yup.. thats about right. I was sure everyone used to say it was the farthest left (or forward) nipple.

 

i have mine hooked up to the second nipple from the left/front. ill hook it up to where the red line goes and see if theres a difference.

 

Hooking the vac advance up to the venturi vacuum, makes it advance as the load increases.
Hooking it to manifold vacuum, makes it retard, as the load increases. Ever watch a vacuum gauge on an engine while driving?
I use venturi vacuum (2nd from front, stock location) and a mild increase in timing. 24-26MPG with a SP12A is very respectable, in my opinion.

 

i agree. i didnt achieve those numbers with my stock port 12a. i guess i need to stay out of the throttle

 

LOL. I have gotten MUCH less, but I was playing on the track all day, and still managed close to 15MPG on the tank.

 

You guys know that you want timing to advance a lighter engine loads including idle and retard at higher loads right? It's because of fuel mixing/flame speed characteristics under the different operating conditions.

 

If that were true, which it isn't, Mazda would have hooked it up that way.

 

ive seen people doing this mod and removing the charcoal can. anybody know any benifits or disadvantages that would produce?
sorry if im thread jacking

 

so if one where to have a locked dizzy at full advance that would not hurt the engine or its performance? (I question this because people say the retarded timing is for emissions only)

 

Locking the dizzy is done for turbo applications.

The vac advance is there to increase low end torque, provide responsiveness, and improve gas mileage.

If you have the dizzy advanced at idle due to vacuum (hooked up to wrong nipple), then you will not have any more room to advance when you get on the throttle. You have defeated the system.

If the vac advance only starts when you get on the throttle (hooked up to correct nipple), then you will gain the benefits of the system.

If you try swapping the vac lines around, keep in mind that you will probably have to fix your timing and idle mixtures to see the final result. If you started with constant advance, then you were tuned for idling at an advanced timing state and your mixture levels will reflect that as well.

So, get the vac line fixed first. Then set the timing. Then set the idle mixture. Then compare results.

 

It is and they did. There's ignition timing diagrams for both the vacuum and mechanical advance that show the way it's supposed to work. They're in the shop manual if I remember correctly. I'll have to look for them, but I'll post when I find them.

 

I completly understand the system, I have a locked dizzy cause I am getting ready to turbo, but since summer got here earlier then I expected I was hoping to autox it N/A.

I was going to get another dizzy, but if its only for emissions (as one claims) Ill just set it locked at full advance, but I doubt that is safe.

 

so what carb do you have to get that mpg???

 

I run it with no rats nest and no vacuum advance so all i need to do is run a vacuum line to the second nipple from the left? I plan on doing the timing again to so should i do with the vacuum line or without it and then put the vacuum line one?

 

As long as your car is idling at the correct speed, and the vac advance is set to ported vacuum, then it shouldn't matter if you disconnect it for setting the timing. Now having said that, I recommend that you disconnect it and plug the nipple just to be on the safe side.

If the vac advance is getting vacuum at idle, or if the idle speed is too high, then you could be setting the timing while it is slightly advanced. That is what you want to avoid...

 

Stock Nikki, on a ported engine. Exact nature of porting is unknown. I've had both the intake and exhaust off. Die grinder evidence is very easy to see, and feel in both sets of ports.
I also have a RB street header, it had the stock exhaust manifold when I bought the car.

 

Both the vacuum and mechanical advance increase the timing as load/rpm increases.
Leading vac advance is around 5*, while the Trailing vac advance is closer to 15*. The mechanical advance is practically identical between Leading and Trailing. Running too little of a split between L&T, with the vac advance hooked up, can lead to the trailing firing ahead of the leading. Not good. This is why the factory split is 20* at idle. When the vac advance kicks in, it becomes closer to 10* split.
The timing must advance as the RPM increases, as the flame event still takes the same amount of time to complete, but has less time to do so.
Locking the dizzy, and setting the timing at full advance, will insure max power at high RPM, but will actually lower the power at lower RPM. Racers don't care about lower RPM. On the street, lower RPM is more critical.
I run about a 15* split, with the leading set around 5*BTDC, and the vac advance hooked up to the factory venturi vacuum port. Those numbers aren't exact, as I don't have my stock pulley remarked, so I use a timing light with an advance wheel, and get it as close to those settings as I can.

 

Here you go. This is from page 5-36 of the factory shop manual.

Notice how the timing advances with higher vacuum. This means that the smaller the throttle opening, the more advanced the timing.

There are some pretty good sources out there for learning the ins and outs of ignition timing. I found "How to Tune and Modify Engine Management Systems" by Jeff Hartman to have a really good, clear explanation that even a novice can follow.

 

That proves my point. If you hook the vac advance to manifold vac, it becomes a vacuum retard. Manifold vacuum drops as you open the throttle. Venturi vacuum increases, as the airflow thru the venturi's increases, hence the name.
Those diagrams show the total advance of the mechanical and vacuum advance individually, but they are combined to achieve the total advance.
Mechanical is 12.5* on both Leading and trailing, while the vacuum is 4.5* L, and 15* T. Total advance becomes 17* L (12.5 + 4.5) & 27.5* T (12.5 + 15) (12A figures quoted, as that is what the title of the thread is dealing with).
Anyone who say vacuum advance should be hooked to a manifold vacuum source, is wrong. I don't care who they are, or how many books they may have written.

 

You may not be reading that plot correctly. Please refer to the shop manual for the correct vacuum feed line. As the throttle is opened, the vacuum advance decreases by design.

 

I believe you have that backwards...

 

U have that backwards.... It's a vac advance system. Not vac retard...

It's a venturi... The more air that passes over the opening the higher the vac. IE the more air your engine sucks in the the larger the vacuum on a venturi port. Which is the port the distrib is suppose to be connected to. So as I romp on the gas the more vac the distrib sees and higher my timing will go...

 

The correct vacuum feed port, is the 2nd from front. Looking at the spacer will reveal that port goes up, into the base of the carb. Inside the carb, that passage goes to a point just above the throttle plates. Venturi Vacuum.

As for the question asked in the title of thread, the trailing timing is set at a retarded state during idle, to help clean up the idle emissions. Leading is also retarded, to a much lessor degree, for the same reason.
Under load and higher RPM conditions, both are advanced to help performance, with a side effect being a better burn of the air-fuel mixture.

Turbo applications do, indeed, retard the timing under high RPM high, load usage. This is to prevent detonation. Not much of an issue with an N/A, unless the static timing is set far too advanced, and the air-fuel mixture is very lean.

 

http://www.classicchevytrucks.20m.com/vacuum.htm

http://www.highperformancepontiac.co..._advance_tech/

RPM advance and load advance are separate and occur at different times. They don't necessarily coincide. I'm sure several folks will be confused by this as it's counter-intuitive.

 

Oh man, now im confused.

Im about to go out and set the timing on my car, so I read over this:
http://www.yawpower.com/pultime.html

Which says that the vacuum advance works off manifold vacuum, and advances the timing at idle.

Is there a straight answer for this???