Originally Posted By: wag123
I beg to differ 440. I have a LOT of experience in this area.
Ported verses manifold vacuum control of the vacuum distributor advance is a much discussed and widely misunderstood topic. From Lars Grimsrud... "Ported vacuum was used as an emissions control method to retard timing at idle (by eliminating vacuum advance) in order to reduce hydrocarbon emissions."
Just because the vacuum advance is plumbed to the carburetor does NOT mean that it is ported (throttle plate controlled) vacuum. In most cases, this is just a convenient place to put a DEDICATED
manifold vacuum port (note in a couple of the attached articles that they tell you NOT to run a "T" in another manifold vacuum line to get your vacuum for the distributor).
To back up what I said, here are some articles...
http://www.460ford.com/forum/showthread.php?t=117504
http://w2ner.com/distributors.htm
http://www.camaros.org/pdf/timing101.pdf
http://www.lbfun.com/warehouse/tech_info/timing & vacuum advance/vacuum_explained.pdf
http://www.lbfun.com/warehouse/tech_info/timing & vacuum advance/Vaacuum_Advance_Specs.pdf
There is a wealth of additional information on the internet about this topic.
I can attest to what they talk about in these articles from personal experience over the last 45 years. There is one thing mentioned in one of the articles that I disagree on, I have found that you should NOT have full vacuum advance at idle. This can cause problems like the OP is experiencing. Most vacuum diaphragms have enough of a response delay in them that they won't cause a idle fluctuation at small idle vacuum variations.
I can also tell everyone from personal experience that IMHO, on a carbureted street driven non-emissions-system-equipped high performance vehicle, you should ALL be using a vacuum advance distributor and you should ALL be using manifold vacuum to control it.
Don't know what to say, other than "my ~35 years of experience is in direct opposition to yours." Mopar NEVER used manifold vacuum. I have less experience with Ford, what I do have is similar. I indeed know that most carbs have a ported vacuum and a manifold vacuum fitting... but at least 80% of the stock setups I've personally dealt with used the ported vacuum nipple. I know for a fact that all the following used ported vacuum from the factory: '49 Plymouth flathead, '66 Plymouth 361, '66 Dodge 383, '65 Chrysler 300L, '69 Coronet R/T 440, '69 Ford Ranchero 302, '73 Satellite 318 (with OSAC- Orifice Spark Advance Control- added for emissions), 74 Mercury Comet 250 1V, 74 Dart 318 (with OSAC added for emissions).
OSAC was a driveability and fuel economy nightmare intended to reduce NOx. It took ported vacuum as the input, and once the engine was at operating temperature it routed the vacuum signal through a check valve with a TINY pinhole so that it took many seconds for the distributor advance chamber to pump down, but the vacuum would all be dumped very quickly when the vacuum signal went away. So a short stoplight-to-stoplight run would never let the vacuum advance engage at all. It pretty much took vacuum advance out of the picture for city driving, and you only got full advance during steady cruising on the highway. And one blip of the throttle would dump all the built-up vacuum and you started over again. Not to mention the fact that the orifice would clog easily, which just killed the vacuum advance ALL the time.
Incidentally, there's one other thing that's dead wrong in the article you quoted. It says that using ported vacuum is an emission control device used to lower hydrocarbon emission. Using ported vacuum at idle lowers NOx, and actually RAISES hydrocarbon emissions. Running a ton of advance at low RPM creates higher peak combustion temps, which directly contribute to NOx emissions. Simlarly, a later spark can result in less complete combustion, raising unburned HCs. So its really not an emissions winner either way.