Revision as of 04:11, 27 November 2006

Engine ignition timing (distributor timing) is the foundation of any tune-up. It is generally the first step you need to perform as all other tune-up and carburetor adjustments depend on ignition timing being correct.

If you have the distributor out of the engine, see Installing Distributor.

1 General Procedure
2 Ignition Timing Specifications
- 2.1 North America
3 Setting Dwell
4 Setting Timing with Timing Light
5 Setting Timing by Hand (for us 'po boys')
6 Vacuum Advance - To use or Not?
7 Fine-tuning the Timing - Relative Timing Marks
8 Advanced Timing

General Procedure

Check the gap on the plugs then put them back in. Spec is 0.028-0.032 for the A12. I recommend 0.035 for the smoothest idle.
Set the point-gap or dwell. Use a dwell meter if at all possible as it is much more accurate than the points gap, Set dwell 49 to 55 degrees. If the points are brand new, a feeler gauge can be used to set the gap to 0.018-0.021 in. Incorrect Dwell can cause rough running, as does bad points. For example, it might only run at high idle, and not very smooth at that.
Double-check the timing. Spec is 5 degrees for the US A12 and should be safe for any A12 engine. This must be done after setting the dwell -- because changing the dwell affects the timing, but not vice-versa.

Ignition Timing Specifications

North America

North America
static timing	5 degrees BTDC
point gap (new points)	0.018-0.021 in
dwell (new or used points)	49-55 degrees
Spark plug gap	0.28-0.32 in

Setting Dwell

Setting Timing with Timing Light

Setting Timing by Hand (for us 'po boys')

If you don't have a timing light ...

Vacuum Advance - To use or Not?

Fine-tuning the Timing - Relative Timing Marks

Note the timing marks on the A12 plate.

Advance and Retard marks

These are NOT used to time the engine (the marks on the crankshaft pulley are). But they are very useful for advancing or retarding the timing a degree or two. After installation, you can look down past the #1 spark plug, between the engine and the distributor to view the marks.

Advance it two degrees (turn distributor clockwise), test drive, repeat until it pings at part throttle
Retard it one degree (turn counter-clockwise) to remove the pinging

Once you have this, you can note the crankshaft timing and next time just set it using a Timing Light.

Advanced Timing

Base timing

Tuning for Power

Dialing in Distributor Spark Curve

Mechanical advance

Vacuum advance

Retarding, for boosted engines

Fully electronic spark timing

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Ignition Timing