I used to send out a complete package on how to do this right, but always got hammered by the "lock it out" and "ported vacuum" know it all's.

The right way is to set the degrees available in the vacuum advance up to add to the conservative initial timing, then add those vacuum advance degrees by full manifold vacuum.

Example,\: 16 deg/BTDC INITIAL, 8 deg?Vac Adv, gives 24 deg?IDLE degrees. Easy starting best idle, best carb tuning.

For 8 degrees, limit the pin travel to .086 inch, 10, .110/.112, 12, .130 inch

Also, if the carbs are inline, the front carb should not have idle circuits that are adjustable, no air screws, it is only a continuation of the rear carb's secondaries.

By The Way, your "super tuner mechanics" are rank amateurs, they are like so many others, they do not understand proper timing control..

 

^^^^This

 

^^^^ x2 This

 

X3. that adjustable cammed stop is trick. Do you sell them?

 

^^^^ X4 what Dave Ray said ^^^^

Vacuum advance is a wonderful invention and my distributor is set to include about 10 degrees of it. You normally need to modify the distributor to get 8 or 10 degrees of vacuum advance, Many come set with up to 20 degrees which is too much. Without a dial back timing light you will not be able to tell what you have if you just plug it in.

That adjustable cammed stop in Dave's pictures is made by Crane not sure if someone else make it now and available at Jegs or Summit.

I only see this big one that is way too complicated.

MSD Ignition 84281 MSD Advance Kits | Summit Racing

Free Shipping - MSD Advance Kits with qualifying orders of $99. Shop Distributor Advance Kits at Summit Racing.

www.summitracing.com

You can make a simple metal tab to screw onto the vacuum advance.

 

If Dave ok’s it I will send you or post the instructions he used to send out but it’s his call.

 

Would love to see the instructions.

 

I will throw my vote in with Dave also. I simply cannot understand not running vacuum advance on a street driven engine.

 

Now i am also going to throw in my two cents worth. You ABSOLUTELY NEED vacuum advance on a streetcar regardless of anything i agree with Dave Ray a TOP Flight ignition man and i don't know him and Bill K and i don't know him either but i have always read their posts and they know what they talk about. The old G/M engineers did two things really good that were a marvel of engineering better than Ford and Mopar and those two things are the Delco window distributor and the Quadra jet carburetor. Stop and think have you ever seen any G/M special hi perf engine both big block or small without a vacuum advance distributor from the factory. That alone tells you its mandatory. There now i have said my rant thank you. Alex

 

Pretty sure The Big 3 put vacuum advance on the car for a reason.

 

I agree with everyone's endorsement of the vac advance. Dave Ray really knows his stuff. I've used his mod and it made a huge positive difference in how the car runs. If you have not read the article I'm posting below, it is worth your time to do so. The guys you hired to work on your car must only work on drag strip cars that work just fine without a vac advance. They shouldn't work on street cars.

 

OP

Ray Charles can see that everyone posting agrees VA is needed for a "street car". Your "expert" shop only knows how to do WOT (wide open throttle) tuning which is fine for the drag strip but sucks a$$ for a street driven car.

Do yourself a favor and set up your dizzy, and adjust your base timing, according to Dave's 411. In addition note the VA stop plate mod and adjustable VA can you should get

otherwise you will get crap milage, load up your plugs and have idle issues as you do now.

 

This is a good thread. I am new to the old school parts (LS guy for 20 years). I have been trying to set mine up and struggling with the carb and dist. The hex key adjuster on my Accel dist is FUBAR, so I am looking at options to fix it or replace it. Setting base timing is easy, but I really don't grasp all the other timing setup stuff.

Can I buy a distributor which is already set up like you guys mention? Seems like if it is a common way to set them up they would be on the shelf somewhere? This 850 carb is another story. Would love to go EFI but the wallet is empty for now.

If I need to buy a new distributor, can someone recommend a good VAC advance unit? I am nothing but street driving, 470cid, roller cam, 5-speed.

