Quick timing question
 

I've got an 84 ford F150, 4.9l 300. I did the DS2 conversion, question is Which vacuum port should i hook the dizzys vacc advance up too?

I've got it hooked up to a ported source right now (it only supplys vacc as engine speed increases) and the truck runs kind've shotty

If i hook the advance up to a full source of vacc it runs smoother, but that could be because the timing is being advanced.

What did you set your timing to?

Vacuum advance should always be hooked to a ported vacuum port ABOVE the throttle valve in the carburetor. This is so that the vacuum advance pulls the timing and advances it as the motor's RPM increases. If you hook a vacuum advance to manifold vacuum, the timing would actually RETARD as you gave it throttle off idle because manifold vacuum drops as RPM increases.

You say the truck runs smoother when it's hooked to manifold vacuum, this tells me that it's pulling advance from the distributor. And, if it's doing that, chances are you've got extremely retarded timing. Enough so that the manifold vacuum is pulling your initial timing just high enough off idle to get it in the ballpark range.

My 1983 4.9 came factory with Duraspark II, and it was timed at 6* BTDC according to the label that was on the valve cover. It might be different for your motor, because each individual motor had a wide array of calibration codes.

Also, since you've pulled the computer/etc. and put in the DSII distributor, did you have the distributor re-curved to run the correct vacuum advance without the emissions equipment (I assume you undid all the emissions equipment like the EGR)?

Yes all of the old emissions has been removed, i bought the truck and somebody already started this process so i didn't have much choice but to finish it. Yeah i thought i should have the advance hooked to a ported source.

However, i did not re-curve the dizzy, what do you mean?
I've heard the timing should be 10 degrees BTDC, i have mine set at 11, engine seemed like it wanted to run best there, and that's where i got the highest/ smoothest vacc gauge reading of 16/5". I didn't want to set it higher than that, without the EGR I'm afraid of pinging this thing to death.

One other thing, the EGR valve is still physically there, but it's dead weight. I've got the vacc nipple plugged, but i haven't managed to plug the exhaust too or from the EGR plate.

Also when the engine warms up it wants to idle around 1500 RPM, thats thanks to the feedback carb.

I disagree. Ported vacuum is for emissions purposes - with less timing the engine runs hotter at idle and produce less NOx.

If it runs better with the line on manifold vacuum, leave it. I run the 300 in my Bronco that way. And the 331 in my Mustang.

Mine won't run on ported vacuum without opening up the throttle plates to get it to idle. Runs skookum on manifold vacuum.

On the flip-side, I run mine on ported vacuum.

I had it on manifold vacuum for a while, but I was having bogs off the line. Turned out, the reason was because the throttle blades were closed so much at idle that the transfer slots in the venturis were completely unexposed. So, when I'd step on the gas, there was a lag between moving the throttle to when the velocity in the venturi would pull on the transfer slot and get the fuel flowing.

When I moved it on ported, the idle RPMs dropped from 700 to around 300. I was able to open the throttle blades some to bring it back up to 700, which exposed the transfer slot again, helping to get rid of the bog.

Who knows, maybe that's because it's a 600cfm carb.


In the end though, use whichever your engine likes best.

Adam, the one answer you got has it exactly backwards. While the centrifugal (often called mechanical) advance system with its springs and weights, advances the timing to keep up with engine rpm, the vacuum advance system would seem to be working backward if you don't understand its job. High manifold vacuum exists when the throttle plate is nearly closed, at idle, or when partly closed, as during freeway cruising on a level stretch of road (which might only require 25-30hp from your engine). Only a small amount of fuel/air mixture is entering the combustion chamber with each cycle, and especially at idle this mixture has a certain amount of residual exhaust gases mixed in with it (true also of the highway-cruise situation if your car has EGR). This mixture burns relatively slow and cool, therefore, the spark has to light the fire early. That early spark is what vacuum advance is for. What follows is a description of how the system works when your distributor is hooked up to manifold vacuum (and ported vacuum is almost the same, as I'll explain farther on).

