""Is it supposed to be actuating the canister at idle?""

Yes it is. He chooses a low rated vacuum can that will keep the timing steady at idle.
I read allot of articles online before deciding to try the manifold vacuum route.

My previous distributor was a FBO unit and when I bought it from Don about 6-8 years ago, he was all about removing the vacuum can and welding the plates closed. Now he sells these vacuum units that are setup for the manifold vacuum.
He told me that if I installed mine at 18 initial, then my idle would show 30 degrees of timing. I was surprised when I installed mine at 18 and the idle shows 32 degrees. The car starts up great with the 18 initial which will be nice on hot restarts this Summer.

Did some checking and found out I was only drawing about 12" of vacuum at idle and the canister isn't responding till 13" is applied. Talked to Don. He had me run the initial up to 30 and see how much vacuum I was drawing. Still 12" at 900 rpms. Have to go to 950 or 1000 to get 13". In any case he mentioned sending it back to him to rework the can.

Interesting thing is, even with 30 initial it fires right up - no kickback.

What is the minimum vacuum required per Don?
I was thinking it was 10"hg.

Thanks

I have not tuned mine in D yet, so that is why I am curious. When I told him that I had 11.5hg in D with my last FBO distributor, he didn't seem to think that the vacuum reading was a problem. That is why I was thinking that 10 was the low number.
Thank you for the info!!

There should NEVER be vacuum advance at idle; this would result in unsteady RPM. The vacuum canister should always be connected to the PORTED spark fitting on the carb, which should have ZERO vacuum with the throttle closed.

Rick

Don changes the can to allow the use of manifold vacuum at idle. I now run this distributor which is connected to manifold vacuum and my N idle is rock steady.
Note: Once I know what the lowest rated vacuum rating is per FBO, then I will tune my in D idle to make sure that I have at least that amount so that my in D rpms (with brake applied) does not fluctuate.

If a distributor has a higher rated can and you try to hook it up to a low manifold vacuum, than I can understand why the idle would wonder. But if the distributor is setup to run manifold vacuum (lower rated can), and you have enough vacuum at idle (or D for an auto), then there should not be any issues with the rpms fluctuating.

I found this description on the net:

""Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.""

I decided to swap intake and carb to something a little more street friendly (where, honestly, it spends all it's time) and then re-approach tuning Don's system. He never really volunteers an explanation up front as to his theory behind using manifold vacuum with one of his systems or how it differs from the orthodox use of timed vacuum (which consequently results in a lot of arguments amongst gear heads).

His theory (and I'm not defending it, just restating it) is you take your initial and add a certain amount of vacuum advance at idle to arrive at 30* while the car is idling. That extra timing is added to your mechanical under part throttle/cruise conditions for a more efficient burn of the mixture and drops out when you mash the throttle - where only mechanical is in play.

FWIW, here's a copy and paste from his (copyrighted) tuning guide that goes into more detail:

"We first need to understand that lean mixtures, like at idle and light throttle pressure or low speeds, burn slower than rich mixtures. These lean low load mixtures are a result of lower volumetric efficiency and the scavenge ability of the exhaust port to clear the spent exhaust gases from the combustion chamber this dilutes the oxygen levels and slows down the burn rate. This condition requires the flame front to be ignited earlier in the compression cycle (more advance),allowing more burn time so that peak cylinder pressure is reached at about 12-15* after TDC.

If your engine doesn’t have enough initial timing in it your Exhaust Gas Temperatures (EGT) will rise dramatically due to the excessive fuel that remains unburned in the combustion chamber and tries to complete the process in the exhaust runner and into the headers.

So we’ve determined that a lean mixture burns slower so on the other side of the coin a rich mixture will burn faster. So if you have a rich mixture you need to fire the plug later or retard the timing slightly. Remember we’re trying to achieve maximum cylinder pressure at 12-15* after top dead center, before that point we’re into detonation after that point results in drastic power loss.

The second timing event is what we refer to as “Mechanical Advance”. This part of the advance curve is controlled by Engine RPM only and has nothing to do with load on the engine or cylinder pressure, it’s simply RPM activated through the weights and their retention springs. Sometimes this is referred to as “Centrifugal Advance”. When we add the static or initial timing to the amount of Mechanical timing we get what we refer to as Total Timing.

So if we set the initial at 10* and allow the weights to pull in another 25* of timing we would have a total of 35* of timing at the RPM where the weight travel is maximized or they hit what we refer to as the Advance Limiters.

The third and final timing event and probably the most misunderstood is the “Vacuum Timing”. Vacuum timing has nothing to do with total under load timing or performance tuning. Vacuum timing is controlled by “Manifold Vacuum”, as soon as you accelerate the vacuum in the manifold drops and the Vacuum Timing is released and the timing events are then controlled by the Mechanical and Initial timing settings.

So previously we determined that a lean idle mixture requires more timing to get a complete and efficient burn in the combustion chamber than the under acceleration or full power richer mixtures as delivered by the main circuits.

