Vacuum Advance 101

I think you did the right thing. It could be bit's of old hose or it could be the diaphragm deteriorating... As to it putting in alot of advance it can do that(28 deg does seem like quite a lot!) but remember you are not under load, so even @ 2500 you're vacuum can be quite high. In other words, when under load like driving the transmission and wheels and moving the weight of the car you have to open the throttle(carb butterfly) much further to reach and maintain the same rpm as when sitting in park in the driveway. Also, I'm pretty sure it's a smaller allen key than that. You found that @ 2500 you are getting 20 deg of mechanical. Most likely the weights in the dizzy are either 13L or 15L, dbl that to get 26 or 30 deg mechanical. So @ 2500 w/ only 20 deg mech you are not "all in" yet. This is where a recurve comes in, to bring the mechanical advance all in at a more optimum time.

Thanks for the awesome information in this thread Econoline. I have been reading articles sparked by your last post and I am understanding this more that I ever thought I would.

Couple thoughts/questions for discussion.

1) You say 13L or 15L, however I see most of these showing 10L or 15L so my mechanical should be 'all in' at 20 degrees at 2500 RPM, if it is at 10L. I will be checking when my Pertronix Ignitor comes in this week.

2) I read the article in the tech archive on vacuum advance and it is all about being connected to the manifold vacuum source. If that is the case I would be getting vacuum advance at idle, right? and how much vacuum advance should I be adding to total timing? You mentioned 5-10 in post 7, is that the same with ported and manifold vacuum? Charlie also prefers the manifold (post 17) and mentions 2-4 degrees of timing from the vacuum advance.

3a) What should be my total timing plus vacuum advance be on a Mustang 200 w/ Auto Trans I have read many different opinions and I am wondering what you guys think. Right now, I am thinking it should be between 32-34 without vacuum advance and 36-40 with vacuum advance connected (all at 2500 RPM).
Initial Timing (warm engine) - 12 Degrees (Most everyone agrees here)
Mechanical/Centrifugal - 20 Degrees
Vacuum Advance - 4-6 Degrees

3b) This spec sheet shows 35.5 degrees @ 2000 RPM.
http://www.mustang-s.com/years/1968/196 ... butors.htm

MustangDadDrake":ygawj7dl said:

Thanks for the awesome information in this thread Econoline. I have been reading articles sparked by your last post and I am understanding this more that I ever thought I would.

Couple thoughts/questions for discussion.

1) You say 13L or 15L, however I see most of these showing 10L or 15L so my mechanical should be 'all in' at 20 degrees at 2500 RPM, if it is at 10L. I will be checking when my Pertronix Ignitor comes in this week.

2) I read the article in the tech archive on vacuum advance and it is all about being connected to the manifold vacuum source. If that is the case I would be getting vacuum advance at idle, right? and how much vacuum advance should I be adding to total timing? You mentioned 5-10 in post 7, is that the same with ported and manifold vacuum? Charlie also prefers the manifold (post 17) and mentions 2-4 degrees of timing from the vacuum advance.

3a) What should be my total timing plus vacuum advance be on a Mustang 200 w/ Auto Trans I have read many different opinions and I am wondering what you guys think. Right now, I am thinking it should be between 32-34 without vacuum advance and 36-40 with vacuum advance connected (all at 2500 RPM).
Initial Timing (warm engine) - 12 Degrees (Most everyone agrees here)
Mechanical/Centrifugal - 20 Degrees
Vacuum Advance - 4-6 Degrees

3b) This spec sheet shows 35.5 degrees @ 2000 RPM.
http://www.mustang-s.com/years/1968/196 ... butors.htm

Click to expand...

You have come a long way, but stock advance specs are from the stone age.
The Pertronix is light years above points.
However your advance specs are antiquated.
You need your distributor recurved by a professional for your application.
Such as when the centrifugal advance starts & the correct total at the correct RPM.
Your vacuum advance also needs the correct total for best fuel economy & driveability.