You're parked at a stoplight, idling. You have the initial timing of, say, 10 degrees that you set with your blink-light. Your manifold vacuum is at its peak, say 21"Hg on the guage. So the maximum of, say, 20 degrees of vacuum advance is applied. Since you're idling, the centrifugal advance has not done anything to the timing, and probably doesn't below, say, 1300rpm. Your total timing, initial plus vacuum, is 30 degrees.

Light turns green and you get on the gas hard to beat the guy beside you to the freeway on-ramp. You still have your 10 degrees initial timing, of course. But now your manifold vacuum has dropped to, say, 6"Hg, and you have no vacuum advance. You don't need it yet because the throttle plate is nearly wide open, and the combustion chamber is getting lots of fuel/air sucked into it (and the EGR has shut off), and this will burn hot and quick, and doesn't need any extra advance . . . except for the fact that your engine revs are climbing, so even a fast-burning fire has to be started earlier to keep up with piston speed. So in this situation, accellerating hard up the freeway on-ramp, you have your 10 degrees initial advance, no vacuum advance, and the mechanical advance steadily increasing to its maximum of, say 23 degrees (which it will reach at, say, 3000rpm), for a total of 33 degrees.

Now you have left the other guy in the dust and have eased off, and are cruising along the freeway, loafing along at 60mph and not using a lot of throttle to do it, so you have quite a bit of manifold vacuum once again. In this situation you have, as always, your 10 degrees of initial advance, plus your engine speed is high enough that your centrifugal advance is adding another 20 degrees, and because the vacuum is high, the vacuum advancer is adding yet another 20 degrees, for a total timing of 50 degrees (and some engines get considerably more).

Nowdays, of course, all of the advancing and retarding is done by computer, but it still is supplied with inputs for engine speed and engine vacuum as well as lots of other inputs.

As regards ported vacuum and manifold vacuum, they are essentially alike in that they both access the intake manifold vacuum. Ported vacuum merely delays the access slightly; the manifold vacuum is not applied to the vacuum advancer on the distributor until you add just a slight amount of throttle above idle. Opening the throttle slightly makes the lower edge of the butterfly valve in the carburetor uncover a tiny drilled hole or slot, which now has access to manifold vacuum. Some cars and some carburetors have a port higher up on the carb called "venturi vacuum" but it is for other purposes. Hooking the hose to ported vacuum ONLY affects the timing at idle or while your foot is entirely off the throttle, nowhere else. It's an emissions gimmick, and to keep the explanation short, its ultimate effect is to make the exhaust hotter, mostly to keep the catalytic converter lit while you're idling at a long light . . . or trying to pass a sniff-test. Even the engineers who designed ported vacuum don't much like it, and they would tell you (one did so, but I forget where the interview is) to hook the hose to manifold vacuum to make your engine run a little better and cooler, and just hook it to ported vacuum to pass a sniff-test.

The first part of this is generally right- stock configuration would be to use ported vacuum.

However, using manifold vacuum does not retard timing off idle. Ported vacuum and manifold vacuum are the same. The only difference is that there is no vacuum signal from the ported source at idle, because the throttle plates are closed. Off idle, they are identical.

"In the end though, use whichever your engine likes best." There it is.

[quote]

Vacuum advance should always be hooked to a ported vacuum port ABOVE the throttle valve in the carburetor. This is so that the vacuum advance pulls the timing and advances it as the motor's RPM increases. If you hook a vacuum advance to manifold vacuum, the timing would actually RETARD as you gave it throttle off idle because manifold vacuum drops as RPM increases.

(Quote):
The first part of this is generally right- (end quote)


No, it is generally wrong. Vacuum advance does NOT increase timing with rpm. The only part of the first quote that is accidently correct is the last sentence; vacuum advance adds timing when manifold vacuum is high. That is exactly what it is supposed to do. And that is also exactly what ported vacuum does because it also is actuated by manifold vacuum, but not until the very bottom edge of the throttle valve opens slightly, uncovering the vacuum port.