So to achieve this, the vacuum unit is connected to a “MANIFOLD or CONSTANT” vacuum port and can be set to add anywhere from 5 to 20* more timing in the motor when high vacuum readings or low load conditions are present. So now to calculate the total timing at idle or low load we add together the static or initial, the Vacuum timing and whatever mechanical timing we have based on RPM. Going back to the previous example you would add the 10* initial setting and say 15* of Vacuum timing for a total of 25* at idle or low load, low RPM cruise. Because the vacuum timing is progressive to and directly related to manifold vacuum as the manifold vacuum increases (Light Load) or decreases (As you accelerate) the timing in the motor changes accordingly to keep the efficiency at optimum. The mechanical timing will come and go as engine RPM increases or decreases..."

I tried calling Don today and he was on the other line with an overseas vendor and did not have time to talk. But I emailed him my question on what is the lowest engine vacuum that his distributor is setup for, and he was supposed to respond to me tonight. I am hoping that he will give me a straight up answer (10"hg, 12"hg, etc.)
Once I know this answer then I can start tuning my in D mixture.

Thank you for adding his tuning response to this thread.

The above theory makes sense in general but it seems like a lot of weird work to solve a basic problem. Why not just richen your idle mixture properly so you aren't cooking your exhaust? It's a turn of a couple screws. The quote above makes it sound like a lean idle mixture is a given and everybody's been making their headers and manifolds glow like uranium until this idea came along.

I decided to try the manifold vacuum to see what changes occur in the cruising/idling A/F readings, idle quality (lower idle speed and higher vacuum reading), improved cruising torque, and I like the idea of having a lower initial timing for hot restarts. Having manifold vacuum is like having a start retard setup. During cranking mine will see 18 degrees of timing which will help on hot restarts in the Summer. Then when the engine fires the timing will jump up to 30-32 degrees at idle.
My current idle A/F is richer than most in the 13.5-13.8 range so that when I put the tranny in D with the brake applied, my idle does not go lean. I know that there are members running their idle A/F's into the mid to high 14's ( I assume that these are stick cars) but I can not get mine that lean and keep a good steady idle while in D, well at least not yet. I hope to start driving the car in the next week or two and get my tuning dialed in.

There is more to it than just making the idle richer. There are other benefits that I want to pursue from the manifold vacuum theory especially on my street driven only car.

Rick's advice is basically correct for hot rodded engines. Although many of us bristle at absolute statements like that (and like Don's), for someone publishing to a relatively non-tech audience, its understandable.

SecretChimp has asked the right questions. In a situation where exhaust gas is not clearing out of the combustion chamber (typical with both hot rod and race engines) richer than stock settings get a better burn. Additionally, the relationship between AFR and load is not linear. At low load its a bit richer than stoich, then goes leaner - sometimes leaner than stoich somewheres around 60-80% of max load, then richer again to max load. This is for max power and efficiency as shown in the graphs here.

A really good and detailed explanation of the choice between ported vs. non-ported advance can be found in this wide-band tuning thread starting at post 10 second paragraph & more in post 13. Also check out 'Shrinker' and 'Sasquatch' posts on the next page of that same thread about the history of ported vs non-ported use by the manufacturers.

Mattax,
Thank you for adding that to the post!

I heard from FBO and they suggest that the NEUTRAL minimal vacuum reading is 12" (I am currently seeing 15"hg). Since I have an auto, then I need to keep my in D (foot on the brake) vacuum reading to at least that same 12" mark to keep the vacuum can happy.

Thanks again to all.

Abody:

""not sure how he set yours up but he set mine up to actuate the vac canister at idle. i have over 30* of timing at idle. it idles rock steady at 750 rpm.""

How much vacuum are you getting at your 750rpms?
Is this in D or is this a 4 speed?

------------------------------------------

Cammando1:

How much vacuum are you getting in N (and in D if you have an auto)?



thanks

not sure to be honest. can't find my gauge. must have lent it to some one and never got it back.

my car is an automatic though. box stock 360 so if i had to guess i would say 15-18* at idle in park. but thats just a guess.

Thanks Joe,
I am just trying to get a feel for how important the 12"hg target might be. I am going to be real close to that number in D at my current rpm. My previous setup gave me 11.5"hg with 24 degrees initial, so I am hoping that the extra 6-8 degrees at the same rpm will get me over the 12"hg mark.

I hope to get some time this weekend to find out.

I finally got some time to tune the carb today and what I ended up was 32 degrees of timing in N and also 32 in D (foot on the brake). I got 15"hg in N and 12"hg in D. I had to turn up the in D idle rpms just a tad to get the 12"hg but the vacuum and the idle are rock solid.
It still likes the A/F in D around 13.5. Once I approach 14.0 in D, the engine just starts to shake allot.

I was worried too because mine were closed pretty far. It raised the idle rpms by 200 but I was able to lower back down with out any problems.
Just make sure that you can get the min 12"hg in D. I was first showing 11 3/4"hg and the vacuum and idle was still steady. But I didn't feel comfortable with running with less than FBO suggests.