Vacuum Advance...Ported vs. Manifold...Thanks Rich Weyand

[BBM3]

I stumbled on a link posted by Rich Weyand.
http://www.hotrodders.com/forum/distributor-tuning-theory-part-1-a-59033.html

Here is a nice PDF version:
http://www.camaros.org/pdf/timing101.pdf

I have a non-emission controlled 350 with a previous owner installed Edelbrock 1406. The PO connected the vacuum advance to the ported tap on the 1406.
I followed the advice of Rich and moved my vacuum advance from the ported to the manifold tap.
I suggest reading the article. It makes so much sense and my truck now idles and cruises under part throttle so much better.

I have a question:

I set the idle mixture using a vacuum gauge connected to manifold vacuum while the vacuum advance was connected to the ported tap.
Initial timing ~12 degrees BTC.

Should I reset my idle mixture now that I have changed vacuum advance from ported to manifold?

[VileZambonie]

Yes

[rich weyand]

Absolutely.  A rich mixture burns faster and doesn't need as much advance.  So if you were on ported, and they tuned the idle mixture, it's way rich.  You should be able to lean it out (slower burn) a lot since you are giving it advance at idle, which allows longer for the charge to burn.

[BBM3]

Okay... Yes and Absolutely.
Idle mixture actually seems / smells leaner after switching to manifold vacuum.
I set the idle mixture as lean as possible using a vacuum gauge and my human hearing tachometer prior to moving vacuum advance from ported to manifold.

So I need to reset the idle mixture. This time I will also use a real mechanical tachometer.

Thank you.
-BB

[rich weyand]

That's because the burning in the cylinder is more complete than with the later spark.  If you still had the AIR pump on it, it would be cleaner with ported, due to the late combustion that occurs in the exhaust runners and exhaust manifold.

[BBM3]

I understand 100%.
What I didn't know was that there was such a thing as ported vacuum. I would have never gone to my truck to find that my vacuum advance was incorrectly connected if not for your article.
I am sure my truck never had an AIR pump. It does have an Edelbrock 1406 that someone choose to use the wrong vacuum advance port.
Regardless, It is now correct and runs like a champ thanks to your informative article and help.

Thank You,
-BB

[rich weyand]

Yeah, the Edelbrock instructions clearly label the two ports for ported (emission-controlled engines) and manifold (non-emission controlled engines), so not only did the PO not know anything about ignition systems, he couldn't read either.

[BBM3]

Yep!
I have to take some responsibility.
When I read the manual and watched the video to learn how to set the idle mixture I overlooked the vacuum advance part.

I make no excuses for the mistake of the original installer. I did RTFM and missed it too.

So READ THIS:
http://www.camaros.org/pdf/timing101.pdf

[philo_beddoe]

If I remember correctly, after I installed the factory original intake and 4bbl qjet, Rich set me straight on all that vacuum advance/timing/ported vs. manifold stuff as well. No more confusion!  Good job!!