No no no, rpm and load are abosolutely NOT synonymous. They are completely different aspects of engine operation. Maybe that's why you don't think you're saying the same thing as REVHED.

 

Again, you're screwing up load with RPM thick skull. I can make the main circuit open up in my carb by opening the throttle in my driveway and not move the car one bit.

 

Not sure what thread you're looking at...here is the first post from it with no mention of manifold vacuum:

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Why did mazda retard timing at idle on carb'd 12A?
Ok not technically retarded, but not advanced.

After doing the "rat nest removal" I realized I'm applying vac to the distrib all the time. (advancing timing) With the rat nest removal at idle there was no vac to the distrib.

I like the way the exhaust sounds with no vac. It's more of a deep pinging throaty note, but the idle seems less stable at low rpm like this. I could easily hook up a solenoid to return the car to the factory no vac idle setting. Just wondering why mazda decided this was beneficial?

 

Ummmm, didn't I just say that they are not? WTF???

 

Sorry, I misread. I am just trying to figure out where you're coming from so that I can help. You're statements are correct about how manifold and ventury vacuum work but somehow you're coming to the incorrect conclusion.

 

This reads to me like a question on how the vacuum advance works.

 

Yes, but no mention of manifold!! That's because it operates off of venturi vacuum, i.e. it is not advancing it at idle because the throttle plate is not open then! Once it opens up the vaccum source is at the base of the venturi - the whole Bernoulli's principle idea that I previously mentioned. The whole basis of revhead's theory is that it operates off of manifold vacuum which is incorrect.

 

Ah, OK, now I see where you're coming from. Maybe these will help.

http://www.highperformancepontiac.co..._advance_tech/
"Vacuum Advance Basics
A heavily loaded engine produces little vacuum while the highest vacuum levels occur at sustained light-throttle cruise. But for maximum efficiency in either instance, peak pressure must still occur in the desired ATDC range. A heavily loaded engine, however, will likely detonate with the high levels of spark lead a lightly loaded engine can tolerate, so a load-sensing unit that uses engine vacuum to advance spark in light-load conditions is added to the distributor. As engine load increases and vacuum levels diminish, vacuum advance reduces spark lead to lessen the risk of engine-damaging detonation. "

http://www.stoveboltengineco.com/howto/vac.htm
" The basic reason for the change in optimum timing at light loads is that when operating at light loads, the mixture is leaner for fuel economy and less dense because of light load. These conditions cause the charge to burn slower, and thus, to reach peak pressure at optimum point in the cycle, the spark must be initiated earlier. Failure to do this will result in "retarded" spark timing and all the aforementioned losses"


http://en.wikipedia.org/wiki/Ignition_timing
"Vacuum advance works by using a vacuum source to advance the timing at mid engine load conditions. Vacuum advance is diminished at wide open throttle (WOT), causing the timing advance to return to the base advance in addition to the mechanical advance.

The most common vacuum source for vacuum advance is a small port located in the wall of the throttle body or carburetor adjacent to but slightly upstream of the edge of the throttle plate. In carburetors having primary and secondary throttle plates, the port is located in the primary. The effect of having a hole here is that there is little or no vacuum at idle or at wide open throttle, with the vacuum signal peaking at part throttle opening."

 

If you agree that what I am saying about manifold and venturi vacuum then I really don't see how you are arriving at any other conclusion. Take your own advice, the first google article that comes up and that has been quoted many times by you guys disproves your own argument! It is not worded in the best way, but it still spells out when vacuum advance works, high rpm and over & above mechanical advance which maxes out at 2700-3500.

" 2. Since fuel burns at a relatively fixed rate (i.e., time to burn is in milliseconds), then the faster the engine runs the earlier the fuel must be ignited so that the millisecond time frame will remain constant. Thus, mechanical timing advances are used. A device, usually a centrifugally activated weight control balanced by a spring, advances the timing proportionate to engine RPM's. The faster the engine runs, the more timing advances. This is true up to approximately 2,700 to 3,500 RPM's. Mechanical timing is critical to acceleration and power response in a gasoline engine. This timing is added to the static timing.

