After doing a swap to an overdrive trans, and wanting to tap into some of that part throttle fuel economy and doing a little research, I decided to hook up my vacuum advance. I swapped a GM cannister into my HEI that limits the amount of advance added to 10*. I did a search on this site and found a lengthy and interesting posting, with a lot of the expertise coming from "Ignition Man". In short, what I got out of it was that in a modern performance engine you should limit the amount of advance and run it from manifold vacuum rather than from a ported source. So that's what I did. Problem is, though, that with the full manifold vacuum, it brings the idle speed up so high that even when I back the idle adjust screw all the way out, it's still too fast. I have a Holley 650 DP 4777-2. Any thoughts?

PS I figured what the heck and stuck it on the ported source for now.

(As you have already noted) If you hook up the vacuum advance to manifold vacuum, you must reset the idle speed and mixture adjustment as it has a big affect on the idle settings. You may need to verify that the tiny fast idle cam adjustment screw on the other side of the carb is not holding the throttle blades open a crack causing the primary idle speed screw to have no effect. This is hard to see part of the choke assembly. Also, the secondaries have a stop screw that can be adjusted although it will likely involve removing the carb to get to it as it is under the baseplate. Check out the other screw first to see if it is holding the throttle blades open a bit, I had that issue with my 650 DP.

-Mark.

Thanks stingr69. My carb doesn't have a choke, so I think I'll need to look elsewhere. Do you think the secondaries could be cracked open far enough to cause this? Can I tell just by looking down the throat? It's a used carb that I resurrected from a box of parts, so who knows what's been done to it.

It definately can cause the problem. I had to pull my carb to back off the stop once or twice. Some folks advise you can install the screw in the hole upside down allowing you to gain access from the top but I have never tried that. You need to check for vacuum leaks etc.

If this were me, I would be pulling the carb off to verify that the butterflies are able to fully close. If the baseplate has any binding for what ever the reason, the throttle blades may be cracked open.

-Mark.

Figured out the problem. I took off the carb, and checked the set screw for the secondaries...it looks like it had been screwed in a bit. I did as you said stingr69...removed it and re-installed it from the top. Future adjustments can now be made without pulling the carb...thanks for the idea!

Anyway...re-installed the carb and it still idled too fast, so I got out my big Box-O-Holley parts and found another DP baseplate, and did a little comparing. What I found was that someone (remember-I got this carb in pieces-used) had bent the short linkage rod that connects the primaries to the secondaries. They put a bend in it such that it wasn't allowing the primaries to close as far as they should. I replaced that rod and now I'm back to having an adjustable idle. Got the vac advance hooked up to manifold vacuum and idling @ 800.

Any idea on how that secondary set screw should be set? I currently have it set so it has no effect.

Thanks again for the help, Stingr.!

Well, nailed that one I guess smile.gif

Aside from the use with a very large cam, I beleve the secondary stop screw should be set to just ever so slightly prevent the secondaries from fully closing. They might stick in the bores if allowed to close all the way. You need to set it so they are just a SCH open. :D If the idle can be set that way, you are fine. Let us know how it works out.

-Mark.

I know there were some pretty heated debates here once upon a time on whether manifold or ported was the way to go. I-man obviously believes in the manifold route.

Personally, I prefer ported for mild engines and small cams, and manifold for engines with lumpy cams that won't idle. My reasoning is this - manifold vacuum DOES significantly increase idle speed which in turn changes the postion of the primary throttle blades at idle when you reset idle speed to your liking.

The problem is this . . . when a cam is lumpy and the engine will only idle at higher speeds the idle screw sometimes get screwed in so much that the throttle blades get opened to the point where they fully expose the idle transfer slot --- then the engine winds up idling on the primary jets. This is bad news because then you either wind up trying to compensate by cracking the secondaries open a hair more OR you wind up drilling holes in the throttle blades. manifold advance in this situation helps by increasing idle speed allowing you to close the throttle blades enough to get the engine idling back on the idle circuit.