I have a basic understanding of these systems but I've got to defer to Bill, he's an expert at timing distributors. It's not just when it's all in and how much timing you can tolerate it's the arch of the curve as well. It takes help from someone like him or a lot of patience and trial and error on your part with a pair of pliers, a set of different springs, some graph paper, basic record keeping and a dial back timing light to get it perfect for your combo.

Yes, if you have a 10L weight in there you would be hitting your maximum mechanical advance. Easy test, run it up to 4K and see what you get. If you have a 10L weight, there's another weight on the other side that's likely larger. The difference in the weights is the length of the gap in them and that only one, the primary weight, is hitting a stop in a DS2. It allows a preset amount of travel to the stop, i.e. a 10L has a smaller gap than a 17L. The change per degree is linear, each degree = x # of .xxx of inches of gap. If you took one apart it would be more obvious. The primary weight being used is the weight with the lighter spring that's got the stop tab in it's gap. 2 weights are used for balance and to shape the curve.

There are 4 different variables. Those variables being the 2 springs and 2 slots and the curve is the interplay between them at any given rpm and the rate or pattern of accent or decline when pushed and pulled any given way. You can bush the stop to limit the slot or you can grind/file a slot open or weld/file one closed shorter. And try different springs. You can bend the spring tabs to fine tune the pressure, ect. They put slots in the plate for you to use a flat screwdriver or needle nosed pliers to do just that. Otherwise you have to disconnect the vacuum advance arm, pull the pickup, knock out the roll pin(noting which hole it was in) and pry off the reluctor, then pull the plate and finally, mess with the springs and weights. Then put it all back together again. Everytime.

Bill the distributor guy is correct...setting the advance CURVE is most important, and he has the SUN machine to do it. Our old dizzy is not controlled by electronics/computer but mechanical controls. That is swing-weights, vacuum, and initial setting. WE are just guessing while Bill has a machine that accurately shows where things are happening. My engine works great with 12 degrees initial, the limiter in the dizzy set at 10 (20 degrees crank), and manifold vacuum. My engine is tight and vacuum is very good. I also run a Pertronix but points are NOT the problem many think. The spark from an electronics ignition is hotter but much shorter than points, due to the inherent design. Points spark is like a campfire flame...not as hot as a lightening bolt, but it hangs around longer. Because the fuel-air mixture is swirling around in the combustion chamber, a short flash may not ignite it, but a longer burn may. If 15,000 volts ignites the mixture, the additional 35,000 volts is not used, so is excess. MSD stands for Multiple Spark Discharge, which was created to simulate a points longer spark. We are talking about very short time-frames but important inside a combustion chamber under pressure. Points worked very well for 75 years and was improved during that time, and with todays hotter coils and better made components, we get more spark to the plugs with updated components. Pertronix simplifies things and cuts down on tuning, but you will not feel a seat-of-the-pants improvement with it...unless your ignition had a problem to start. Read the info for ACCEL, Mallory, Pertronix, etc. and you will see they help in upper RPM situations, which our SIX probably will never see. Hot Rod did a test and proved good points can stay with electronics up to about 5,000 RPM, making as much HP/TQ. The key is a GOOD tuned points system...which Bill can tune. I have nothing against modern electronics ignitions, they work great, but came about to work with computer-controlled Fuel Injection, transmission shifting, help with emissions, and cut down on customer tuning. A properly tuned points system will get you safely across the US and back between tune-ups, and it will do so just as well as any electronics system at our speeds with out an O2 sensor, MAP, MAS, TPS, or any electronics. I have argued this with engineers and finally got one to admit, With a classic engine, at normal driving speeds, using modern components in the dizzy, better wires and plugs, little is gained adding a computer system. My modified 200 gets 25 MPG with 89 octane gas, running 75 MPH in the fast lane, using the Air Conditioner all the way, loaded with luggage for a long trip and hauling 2 old over weight Hot Rodders.

looks good...
intake opening @ the head light? Where?
What kinda 'bucket' or scoop does it have?
Nice to see the red steada the lght blue...
(I luv pic!)