 

T

Can I buy a distributor which is already set up like you guys mention? Seems like if it is a common way to set them up they would be on the shelf somewhere?

If I need to buy a new distributor, can someone recommend a good VAC advance unit?

No there is not a off the shelf dizzy set up like Dave Ray 411 shows. He did (read IDK if he still does) do the mods on a stock dizzy for x$.

If you see/read his 411 ( I believe it was posted as attachment in this thread) you can do it.

I used my stock points dizzy (running a Perronix) and did it but that was when you could buy the VA detent stop plate/adj VA from Crane or Accel. Adjustable VA is still available but you need to make a VA stop plate. Pretty simple how to in Dave's 411

You really need to get timing sorted before tuning carb...your 850 may be fine but without being able to control when and how much VA a dizzy provides (usually stock VA is to much) you just can't get the tune for partial throttle and idle (95% of street car use) nailed down. WOT and no VA is fine if its a drag car

 

I have been on the fence between connecting to manifold or ported vacuum. There will always be pros and cons to any setup. For my car, I get a better (more stable) idle by setting up my idle mixture connected to ported vacuum. Ported will give you the exact same vacuum as manifold vacuum (except for when your idling)

 

Interesting, I have never tried to use ported vac, I always use full manifold to set my idle screws.

Always full manifold vac for vac advance though.

 

IIRC, ported vac was done for emission reasons...IMHO full manifold vac is what you want to use for VA

 

ported makes sense when you run a loose converter and/or in high altitude and run 24-26° (the idle timing you aim for) already via initial timing.

Then additional vacuum timing would put you to 34-36 idle timing which is not the goal for idle timing, but for cruising its still welcomed.

So then ported is the way to go in my opinion..

Vacuum timing - full or ported - should be used to top up your existing initial timing to end up with 24-26° idle timing.
whatever version of vacuum timing accomlishes that..

 

Thanks for the advice. So I decided to order a new distributor to handle that issue.

Yesterday I tried setting my 4-corner idle and I can't get the ported vacuum to zero with this Proform 850 (stays around 5 inches of vac). I can try to screw back the idle set screw but then the car won't idle. Base timing at anywhere from 10-14 degrees and same result. I saw somewhere about pulling the carb and tweaking the secondary side open just a hair for idle as this will not overexpose the port vacuum. Any advise? Manual trans.

 

 

Love the hack job with all the stacked up washers on the motor. Not to mention the fuel hoses. Does this guy have an engineering degree?

 

Here's a good explanation of Ported Vacuum and why you don't want it from the article linked above:

Now to the widely misunderstood manifold vs. ”ported” vacuum aberration. After 30-plus years of controlling vacuum advance systems with full manifold vacuum, that “free” indicator of engine load and fuel mixture, along came early emission control requirements (seven years before catalytic converter technology was introduced), and all manner of crude band-aid systems were introduced to try to reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these crude, but effective systems was GM’s Air Injection Reactor (A.I.R.) system, which pumped fresh air into the exhaust ports to “afterburn” pollutants in the exhaust manifolds. The key to making this system work at maximum efficiency was retarded spark at idle; with retarded idle spark timing, the “burn” begins late, and is not complete when the exhaust valve opens, which does two things that were important for emissions. The incomplete burn reduced combustion chamber temperatures, which reduced the formation of oxides of nitrogen (NOX), and the significant increase in exhaust gas temperature ensured rapid “light-off” and combustion of the hydrocarbons in the exhaust gas stream when the fresh, oxygen-carrying air was introduced from the air pump.
As a result, these engines ran poorly, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to “run hot” at idle; cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it. “Ported Vacuum” was easy to implement – they just moved the distributor vacuum port orifice in the carburetor from below the throttle plate (where it was exposed to full manifold vacuum) to above the throttle plate, where it was exposed to manifold vacuum only after the throttle plate opened. This meant that the vacuum advance was inoperative at idle (retarding idle spark timing from its optimum value), and these applications also had very low initial timing settings; they were usually set at four degrees before TDC or less, and some even had initial timing settings as much as two degrees after TDC. The vacuum advance still worked at highway cruise, but not at idle, which caused all manner of problems. “Ported Vacuum” was strictly an early pre-converter crude emissions strategy and nothing more. Don’t believe anyone who tells you that ported vacuum is a good thing for performance and drivability – it’s not. Anyone with a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don’t understand what vacuum advance is, how it works, and what it’s for. There are lots of long-time experienced mechanics who don’t understand the principles and operation of vacuum advance either, so they’re not alone.

