iginiton timing questions

A lot of your questions depends on whether it's a stock dist or one that has had the advance diddled with

Some stock, expecially '72-3-6, etc, "smog" distributors, can have a loooooonggg slow advance, and be slow to "kick in." Some of those started with a VERY low initial timing, sometimes zero or on some cars, even AFTER TDC

Also, some of us set (check, recheck) our timing at a high RPM and just set it for TOTAL mechanical advance. In my own case, I don't presently use a vacuum advance.

So it depends on the "curve" that is in the distributor

So you have:

low idle speed, before there is any vacuum at the port (should be disconnected to check timing

and low enough idle that the mechanical advance is NOT activating

what you have there is known as "initial" advance. This is the figure that is specified on your engine "smog sticker" if you have one

NEXT, is "when does the mechanical advance" start to kick in? This depends on the SPRINGS that are installed.

Next, is once the mechanical advance DOES fully advance --and the RPM that happens at depends on the SPRINGS......

that is "total" advance if you have not vacuum advance, and IF you have vacuum advance, this figure will be when you have your foot planted on the floor --because ever if you run vacuum advance, there will be none under this condition.

The amount (degrees) of mechanical advance depends on the length of the slots which the mechanical weights work "in." These slots, on a stock dist, must be ground, brazed, filed, shortened (patiently) and usually by the use of a distributor machine until you "get" the advance curve you want. Many "performance" distributors, including the one I have, have screws or other methods by which you can "easily" change the amount of mechanical advance

The above, "foot on the floor" and engine at high enough RPM that the advance is "all in" is the "full power" condition, and will be the "initial" advance plus the MECHANICAL advance.

Now, in addition to the above, when you are cruising down the road at a high enough RPM to generate vacuum at the port for the advance can, AND have a light enough foot on the throttle (not going up a steep hill, passing, or just "lead footing")

The vacuum can will ADD (depending on the can) 10* or so

So the ultimate "TOTAL" advance is INITIAL + MECHANICAL + VACUUM

You can plot your dist curve yourself by either degreeing your balancer or buying the proper size "timing tape." Then use a tach and timing light to plot out what RPM gives you how much advance.

In my case, I have a performance Accel dist. that has somewhere around 22* "in the distributor" meaning the mechanical advance.

In my case, I twisted the dist, revved the engine, back and forth until I had 38* at high RPM. This means that at idle I have 38* -- the 22* in the dist, so my initial is around 16*. I don't recall offhand, how "fast" this kicks in, but it's much much quicker than a "stocker" smog dist.

Another thing you must be careful of, is that some dist. specs are specified in "distributor degrees" instead of "crank" degrees. The 22* I mentioned in my dist. are "crank degrees"