3. The third timing, pneumatic (vacuum) - mechanical, provides an extra measure of timing over and above static and any mechanical advance. The vacuum advance extends the burn cycle even longer so that all the heat energy may be absorbed by the pistons resulting in more power yield per measure of fuel and more mpg. The sensing device is a spring offset, vacuum activated diaphragm. This diaphragm receives its vacuum signal from the carburetor or intake manifold. In many vehicles this signal is modified by the transmission gear selector position, by coolant temperature sensor(s), by vehicle speed sensor, or by combinations of the above. Early computer assisted vehicles retained the vacuum advance system but employed the computer to modify the signal to the diaphragm, thereby optimizing the system performance."

And further explains why it is above the throttle plate and the conditions that exist at idle:

"Vacuum advances are sensitive devices. By design, vacuum advances are spring offset to give zero advance. Only when a strong enough vacuum signal is present will it advance the ignition timing. The higher the vacuum signal, the greater the timing advance. High vacuum signals are indicative of low engine loads at a closed or nearly closed throttle position. At idle, when the throttle is almost completely closed, vacuum is highest; however, during idle, no vacuum signal is present at the vacuum advance. This is accomplished by tapping the vacuum for the advance system above the throttle plate."

I think this is where you and revhead are getting lost, due to the wording or lack of detail by the author. Read it again very carefully, and you will see that while he is saying that "high vacuum signals are indicative of low engine loads at a closed or nearly closed throttle position" he is talking about manifold pressure there, and then in the next sentences pretty much explains why that does absolutely squat for the vacuum advance system due to its location. Put those 3 paragraphs together and voila, you will see why vac advance works after mechanical advance peters out, and that has nothing to do with manifold vacuum since it is much, much lower than venturi vacuum at those higher RPMs.

 

The article you site is pretty poorly written.

How about this one?

http://www.classicchevytrucks.20m.com/vacuum.htm
"The vacuum advance system is responsible for increasing

ignition advance based on engine load. The lighter the engine load, the greater the advance. The greater the engine load, the less the advance. The vacuum advance

consist of a diaphragm within a metal housing. Connected to one end of the housing is a source of manifold vacuum. On the other end, a plunger is connected to

the diaphragm, wich, in turn, is attached to the distributor breaker plate. When the engine is lightly loaded, such as at idle and when cruising, manifold vacuum is high

and ignigtion timing can be advanced without causing detonation. The high manifold vacuum pulls on the diaphragm. In turn, the diaphragm plunger pulls

on the breaker plate, causing it to rotate. Since the ignition trigger (breaker points, magnetic switch, and so on) is mounted on the breaker plate,

it begins to open earlier, relative to the distributor shaft. And this causes ignition timing to increase.This increase in ignition timing subsdtantially improves

fuel economy. If the engine is accelarated, manifold vacuum drops. This allows the vacuum diaphragm, and in turn, the breaker plate, to ease back

toward their neutral states, reducing ignition timing."

 

Or this one:

http://www.familycar.com/Classroom/ignition.htm
"Vacuum advance uses a vacuum diaphragm connected to a link that can move the plate that the points are mounted on. By sending engine vacuum to the vacuum advance diaphragm, timing is advanced. On older cars, the vacuum that is used is port vacuum, which is just above the throttle plate. With this setup, there is no vacuum present at the vacuum advance diaphragm while the throttle is closed. When the throttle is cracked opened, vacuum is sent to the vacuum advance, advancing the timing.

On early emission controlled vehicles, manifold vacuum was used so that vacuum was present at the vacuum advance at idle in order to provide a longer burn time for the lean fuel mixtures on those engines. When the throttle was opened, vacuum was reduced causing the timing to retard slightly. This was necessary because as the throttle opened, more fuel was added to the mixture reducing the need for excessive advance. Many of these early emission controlled cars had a vacuum advance with electrical components built into the advance unit to modify the timing under certain conditions."

 

It doesn't matter how many articles you show them saying exactly the same thing they still won't learn.

People like this are a lost cause.