Now when running a manifold vacuum source on a mild engine the exact opposite occurs --- you wind up closing the idle screws so much to achieve your desired idle speed that the engine tends to stall and idle speed gets unstable. Idle speed winds up being affected by idle vacuum and the vac advance the vacuum creates.

Just my opinion.

I appreciate the opinion, Eric! My brother, who is much more a gear head than I am, said the same thing you did. My cam idles like my dad's old 70 Impala - if you weren't sure you'd started it you wouldn't even think it was running. Well - OK -not quite that bad. But I get 15" Hg. in neutral @ 800 RPM, 11" Hg. in gear. Putting a timing light on it, I found that in gear, the vac advance hasn't kicked in yet, and I'm still at 12* BTDC. In neutral, the vac advance IS activated, and I'm at 22* BTDC. (I have a 10* can) So really, as it is now, I gain nothing by running manifold vac. I ordered a different vac can that is fully activated at 8" Hg., and I'll see how that works. I'm interested to see what effect manifold vac advance will have on engine temp in around town traffic. Last month there was a thread on vac advance, and Stingr69 and JohnZ (Ignition Man in disguise?) had some interesting points to make.

http://www.camaros.net/forum/ultimatebb.php?ubb=get_topic;f=4;t=006587#000000

I ordered the can he mentioned, and hopefully I'll be able to achieve a steady idle speed with it. If not....back to ported! Thanks again, everybody, for the advice!

Oh yeah, one more question....does ported vac mimic manifold vac once it kicks in?

BPOS out.

LOL :D JohnZ = Iman in disguise. :D haha.

Ported vacuum advance only provides vacuum at part throttle - cruise RPMs. At idle and at WOT it provides no vacuum at all.

Manifold vacuum provides vacuum at idle AND part throttle. Again, nothing at WOT.

Once again, I would stick to ported for a mild combo and use manifold only for a lumpy cam. It won't hurt to play around with both though - have fun.

Thanks, Eric. Actually, I should have stated that last question more clearly - would the ported vacuum reading be the same as manifold vacuum reading at cruise at say, 2,000 RPM? i.e if you had 25" manifold would you also have 25" ported, and would they both drop the same amount if you stab the throttle? Just curious....

I'd have to go thru it all over again to be sure it's right but...
The normal position for timing advance seemed less reactive and bit below in vacume. But I may of had the manifold port off when trying the normal advance vacume port position to decide. Kinda glad I went with the riser section of the manifold. Reading this makes me feel I choose right.

Here is the issue...If you have a vacuum can hooked up to manifold vacuum and have the correct can so that it is fully deployed at idle vacuum levels, you don't realy care at cruise what the signal looks like because the vacuum at 2200 RPM cruise will be way over the amount of vacuum needed to fully deploy the vacuum advance. Ported or manifold vacuum hookups should both be fully deployed at cruise. The only time it is "functionaly" different is at idle. You might see some loss of signal using ported vacuum but nothing operationaly significant as the vacuum is way over the amount needed to fully activate the can.

Everything over a mouthfull is wasted! :D

-Mark.

I read JohnZ's post in that other thread and I have a question.....

If the only difference between ported/manifold vacuum for the vacuum advance is that at idle you don't get the advance with ported then how can it impact all of the things that he mentioned? I can see it impacting idle temps, but fuel economy, peak engine performance and driveability? How much of an impact could it have on fuel economy? How could it impact peak engine performance since it's not being used at WOT? Driveability, the only way that I could see this is that there would be a smoother transition from stop to cruise.

Here's something else to consider. When you combine the effects of mechanical and vacuum advance I personally believe the two vacuum sources will behave differently.

Look at the following scenario - car is idling and the driver mashes the gas suddenly (drag race, getting on the freeway, etc). What happens to TOTAL timing when connected to manifold vacuum? ported vacuum?

