Fundamentals of ignition timing

[Yeoman]

Installed Pertronix in my '71 1275 2 weekends ago, runs great. Hit some serious bumps in the road today, distributor jumped out of the block because the blockhead PO (Present Owner) forgot to tighten the clamp bolt upon completing the job. Got it running, went straight home. Ran better coming home than it has been. Set the timing as soon as I got home, and now I have these questions:

Idles faster when set by the book (10 BTDC, 1000 rpm, vacuum disconnected).
Idles smoother, less popping out the exhaust when set to 15 BTDC.

What will the consequences be of leaving it at 15 BTDC? Will it run quieter when retarded like that? Are the timing marks accurate on these engines? Any chance that the 15 mark is really 10 BTDC?

What would the consequences be of running it at 20 BTDC?

Thanks.

 

[r67cat]

Yeoman, the problem with too much advance is spark knock which can cause engine damage, the timing marks should be accurate. In the days before timing lights we would set the advance for the smoothest idle, and if the engine, under load, would knock we would just back off the timing until the knock was gone. Remember that you will get additional advance from the vacuum and the mechanical advance. I think the total advance is listed in the service manual, that will give you a factory reference. Rick