 

The guy in the video says, and demonstrates, that ported and full manifold do the same thing EXCEPT at idle. At idle ported does not have any vac and will not add advance.......OK, but doesn't the added timing of manifold vac aid the engine running cooler at idle, situations like stop and go traffic?

His reason for favoring ported is that manifold vac adds way too much timing and....HE says you have to decrease your static timing so you don't end up with too much. NO NO NO.

I bet if he were to be shown the benefits of a hard stop vac advance limit, used with full manifold vac, he might just change his opinion.

 

I'm not trying to convince anyone that ported is better than manifold. Across the board, manifold is proven to be best, I used manifold for years in my z28 and my L78. I get a better idle now with ported maybe it's just the way my engine is setup. I never had an overheating problem before and still don't now, I also don't find myself driving in stop and go traffic often enough for that to be an issue. I may try tweaking to the carb to see if I can get a better idle with manifold, but for the moment ported is working fine.

 

using ported vac you would need more "base" timing...which in turn is then to much at part throttle because vacuum advance is + more timing, especially when you get to 2500-2700 rpm

But you need to be able to control "how much" VA your can is adding which is WHY you need a stop plate on VA

Once you have the VA & timing dialed in you then tune carb and all works as good as it can

 

Please remember, EVERY factory points GM distributor with vacuum advance came with a section of rubber hose, cut to be a limiter on the vacuum advance pull pin.

Through the decades, those rubber stops rotted away, were pulled off by persons that had/have NO idea as to how timing really works, and the BIGGIE, NONE were installed on any manufacturer's aftermarket vacuum advance distributors.

So, the aftermarket replacements had, actually way too much timing advance in their replacement vacuum advances. I have seen 30 degrees in a 16 degree emissions spec advance, because the rubber stop wasn't even there.

On the large HEI, ALMOST all were made without the rubber degrees limiter.

 

i have shared a very similar experience with you other than I was running a locked out distributor that I was not happy with. My problem was a bit different from yours but never the less still a problem that had me making a decision to change distributors. There is an alternative to the suggestions to the advice here. Having said that I would preface my comment that I highly respect the advice you have received from some very knowledgeable and experienced members on this forum. I was just about to order a new MSD vacuum advance distributor for my 620hp version Mark Jones 467 when I accidentally discovered the alternative. I first need to say that Mark used my locked out distributor that I supplied. I already had it when he built my motor. What I accidentally ran across was Progression Ignition. Not sure how many folks here have seen or heard of it but I’m highly impressed. It is an electronic distributor with electronic vacuum advance, app tunable high quality distributor. It has its own specific ID code, it has a built in rev limiter, and a anti theft lock out that will not allow the cat to start. Here is how simple the install is: Follow the install instructions, down load the app, input your ID code, answer the questions, start the car and it creates a real time adjustable timing table right on your phone or laptop. I swear it must be made by MSD with Progression internals. When you call you speak to the owner. He told me the design is by himself, his dad and his brother. I highly recommend going to the website. This is a copy of my timing table and a link to my car. It is before the new distributor install. iCloud

 

For all the back and forth banter about ported vs manifold vacuum I just want to know one thing:

How many of you actually checked the difference with a timing light? That should tell you what you want to know. Disclaimer: I don't know **** but I know this much because I just learned it through trial and error.

I recently replaced my original intake with a new Edelbrock and had to remove the distributor in the process. I didn't have a timing light at the time and just eyeballed it. Upon test-driving, things were ok but felt a little sluggish/rich/retarded. I then got a timing light and dove down the ported vs manifold rabbit hole.