Have you got a carburetor on the shelf, one that's new enough to have a ported vacuum nipple? You'll see (suck or blow through it) that it connects to a little hole at the very bottom edge of the throttle valve, not "ABOVE" it.

Next you could hook up a vacuum guage to your engine (manifold vacuum), and connect your blink light. With the engine warmed up and idling, read the vacuum guage, which will be at its highest point. Then see where the timing is. If your vacuum advance is connected to ported vacuum, the reading at idle should be the same as the initial timing figure. If your vacuum advance is connected to manifold vacuum, the timing mark will be a long distance around the harmonic dampener in the advance direction (an adjustable timing light with a dial on the back is a great tuning tool and will let you find out exactly where things are happening). Now open the throttle slightly; if your vacuum advancer is hooked to ported vacuum, you'll see with your blink light that the mark has moved to a considerably advanced position . . . because NOW your distributor is getting a vacuum, manifold vacuum. The reading on the vacuum guage will still be high. Now wing the throttle quick, . . . and watch the vacuum guage drop. Your timing will drop too, because the vacuum advancer isn't getting much vacuum. You can also disconnect the vacuum advancer entirely (plug the line) and move the throttle back and forth and with your blink-light observe the centrifugal advance system doing its thing.

<!-- / message --><!-- sig -->As far as using what your engine likes, okay, but if your engine is properly adjusted, manifold vacuum is what it will like. Most systems on engines in the last thirty-five years require multiple adjustments if any one part is altered.

A good example is EGR. Guys will delete the EGR system hoping for slightly better power and fuel efficiency; they could get those good things for sure, but they have to understand the whole system. Spark timing is advanced in an engine with EGR because the exhaust gases polluting the fuel/air charge slow and cool the burn (which is the intent of the device). If you only blank off the EGR, without recurving the spark timing, your timing will be too advanced and you'll probably have an engine that now pings.

This happened to me long ago when I blanked-off the EGR in a Mopar 318 and suddenly got lots of ping. As a temporary measure, I backed down the initial timing slightly and disconnected the vacuum advance completely, and drove the car without any ping, Now, you therefore could say that I had done what the engine liked. But it liked it better when I put a pre-EGR advance curve in the dizzy, including the vacuum advance. Still no ping, but with better power and a lot better fuel economy.
<!-- / sig -->

"But it liked it better when I put a pre-EGR advance curve in the dizzy, including the vacuum advance." There it is #2.

Duraspark_distributor_recurve_instructions_index

FORDSIX PERFORMANCE &bull; View topic - Recurve Results

Wow, great write up seattle smitty. Trust me i'm not new to the mechanical seen, but i believe what you just typed up would explain to ANYBODY timing in a nut shell, and they'd understand. Great write up, i have one question however, i didn't recurve this dizzy and I'm going to pick up the spring kit to do so, for the time being should i leave the vacc advance on a ported source so i don't ping this engine to death?

Also, when i purchased this dizzy i sourced it for a 79' truck, so it would be an EGR curved unit.

to quote from smitty's first post above:

"Hooking the hose to ported vacuum ONLY affects the timing at idle or while your foot is entirely off the throttle, nowhere else."

so- if your truck is pinging, you need to back off the timing.
changing which port you're using won't make a difference.


to quote from smitty's second post above:

"This happened to me long ago when I blanked-off the EGR in a Mopar 318 and suddenly got lots of ping. As a temporary measure, I backed down the initial timing slightly and disconnected the vacuum advance completely, and drove the car without any ping"

the process he describes above should hold you long enough to recurve the distributor.


if you're going to recurve the dizzy, you need to read through these two articles:
FORDMUSCLE webmagazine: Timing is Everything - Distributor Curving for Maximum Power

gofastforless.com
i can't link directly to the article on the second one, for some reason. hit the link, then click "Ignition", then "Ignition Curve".

