Vacuum Advance and ported vs. manifold vac.

[Charlie Chops 1940]

When I recently had the ZZ4 out for the tranny fiasco, I went back to my Holley 600 vac. secondary carb from the 570 Street Avenger that I had to try. The 570 worked pretty good but had an annoying off-idle stumble that I couldn't get rid of. I rebuilt the 600 that  has always been the carb of choice on the 3 engines that have been in the '40 over the last 27 years and put it back on. I ran the vac. advance off the port above the throttle butterflies. Checked the timing and set it at 10 BTDC and then checked for total and got only another 13 or 14 degrees. That seems a pretty short curve.

I guess I haven't been paying a lot of attention to the ongoing ported vs. full manifold for vac. advance but it sounds like I need to try full manifold vacuum and see what it does, huh? The dizzy is the vac. advance HEI supplied by GM with the engine.

Any thoughts?

Charlie

[EMSjunkie]

Hey Charlie, I would recommed manifold vacuum for the dist.

check out this by Steve Jacks, I read it and it made sense. went and changed my set-up around, and it works like a dream.

http://www.chevytalk.org/threads/showflat.php?Cat=0&Board=UBB64&Number=1105501&Searchpage=1&Main=1105264&Words=%26quot%3Bcamaros.net%26quot%3B&topic=&Search=true#Post1105501

hopefully Steve will read this post and add ALOT more intelligence to the questions.


Vance

[Charlie Chops 1940]

Thanks Vance, That's what I was looking for I believe. I'm printing it now.

Charlie

[EMSjunkie]

hope it helps you out.

it sure made my coupe run better.

I didn't wanna say it out loud, but that Steve Jacks feller is pretty smart.

didn't want him to think we like him or anything.  


Vance

[HOTRODSRJ]

hope it helps you out.

it sure made my coupe run better.

I didn't wanna say it out loud, but that Steve Jacks feller is pretty smart.

didn't want him to think we like him or anything.  


Vance

Awwwwwwwwwwwh Shucks!  I like you guys tooooo!

First, let me give credit where credit is due. The article that you linked to is actually from John Z over at Camaros.net.  This is one of the best expressed tech explanations that I have ran across in years...so it's John's discertation....not mine.  But, it's exactly what I have learned and know...and in perfect form.  Another link here... takes the garbage numbers out that were somehow scrambled in the text copy.  http://www.camaros.net/forums/showthread.php?t=70263&highlight=vacuum+advance+101

Also, my experience with the ZZ4 HEI is that you will definitely need a full time vacuum advance for this...but it also has a tendancy to allow too much vacuum advance under light and moderate throttle conditions.  So, a Crane adjustable/limiter is usually needed...or it will sometimes detonate or ping due to the excessive total vacuum advance.  Limit it to 12 degrees please.

Also, the Edelbrocks AFB carb can have an "off throttle" hesitation due to the booster springs that come in the carb by default (orange).  The ZZ4 will make at least 18" of vacuum and need the highest booster spring (silver) or stiffest one you can get.  I suggest you also make this change and watch your hesitation disappear.

My two sense.

[EMSjunkie]

Charlie, I had a Holley 4160 that had a off-idle stumble.
turned out to be the float bowl gasket was partially blocking the accelerator pump orfice.  :x

wallered out the gasket a little, seemed to help alot.

good luck, keep us informed on the progress.


Vance

[Charlie Chops 1940]

I rerouted the vacuum line to manifold vacuum, rechecked and set initial at 10. Sounds crisp but I probably won't get it out til tomorrow. It has always run good at 10 initial but I will run a map of different settings and see where kickback and/or pinging sets in.

Stand by for news.

Charlie

[timkins]

OK, which is which?  Which port should the vacum advance be connected to on the carb? Please explain where to pick up manifold vacum and ported vacum.

[Mikej]

The easiest way to tell is at idle manifold vacuum is sucking and the port vacuum won't be. Just pull the line off and put your finger over the hole. On the port fitting as you increase the throttle you will feel the suction.

[1FATGMC]

Good piece Jack, thanks.

I'm going to check, but I think I've been running mine all of these years on the ported.  I remember guys talking about running hot and your suggestion to cure it was to run manifold vacuum.  I was never running hot underway, so thought why change.  I do run hot idling and can now see where changing sure might help that.  Sounds like it could also possible help mileage some .

I've been running 12 deg. initial and wonder if I might have to drop that back a couple degrees if I end up with more initial now with the vac. not on the ported side?  Guess I'll just try it and see.

c ya, Sum

[Charlie Chops 1940]

OK, which is which?  Which port should the vacum advance be connected to on the carb? Please explain where to pick up manifold vacum and ported vacum.

In general manifold vacuum is a port below the throttle blades in the base plate although the power brake tsap is another source. Ported vacuum is above the throttle plate.

Charlie

[Skip]

When in doubt, pull the distributor vacuum off an intake runner.