Ok, I'm working on Fergus, the 1968 wagon with the 390. The distributor has a vacuum advance thingy on it, which I thought was how timing advanced. However, I was having a lot of trouble reading consistent timing today so I took things apart to test them. If I hook my mechanical vacuum pump up, the distributor advances. It actually advances completely about about 15 inches of pressure. It doesn't really want to un-advance until it's nearing 5 inches. Then I was mucking about wiggling things and it seems that underneath the plate that has the points and all of that, there is a mechanical advance mechanism too.
My question is, why do I have two systems?
If I unplug the vacuum advance it will only barely run. This leads me to believe that I'm always vacuum advanced when it's running and pulling vacuum. If this is true, why is it there? Should I just cap it and set the static timing and let the mechanical system work it's magic?
Google "mechanical vacuum advance". That will help you understand the two different systems.
Since your vacuum advance is pulling that hard at idle, I suspect it's hooked up to an incorrect vacuum port. Probably manifold vacuum. Usually it should be connected to venturi vacuum at the carburetor.
It IS hooked to the manifold. It was when I got it. I suppose I need to figure out which port is the venturi port. This is what happens when you're raised on fuel injected cars.
Mechanical advance is purely speed related- the faster the engine turns, the more lead time you need to get maximum cylinder pressure at the correct time. Vacuum advance is load related. High manifold vacuum means light load, so lean mixture and lower cylinder pressure, so more lead is good. Low manifold vacuum means higher loading, higher cylinder pressures, less lead required.
Lots of cars had vacuum advance hooked to the manifold. It bumps the timing at idle, and tends to smooth things out. Venturi vacuum is not actually what is used- that signal is way too weak to produce anything significant. The normal spot for advance is a passage that runs to slots just above the throttle plates, so you have no advance at idle, but it pulls it all in pretty quickly when you crack the throttle.
Either system works, its just a matter of getting the carb adjusted to accept whatever you have hooked up. It can be a bit tougher to get a stable idle speed with manifold vacuum.
I just found this:
Ford and Chevy used to use a thermal vacuum switch for changing the vacuum source going to the dist. At cold and normal eng. water temps, the dist. got ported vacuum. At very high water temps, the dist. got manifold vacuum to speed up the engine at idle to try to keep cooling efeciency better. I always remembered how to hook them up by remembering the word "C.A.M." . Top port went to the "carb" vacuum, middle port went to the "advance", buttom port went to "manifold" vacuum.
I have one of these multi port switch thingies in the elbow coming off the engine but the topmost port it broken off. It's bypassed by using vacuum from the manifold which would have more advanced timing to bump idle speed to increase cooling. So probably what I should do is figure out which carb port to use and re-set timing? The big problem is that timing is moving all over the damn place and I'm having a hard time setting it.
Yes, but not on a 1968 engine as I recall.
The can itself is simple. The way the vacuum signal is sent varies tremendously. As I said, because of the way yours is acting, I suspect it's hooked up to an incorrect vacuum supply.
Read up on the specific vacuum advance for that exact engine, in that exact vehicle, in that exact year. It all can vary. Not saying it absolutely will, but sometimes it very much does.
I moved the vacuum line from the manifold the the only port on the carb that made sense. I reset the timing to 10 degrees and took it for a spin. I'm quite happy. Drives well, and WOT is a lot more exciting. The car feels happier and timing seems solid at idle.
Sounds like you guessed right. Remember that there are very few things in the world that are less "snappy" than a stock big Ford with a C6 behind it. Don't get too hopeful.
In reply to Streetwiseguy:
I believe the wagon in question was special ordered with a 3 on the column toploader trans and 4.10s.
Is it possible that 68 uses the old Load-o-matic distributor with no mechanical advance? If it does, without the proper spark control valve in the carb and vacuum plumbed properly it won't work at all.
The guts of the distributor are all new. I suspect the whole thing is. With the cap off the rotor clearly has a bunch of motion that is controlled by something under the plate that holds the points and all of those bits.
The 390 isn't exactly snappy, but the 3 speed helps. I'm still not sure that it has 4.10's as first gear will get me to around 40 before I feel like I have to shift before there is an explosion. I need to get a tach of some sort. Second gear, 30 to about 60 is pretty darn exciting wide open. I'm starting to understand this old car, and I like it. I was reading a bunch of stuff about converting to a Ford Duraspark distributor and that sounds like a project for the future when the novelty of points wears off.
Which direction does the vacuum actually pull, in the direction of rotor rotation (retard) or against it (advance)?
Lots and lots of distributors are mechanical advance and vacuum retard. The advance comes in based on engine speed but very high manifold vac (like letting off the gas with the engine still at a high RPM) will retard the timing for emissions. Part throttle will result in a dip in manifold vac, which bumps the timing to advance by reducing the vacuum retard.
There's also another variant: the dizzy retards the timing at idle when there's 20 or so inches of vacuum.
The way it works: the dizzy capsule is connected to manifold vacuum. Under high vacuum conditions it retards the timing, meaning you have to set your base timing with vacuum connected. When the throttle is opened, the spring in the capsule pushes the point or pickup plate making for very quick advancing of the timing until the engine gets spun up enough to make the mechanical advance work. Those can easily be identified because the vacuum advance will operate backwards, so to speak.
Make sure you set the base timing with the vacuum advance unhooked (and port on carb plugged).
Read this regarding manifold vs ported vacuum source http://www.corvetteactioncenter.com/forums/c3-technical-performance/60830-ported-vs-manifold-source-vacuum-advance.html
Our Lemons F-body idles much, much better with manifold vacuum. It also drops less RPM going into gear (auto) with manifold vacuum.
tuna55
PowerDork
5/23/13 1:12 p.m.
Also, if the timing is erratic with the vacuum figured out, you may have a word distributor shaft - I have seen that before and it's annoying.
In reply to ross2004:
That's a fantastic article. Now I just need to figure out how all of this applies to the distributor that I have. I know it was fed through a manifold that screws into the coolant elbow. There were two sources it, both from the carb and the intake manifold. The idea is to change the source of vacuum at high temperatures.
I've got an old Chilton's manual I'm reading through and some other stuff to look at. You guys are helping me to figure it out.
Is there a port in the base plate of the carb you can use? That dual source deal sounds unnecessary. Full manifold, all the time.
When you read that article, you'll understand why it was done that way....and why it's unnecessary.
One other thing- does the dist. have two ports on the vacuum canister? If so, one should be to advance the dist. and one to retard. Ditch that setup- it was an emissions crutch. All you want it a single vacuum line on the "advance" side of the dist. (pointing towards front of car) that gets manifold vacuum. Set your base timing with the RPMs as low as possible with the vac. advance unhooked. Reconnect the vac. hose and enjoy a better idling, cooling running engine.
