[go2fast]

The 350 in my '68 idles much smoother with ported vacuum than it does with full. I'm running about 10 deg of initial adv and around 36 total. It runs awesome overall. But if I run full vacuum advance at idle it's kinda rough and the exhaust note is not smooth. With ported vacuum (no vacuum adv at idle) it's perfect. The problem is everything I read says I should be running full advance at idle. Should I dial back my initial and run full vacuum advance?

Thanks for any thoughts.

[DjD]

Run what works best for your combo... I would use what you have for a baseline (take note of timing, idle mixture and idle rpm) so you can restore it with ease. Then switch to manifold vacuum, reset your idle and adjust the idle mixture. I bet it smooths right out and my guess is adding the extra timing from the vacuum advance just put you at an rpm where you were on the curve causing the idle to bounce...

[Gary L]

If you plug into manifold vacuum the idle speed should increase, provided you have enough vacuum to pull the can in all the way. What is your idle vacuum and what is the spec for the vacuum cannister? Are you sure you are getting full manifold vacuum? You should not need to dial back your initial w/ full vacuum advance. You could be "dithering" as DjD said above.

[alanrw]

Dennis is correct. If you car runs/idles with ported vacuum, that's great. It is idling off the idle circuit and the engine is happy. With ported vacuum, the second you open the butterflies, vacuum comes into play thru the ported port, so why worry about it?

alan

[marx3]

have you checked how much advance you get in idle with the vacuum advance hooked to manifold? if your initial is 10 and your total is 36, that means you have a pretty long curve of 26 degrees... If your total is 36, your vacuum advance must give about 14 degrees, because 50 degrees ( 36 plus 14 ) is the approximate total the engine can handle at cruise speed with light throttle application.
It is obviously kind of a religion wether you run ported or manifold vacuum to your timing advance. Dont really get why, since the physical laws provide good arguments toward manifold and not ported. If your can is one of those that give close to 30 degrees, then you have about 40 degrees of advance in idle hooked to manifold vacuum, and that may be too much. But don't solve this problem by switching to ported vacuum, solve it by buying an adjustable vacuum can for about 30 $ instead.

[Steptoe]

Like above..it will depend on the specs of the vac advance canister, how many degs is in it, and where it works at in " and what vac u have at idle, cruise at diff speeds, and part to full throttle.
Basically if u use manifold the vac ad should be all in plus 1 or 2" at idle vac
Not being all in causes a rough idle because the vac keeps changing and the vac ad will keep changing the idle advance, that keeps chang the vac advance...a visious circle.

If u have a very low idle vac, up around the 10" mark, and the vac advance works in the 12 to 15" range it will not make any diff at idle and only work at cruise 20+"/part throttle 12/15"

What scews up many is the early and 70s+ vac advance units and dizzy curves are different specs one designed for ported and the other (early) for manifold.
Dont mix them.

[go2fast]

Actually my vacuum diaphragm is adjustable, I just didn't play with it. I have a good 20" of vacuum at idle (mild cam) and I did try to get a good idle (adjusting idle mixture w/ gauge connected) with the vacuum advance connected to full vacuum, but it just didn't really like it. The only problem I can see is less combustion efficiency, so maybe it runs hotter at idle? In 30+ years, this is the first one that just doesn't like vacuum advance at idle. I even had the same carb and cam on a 327 and it was happy with advance at +- 25 degrees at idle.

[Steptoe]

Actually my vacuum diaphragm is adjustable

Again "adjustable vac advances" come in many ranges...its imposble to make one to cover all ranges and specs.
ie from about 3 degs thru to over 20 degs
and anything from 5" thru to 22"

Another thing, as one screws in, loads up the spring it also reduces the amount of total movement reducing the deg range.
For example a common off the self unit thats full open with a vac range from 5" to 12" will have about 10 degs of advance, but wind all the way in will have a vac range of about 10 to 12" and about 5 degs of advance

As with std units, not all adjusts are even
I describe several times in old threads how to mod std units (and how to change cent advance range in GM points and HEI dizzys) and adjustable units, but one must have a unit in the ball park to start off with
A std non adjustble can be made to cover a far wider range than an adjustable.

[The Devil's advocate]

Which adjustable vacuum advance do you have? The only workable one is the Crane, and then, you have to do the "IgnitionMan" modification to the stop plate mounting position to get the advance to work correctly.

You can get a complete set of how to pictures, with tutorials, from Dave Ray (IgnitionMan", all you have to do is e-mail him and ask for the Crane stop plate modification picture set. He is available for the pictures at - sparkman451@earthlink.net

His modifications separate the degrees settings from the rate settings (rate is the vacuum pull through the hose fitting, changed by the Allen wrench screw inside the advance).

If the vacuum advance degrees of travel are not reduced, then, I can see where the idle quality would go to bad in a hurry from over advanceing of the timing at idle. Some of those stock, and all of the adjustable vacuum advances have very large slots in them, resulting in way too many degrees available. Add to that the Crane advance with the stop plate in their position, on the end screw, and every serration loads the spring in the rate part of the advance heavier, moving the advance rate up, sacrificing adjustablility.

I have the Crane adjustable advances on all my GM engines that are carburetted, and use no more than 10 crankshaft degrees of vacuum advance, and always, full manifold vacuum, NEVER ported.

Regards,

Milton

[JohnZ]

The "adjustable" vacuum advance units have WAY too much travel, and provide WAY too much advance. Get one from NAPA that has the right specs for your application - 15* advance at at least 2" Hg. less than your combo pulls at normal idle so it's fully-deployed at idle.

[The Devil's advocate]

Ask Dave Ray for his picture set of the modified Crane advance. His modifications allow the degrees to be completely adjusted, all the way down to zero degrees, two degrees at a time. The Crane advance comes with a plate with a serrated edge. When properly installed (as in the Dave mod, NOT the way Crane says to do it), the pin travel is completely, and easily adjustable, and costs less than a dollar for a screw and washer.

FYI, the Crane advance is the only adjustable advance that will work down to 4 in/hg vacuum. The others won't go down to below 8 in/hg vacuum, don't come with the stop plate, nor 3 sets of mechanical advance springs.

All the Cranes Dave has done for me came out perfect.

If anyone still has questions and/or reservations about the Crane advance, ask Dave for his advice and his picture/text info, you will get the right info on the Crane advance, and a modification that makes it even better, and works, along with one heck of a top notch ignition tech to help/advise you in the bargain. Not a bad deal from a person that figured it out, and doesn't make a penny in giving his info out to people so they can do the mods for free.

Do as you wish, but you are crazy if you don't ask Dave for his advice and help, if you need it.

Regards,

Milton