Join Date: Mar 2002
Location: Austin, TX, USA
Disconnect the vacuum advance and plug the line if you have one. Loosen the clamp on the distributor just enough that it turns with a little bit of effort - too loose and it will move on its own. Have a buddy do the cranking for you. Hold the timing light in one hand (set on zero), and turn the distributor with the other. If you are too far advanced, the engine will turn hard. Too retarded and the engine will crank freely and act like it wants to fire but not run well. Set it at about 10 or 12 degrees and get it started. (You can also preset the light to 10 or 12 and adjust for 0 on the timing mark).
Once it is started hook your vacuum advance back up. Then turn the knob on your light until the timing mark drops back to the baseline setting (same mark as in the previous step). The reading on the light dial is the amount of vacuum advance. Should be about 10 degrees above baseline or 20 degrees advanced.
Then preset the light to about 32 degrees. Rev the engine to 2500 RPM or so and see if your timing mark is close to the baseline. Adjust the timing light to line the mark up on your baseline reading. The light now shows "all in" advance setting, provided 2500 RPM gets you all in.
Things to watch for is at what RPM does the mechanical advance start to kick in. It is a good idea to remove the vacuum advance line and plug it for this test. You don't want the machanical kicking in too close to idle or you're likely to have idle stability problems.
In general the engine will want as much timing you can give it as soon as possible without pinging. Sometimes pinging or detonation is hard to hear, especially with a noisey exhaust. The targets of 10* base, 20* vacuum and 36* total are rules of thumb that must be adjusted for each engine combination.
Use an adjustable vacuum advance can. There is a small allen screw inside the vacuum port that you use to preload the diaphram. This will control when the vacuum advance kicks in a drops out. You can also get stroke limiters for the vacuum advance if yours advances too much.
For mechanical advance, you'll need to fiddle with the springs and weights to tune it.
For the first few test drives listen for pinging. Check your plugs after a few hours of running time and look for tiny balls of melted metal on the electrodes and porcellin as a sign of detonation. Once you figure out what works for your motor, write it down.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI