Bein' paranoid... ignition advance at idle. :)

[JVA]

I have my ignition set to 5 degrees BTDC with the vacuum advance disconnected and plugged. With the advance plugged in, advance jumps to 31 degrees. Is this normal, or is my vacuum advance canister is weak and excessively advancing timing? 31 degrees at idle seems quite, well, advanced.

Much thanks!

[Bobby B]

You need to specify the year, model, engine, etc…..
                                                                 Bobby

[Philippe M. Ruel]

+1 with Bobby. Depending on brand/year/model/engine, vacuum line has to be plugged to "full vacuum" (intake manifold/carb base) or to carb "timed vacuum" plug. To both on some dual advance systems.

The former gives too much vacuum (and advance) at idle if the advance unit is supposed to be plugged to the latter.

[R Schroeder]

My 78 is 37 BTDC at idle, with vacuum advance connected.
This is a factory setting. Again they would need to know which car you are talking about.

[James Landi]

Seems normal... you should have about 17# of vacuum at the manifold at idle  (again may vary some but that's typical for a V8)  less vacuum, or fluctuating, or jittery vacuum indicates possible programs.  Newer models had vacuum advance shutoffs (as mentioned in prior posts)... and the cutoff devices were tuned to engine speed.  IMHO, I enjoy the direct vacuum advance set up to the manifold.  The throttle always seems (to me) to be very crisp.   Happy day, James

[Jason Edge]

Engine, Vacuum and Centrifugal ignition advance has always been one of my areas of interest on the pre-electronic ignition cars as the total Engine + vacuum + centrifugal advance can seem confusing, however, not too hard to understand if you dig into it a bit.  Lucky for the Cadillac LaSalle Club Members John Washburn did the digging and presented this info in a concise easy to follow manner in the Jan & Feb 2014 Self Starter magazine which I keep a reference to on our 1963/64 Cadillac Chapter Help Page (www.help.6364cadillac.com).  It really is going to depend to your year, body style, engine.

Here is our entry, in part, referencing John's excellent 2 article on the 63/64 Chapter site:

Understanding the basics of max vacuum and centrifugal advance, total advance, manifold vs ported vacuum, and how it all works to make your engine run smoothly and efficiently is a complex topic, but have to say that John Washburn's 2 parts look at this subject in the January and February 2014 Issues of the CLC Self-Starter magazine provides an excellent overview.

In Part 1 in the January 2014 Self Starter, he covers vacuum advance includes the part # for the 1956 to 1963, and 1965 to 1966 Vacuum Advance (part # 111 6168 in groups 2.410) but skips over the 1964 one year only vacuum advance (part # 111 6237).

In Part 2 in the February 2014 Self Starter he covers all 3 elements of ignition advance including engine timing, vacuum advance and centrifugal/mechanical advance and how they affect performance. Both Part 1 and Part 2 include charts and stats and illustrations up through 1962 which is applicable to our cars since the 1963 distributor and advance where used for previous years.

[TJ Hopland]

We will have to see what year he has.   

I think a lot of the 425's (77-79) ran direct manifold vac so that could be normal.  Many other years in the emissions era (66+) had some sort for thermal switch that switched between manifold vac source and a 'timed' or 'ported' source on the carb depending on conditions.  Some years in that era had other do dads to alter the vac signal like 70-72? had the speed sensor things.    Its fairly common for people to run direct manifold for the distributor and many times it does work well that way.   

[JVA]

Um, yeah, I can see how the specific model could be important. Oops.

It's a '67 429 in a Fleetwood 75. The FSM says 5 degrees at idle with vacuum unplugged - that's right. I was just curious if 31 degrees with vacuum was normal.