With manifold vacuum the vacuum level in the engine instantly drops to zero (or very close to it) when the throttle is snapped open. When this occurs whatever vac advance that was added to the engine at idle goes away, effectively retarding the timing until engine RPM increases enough for the mechanical advance to take over. If your distributor is specifically curved for a manifold advance the engine will probably accelerate just fine, BUT if you have a slower mechanical advance curve and not enough base timing there is potential for a hesitation caused by too little timing. This would probably be more of an issue with "freeway" gears or a tight converter. One more thing --- with a small-ish carb you CAN actually see a few inches of manifold vacuum at very high RPM so in theory the vac can COULD add some advance at high RPM (not what you want) --- probably more an issue with two barrel carbs, etc.

With ported advance when you mash the gas, manifold vacuum has no affect whatsoever on your timing. Since there never was any "part throttle" when the throttle is snapped open from idle ported vac advance never even comes into play. Mechanical advance adds timing as RPM increases and you're set.

Now I've run both at the drag strip on back to back runs with my Accell HEI and frankly, there was no difference. I have a pretty quick advance curve set up and a 3000 stall (at the time) so my mechanical advance was fully in VERY quickly. I will say though that when running manifold advance on the street my 383 has a tendency to stall when stopping hard (power brakes?) and the idle speed fluctuates more than I like.

Just my opinion.

Eric,

Thanks for the replie(s).

In your scenario outlined above, if running vac advance from manifold vacuum, wouldn't mashing the throttle (and thereby losing the vacuum advance) just take you to the same point you'd be if you were using ported vacuum? Since vac advance should be disconnected and plugged when setting initial (or total) timing.

Anyway - this an interesting topic. Once I group my poop and actually get the CORRECT vac canister, I'll let you know how (or if) it works. The can I ordered doesn't fit my HEI.

I doubt I will get my car to the strip as it's a convert, and I'm not interested in putting a roll-bar in it. I might try and sneak it in at the street legal drags and tell them it's a 307.

Originally posted by Eric68:
LOL :D JohnZ = Iman in disguise. :D haha.

I don't have as many cars as he does :D ;) graemlins/hurray.gif

Not to change the subject, but if gas milage is not a concern is there any reason to use vacuum advance? When i got my camaro it had an Acceel dual point distributor with no vacuum advance. I never changed it. Would I benefit from vacuum advance?

Yes, smoother part throttle starts and when just cruising your engine will just be a bit more driveable... You might even notice a temp drop if your mill tends to run a tad hot.

Please explain where you get ported vacuum from. Is this bottle rear of the carb?

EdsZ - Ported vac comes from above the throttle blades, manifold vac comes from below the throttle blades. A good way to double check is to put a vac gauge (or your finger!) over the vac port. If there is no vac at idle and vac at a steady rpm above idle it is ported. If there is vac at idle, it is manifold. Most modern carbs have at least one of each.

I found a vac can that deploys fully in gear at 11"Hg @ 600RPM. Throttle response is great. Haven't had a chance to test driveability cuz it's been pouring down rain here. (Oregon)

An interesting side note - if you change vac cans in your distributor, recheck your initial timing. The tab on the new vacuum cannister relocates the base plate in my distributor slightly and retarded my timing by 2*.

For those who are interested -

I built an adjustable stop for the vac advance can, and am running my vac advance from full manifold vac. I built the stop and set the limit to 8*, with the help and advice of IgnitionMan. (Thanks Iman!)

Observations:

Throttle response - awesome.

Idle temp - No change

Idle quality - Rock steady

Driveabilty - No complaints.

I was talking to my brother about this whole deal. He has a BBC roller cammed Nova that runs in the 10's (a race car w/license plates) and he set his dist up so that it only adds about 20* mechanical advance, and he sets his initial around 20* BTDC, and runs his vac advance from ported. Hmmmmm.

BPOS out.

I changed mine to manifold vacuum after reading JohnZ's post and the idle quality is much much better! My idle would roam with ported vacuum, but now with manifold vacuum it's very steady! The car seems to run a little smoother from stop to cruise as well!!