 

The manual tells you to test for vacuum at 1000-1200rpm with the car stationary (obviously). If you're trying to tell me there's load under these conditions you're even dumber than I thought. The only reason there is vacuum is because the throttle blade has moved past the port and exposed it to manifold vacuum!

The reason automatics have the port below the throttle plate is because they require vacuum advance at idle.

 

Is it not possible to exhange differing points of view without making it personal and getting all upset? If you think you can make someone respect your intelligence by trying to make them look like an idiot then you are sorely mistaken.

Revhed; you are obviously hung up on the idea that the movement of the throttle plate exposes the line to manifold vacuum. Well, good for you. We are all now very clear on what your interpretation of this is. So, give yourself a pat on the back and tell yourself that you are right and we're all a bunch of thick headed morons. Then go away and let the grownups talk...

I don't care one bit if I'm proven right or wrong, as long as I end up with accurate information. But I will not be swayed by arguments that don't hold water, or by someone claiming that I am thick. I work with logic and information, not emotion.

 

You sir, are mistaken. You are the one who said the main circuit only opened up under load!! I said that was BS!!

Revhead: "What do you think carbs have idle and transition circuits for? It's because there's not enough venturi vacuum to pull fuel through the main circuit. The main circuit only comes into play under load."

That's why I said you are confusing load and RPM, yet again!
I understand how the idle circuit works, and even posted how it works and what conditions exist while it is working. In the same article it only says 2 or 3 times how manifold pressure drops as you open the throttle plate, and returns when the plate closes - facts that you continue to conveniently overlook. Not to mention the last article posted by me, stating that vacuum advance operates above and in addition to static and mechanical timing, at higher rpms. Higher RPMs = throttle opening more = manifold vacuum decreasing towards 0 while venturi vacuum is increasing proportionally to power output.

You two really are a couple of gems, quoting me and having responses that straight away make it apparent that you didn't even read what I said! Unreal!!

 

I'm sorry for the personal attacks. It's uncalled for. I'll try to keep it civil from now on. I just find this extremely frustrating.

Here's two questions I want you two to answer. Forget the other sources (including the ones the Purple82 posted that you completely ignored), this is straight from the factory manual.

1. It tells you to test for vacuum at 1000-1200rpm. The throttle will be just off idle hence high manifold vacuum, no load (nothing to do with RPM but because the car is stationary and the throttle barely open!) and not enough airflow through the venturi to pull a vacuum. How could it could it be connected to anything other than manifold vacuum under these conditions?

2. The diagram shows that automatics have the vacuum port below the throttle plate. If your theory was correct how would the vacuum advance work at all on these cars?

I think where you're going wrong is you don't understand what vacuum advance is for. It's for situations when there is high manifold vacuum like cruising and just off idle. Not, for high load situations.

 

Nice dodge on your rpm/load blunders in your previous posts revhead, can't you just admit it when you've been shown that you were wrong? Its really not that hard for a secure person to do. Lack of admission only further discredits anything you say afterwards, and also garners you no respect.

This article below explains a lot and gives us ground that I'm sure we can come to an agreement on. The good news is that we are not the first ones to ask this question, and it commonly sparks debate among enthusiasts. The reason none of the other sources cited previously in this thread are helping is that they fail to mention that, depending on design and or possible modifications, some cars use manifold vacuum for vacuum advance while others use port vacuum. The author takes two cars that have the same engine and takes several measurements over an extended amount of time, using both manifold vacuum and port vacuum and then posts his results. Keys here are that: #1 manifold vacuum and port vacuum are different #2 the difference between them is the location of the port, manifold being behind the throttle plate and port vacuum being in front of it #3 our cars in stock configuration use port vacuum because the port is above the throttle plate.

From http://www.highperformancepontiac.co...advance_tech/:


When the vacuum-advance unit was disconnected, idle speed decreased slightly, but actual idle quality changed very little. It appeared that the additional idle fuel allowed the mixture to burn more efficiently with less spark lead, and that manifold-vacuum advance was not totally necessary. This prompted us to readjust the carburetor settings and reconnect the vacuum-advance unit to ported vacuum for further testing. The result included a strong, stable idle and seamless transitions without any effect on economy.

