Idle (consistent but low)

  When gauge needle is steady at about 8 to 14 it generally indicates incorrect valve timing. 

Idle (consistent but mildly low)

  A reading lower than normal with the hand steady indicates that rings are in poor condition.

Idle (consistent but mildly low reading)

If normal is 20 inches and needle should go to approx 14 check timing. Spark may be retarded.

Idle (consistent but slightly low)

A reading of 13 to 16 generally indicates incorrect ignition timing. 

Idle (with random mild drop)

When the hand drops occasionally from 3 to 5 points, from the normal reading, it generally indicates a sticking valve.

Idle (with consistent mild drop)

 A burned valve will cause the needle to drop back several divisions each time that particular cylinder operates.

Idle (with consistent slight drop)

Leaking valves also show up on the gauge by the needle dropping back 3 or 4 divisions, whenever that valve opens.

Idle (with occasional mild drop)

Occasional drop as cylinder is firing may denote valve open or plug not firing.


Idle (constant mild drift)

  Poor adjustment of the carburetor will cause the needle to float slowly between 12 and 16.

Idle (mildly low with constant small drift)

When needle drifts slowly between 14 and 16 it generally indicates that plug gaps are too close, or breaker points are not properly synchronized.

Idle (with constant large fluctuations)

This action usually indicates a leaky head gasket.

Idle (with fast vibration)

When needle has a fast vibration between appoximately 14 and 19 it indicates loose valve guides.

Cruising (big variations)

Wide variations of needle increasing with motor speed indicates weak, or broken valve springs.



Opening and closing throttle quickly

reading on an engine in good condition should drop to 2 and come back to 25.

Idle (slow steady drop)

Normal reading at start, but gradually drops, indicates choked muffler.