For my car:

manifold vacuum instantly raised the idle but when hitting the throttle, the timing didn't change at all. No advance observed. Nada.

When I plugged it into ported vacuum, the idle went down a little but when I hit the throttle, it actually, significantly, advanced the timing and when I let go of the throttle, timing instantly reverted back to normal. Seems pretty cut and dry to me.

The big outstanding question for me is this: If ported was really just some relic of ancient emissions standards then why does MSD literature insist on using ported vacuum? (I would follow up with 'why does ported work as intended on my car when manifold appears to do nothing useful?' but I realize it's all in how our cars are setup) Surely the biggest manufacturer of distributors on the planet knows what they're doing. Unlike myself, who I acknowledge doesn't know much.

Once I got a timing light, it was no longer a debate; Ported vacuum advance works as intended in my application: It advances the timing when the throttle plate opens. I'm not sure how else you would expect or want your advance to operate but for a person with very little knowledge on the subject, ported vacuum on your advance seems like exactly what you'd want.

I can't for the life of me figure out why you'd rather just have straight manifold vacuum pressure. With constant vacuum, when do you expect the "advance" part to kick in? I must be missing something.

 

when hitting the throttle, the timing didn't change at all. No advance observed. Nada.

I'm no expert on this, but I have learned a few things hanging around the forum here.

That statement pretty much begs the question ...

Are you saying you have no mechanical advance at all?

How many of you actually checked the difference with a timing light?

I have.

Here is my understanding:

At Idle you want about 20° to 24°.

To get there and not have the engine hard to crank upon start-up ... set the "initial" advance (vacuum disconnected) to about 12°

I have a stop on my distributor to limit vacuum advance to about an additional 12°

This right here:
DSC_1511 by Larry Madsen, on Flickr

With this set-up I am at 12° for cranking and start-up, then as soon as I am running with manifold vacuum, I have an additional 12°, I am at that 20° to 24° advance. Right where I want it at idle.

From there on up the engine is reliant on mechanical advance, which is set up to provide about another 12° all in by something above 3,000rpm.

 

You want manifold vacuum because you WANT VA at idle and part throttle. Base timing (no vacuum) of 10-12 degrees with VA adding 10-12 which gets you 20-24 degrees of timing at idle. Once you mash the gas VA goes away and mechanical takes over on top of your base timing

Ported vacuum was a vain attempt to help emissions by keeping timing down at idle

Aftermarket companies like MSD dizzys assume the users are only interested in WOT performance and don't have VA on dizzys. That is fine for the 1/4 mi but not for street car

If one switched from ported to manifold and saw no timing increase...the VA is not working so find out that "why"

 

To clarify, it probably went up AS A WHOLE when I plugged in into the manifold vacuum. But when I hit the throttle it did nothing to advance the timing.

Ported, on the other hand, worked exactly as I expected it to: It advanced the timing when I hit the throttle. Isn't that the point?

All I'm saying is anyone who is not sure which is better simply needs to use a timing light and they'll see for themselves what the difference is. And yes, I have a lot to learn myself and that's part of why I'm asking the questions.

 

Here. A YouTube video from MSD Performance Ignition explaining it all.

How Ignition Timing Works: Vacuum and Mechanical Advance Explained! - YouTube

 

Out of good faith, I watched that whole video and I don't see how it relates to the specific questions I had. This video didn't back up/deny anything being discussed. Did you even watch the video?

 

OP states when connected to manifold vacuum, the idle is increased. Expected behavior.
The only ways to raise idle are adjusting carb idle or mix screws(which he did’t do), a vac leak(which he doesn’t have) or adding timing. So when connected to manifold the vac can pulls all in. When hitting the throttle, vac drops, can drops out but mechanical increases so net result probably looks like it timing doesn’t change. If held at higher rpm, you can probably begin to see the mechanical curve start to come in, probably very heavy stock springs if a stock 307.
Manifold vac- you are already starting with the can pulled all in.
Ported vac- no vac to the can until you move the throttle blades.