What We Learned
After logging several hundred miles with both Firebirds in their final states of tune, we are completely satisfied with the results. We were able to tame a radical camshaft and dramatically improve temperament with manifold-vacuum advance in one combination. In these instances, the long-duration camshafts required for high-rpm operation have excessive valve overlap, which tends to dilute the incoming mixture with residual exhaust gas. Additional spark lead provides extra time for a more complete burn of the mixture in each cylinder.

Vacuum Advance Combustion Chamber

We also found that relatively stock combinations with limited modifications do not always benefit from manifold-vacuum advance, especially if the carburetor's idle circuit has been modified. It appears that in these instances manifold-vacuum advance will not always improve street manners but can smooth idle. Ported-vacuum advance can then be used to improve part-throttle response and overall fuel economy. If the original carburetor is unmodified or a different carburetor is used entirely, manifold-vacuum advance will likely provide noticeable improvements in all operating ranges.

Conclusion
There is one constant generality about vacuum advance that likely applies to every combination: A suitable amount (whether manifold or ported) can noticeably increase part-throttle response without any negative aspects. But because most of our engines are modified in some manner, neither form is necessarily "correct" for any vehicle. Since vacuum advance offers such distinct advantages for any car driven on the street, we suggest experimenting with both forms to determine which works best for your specific application. Once you find the right combination, you'll wonder why you waited so long to perform your own vacuum-advance experiment! "

 

lol, it's quite humorous to see one of my Pontiac mags that I subscribe to so liberally quoted in a rotary forum...it's a good source, though, because all of those guys working for that mag and Pontiac Enthusiast are hard core racers who like to be able to drive their cars to and from the track....being a street-driven weekend warrior that can set ET records is important to all of them, but now you can see a definite shift toward "how do I get better gas mileage" kinds of articles along with all the exhaustive run-downs on parts numbers and options-types of articles...

 

I have not once mixed up load and rpm or said they're the same thing. It's not my fault you can't comprehend what you read.

Notice how in that article it only talks about vacuum advance helping during idle and light throttle situations whether it's conncected to manifold or ported. You're original argument was that it's connected to to venturi vacuum and increases with load which as you can see was completely false.

I'm glad to see you can admit when you're wrong.

 

Revhead: "What do you think carbs have idle and transition circuits for? It's because there's not enough venturi vacuum to pull fuel through the main circuit. The main circuit only comes into play under load."

 

Ok, you guys seem to be making some progress here, but I still don't understand why ya'll took the rat's nests out in the first place, since it caused all these problems.

But if you kiss and make up and get those mutilated carbs to idle again everything will be OK.

You didn't throw out the rat's nests, did you?

Ray

 

Ray, we all know at this point that your car is stock and you like it that way. My car gives me no problems, has only 34k miles, and is completely reversible to stock if I should choose to do that. However, I have chosen to modify it just like many others on this forum because hey, it's my car and I want to. Nearly every time you post it seems to be about telling everyone that they should have left their cars stock, which adds no value to the discussion. Give it a rest already, please.

 

You missed his real point....maybe if everybody would stop thinking that removal of the rats' nest was actually a good mod, we'd be seeing less of these kinds of threads.

 

I will await your photo tutorial showing how to make the rats nest work with an aftermarket carb! Screw the environment, I want to let my rotary breathe!
Seriously, no matter what the topic is Ray always seems to make a plug for keeping everything stock. I just don't get it.....to each his own.

 

dj's right, I'm the guy who's always saying "What about stock". But he's not completely right, I don't have cats and the last GSL I picked up has a recently installed RB header I'm liking just fine. Now if I can just get the carb in that silver beauty to idle, that is going to be one fine car.

And he might even accuse me of thread hi jacking, but it seems to me that it is reasonable to ask: "Does this mod really do anything to improve performance over the stock set up?" Specifically, is there an aftermarket carb set up out there that will give me the drivability, gas mileage and convenience of the stock Nikki?

If so, I'm all ears.

This thread has been full of very useful information, I hope it keeps going.

Ray