Basic Ignition Timing

 

This data applies to a standard points-type ignition system. GM's HEI will follow the same basic routine. If you have an aftermarket units with a spark booster and/or electronic ignition system, you need to consult your information supplied by that manufacturer.

 

 Ignition timing is a term applied to the relationship of piston travel and moment of spark in the engine. Due to many variables involved, such as compression ratio, humidity, elevations, fuel octane, engine condition, work load, etc. published timing data must be considered approximate with some tolerance permitted.

 

Ignition timing consists of basic and dynamic timing. It is important that point dwell be correct before timing an engine.

 

Basic Timing

 

Basic timing can be checked quite accurately with a timing light. A timing light, when properly connected to the number 1 spark plug wire and the battery. On a Chevrolet engine the crankshaft balancer will have a 'timing mark' or groove. To better see this groove with the timing light you might want to paint the groove white or bright yellow to make it easier to see with the strobe from the timing light. Typically there is a tab on the front engine cover with several marks for degrees of advance like this aftermarket tab. Note the '0' mark and several marking on the 'BEFORE' side of the '0' for degrees of advance or 'before top dead center' (BTDC). Because ignition timing is affected by the points setting the points need to be set properly. See Timing Chain/Cam Gear Timing Marks/Harmonic Balancer for setting cam and crank gears.

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Unless otherwise stated by the distributor manufacturer, the distributor control vacuum line should be disconnected, and plugged, to prevent fuel induction disturbance. Loosen the distributor just enough that you can move it with minimal effort.

 

Connect the timing light according to the manufacturer's instructions.

 

With the engine at operating temperature run the engine at low idle; you can use a tachometer to adjust the engine speed. Many timing lights have various functions built in such as a tachometer, voltmeter, dwell angle measurement, etc.

 

Shine the timing light on the balancer mark and timing tab. Rotate the distributor until the timing tab advance desired matches the mark on the balancer. The mark on the balancer will appear to move up or down under the timing lights' strobe light.

 

When the timing is set to the desired advance, carefully tighten the distributor hold-down clamp then recheck to insure the timing hasn't changed.

 

Dynamic Timing

 

To accurately check and calibrate dynamic timing through all attitudes of operation, a more sophisticated timing light is needed that can measure total timing at a high rpm. These timing lights can run from $75 for a basic digital unit to a little over $100. If you work on your cars a lot a good timing light is a good investment.

 

Vacuum Control

 

When timing an engine the vacuum hoe is disconnected from the distributor and plugged. You might want to check the tech article Timing 101 by GM and Chrysler Engineer John Hinckley on ignition timing.

 

Checking Vacuum Control

 

Connect a timing light and tachometer as usual.

Connect a good vacuum gauge to the vacuum line between the carburetor and distributor.

With the engine at operating temperature and timing adjusted, run the engine at low idle.

Shine the e timing light on the indexed area and observe the vacuum reading and timing light index relationship.

Compare these readings with your engine's published vacuum advance data.

You also might want to take a look at the "How to Read A Vacuum Gauge.'

 

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