Well, Adam, for a starting point, go with Harte's idea; hook the hose first to one port, then to the other (plugging any unused ports) and see what the engine seems to like best. The point I was making to Harte (probably entirely unnecessarily for him:-wink) was that what the engine is "liking" at that point is merely the best of the choices you have tried, and there may very well be better choices yet, especially if you have done any engine modifications.

I tend to expect people on these sites to do some modifying just because all of my pals do, but maybe that's a bad assumption. So if in fact your truck is factory-stock, unmodified, then the advice you got from 1983F150 to hook things up the way the factory did it is very likely your best choice. When the manufacturers changed from manifold vacuum to ported vacuum sources for the distributor many years ago, they made a couple of other small changes at the same time. Unless you are a good enough tuner to deal with all of the large and small ramifications attendant on any single change, maybe you should leave well enough alone, as he told you. Changing from one type of spark generator to another (your DS-II) doesn't change the spark timing requirements. But as Harte implies, real-world results trump speculation every time.

If you ARE modifying your engine, then you need to do some tuning and testing. Rusty's link is a great start, and if you hunt there are other good articles on tuning. From the start of the process you are dealing with multiple variables that don't all act independently but often affect each other. So for instance, while you are adjusting your idle mixture by reference to the vacuum guage, you will see on the tach that your idle speed has changed from where you want it, and you'll have to go back and forth making changes, . . . and then maybe find a new initial timing figure to start from, which opens new cans of worms. Tuning can be a fun challenge or a royal pain, so if you have better things to do, go with '83150's advice and don't try to out-think the Ford engineers.

But if you want to try being your own engineer (because you have a somewhat different set of goals to achieve and compromises to find than those required of the Ford engineers), you need to get a vacuum guage and learn how to use it, one of the best tuning tools ever. A dial-back-to-zero timing light (blink-light) is a wonderful invention; Sears has them, even one with a digital read-out which I know nothing about. You have to have a tachometer you can clip on and read while you're bent over the engine, and later you'll need one in the car, along with the vacuum guage and a stopwatch. One of the things I detest about modern cars is the acres of plastic. I'm dealing with such a car now, one I've modified and for which I must come up with a new timing curve, so I need those dash-mounted guages. But it has a vinyl dash, which isn't good for mounting a pair of good-sized guages. The cars I grew up with had tubular steel steering columns, great for hose-clamping guages to, but this car has a two-piece vinyl cover over the steering column and its great big wire bundle. This is progress?? Well, as a temporary measure you can run wires and a vacuum hose from the engine compartment and through the driver-side window to work any guages you want to duct-tape to your vinyl dash while you get the truck tuned.

Adam, I looked again and see I missed part of what you were asking, about pinging. You did say you had disconnected the EGR. If it is working, and doesn't leak vacuum (use a Mityvac, or just suck on the hose and see if the EGR valve-stem moves in and out), hook it up again for now. I'm not real fond of EGR (any old racer remembers doing his best to AVOID having the intake charge diluted with residual exhaust gases, and EGR does it by design!:-X18). But it doesn't hurt full-throttle performance at all (it shuts off), and doesn't hurt fuel economy a whole lot. And disconnecting it can sure make the engine ping. No, the EGR valve will no longer admit exhaust gases into the fuel/air charge when its vacuum hose is disconnected, and that's the problem; it is the LACK of those diluting exhaust gases that can make the engine ping, because the factory timing curve is set for a diluted mixture. With an UN-diluted mixture you now have too much spark advance, until you do a re-curve.

The more modern the vehicle, the more complicated this stuff gets, because any one alteration affects more parameters. On a computer-controlled engine, you can really goof things up by trying to "improve" on what the factory engineers set up. The car I'm working on now is mid-eighties machine with a real simple first-gen computer. Pretty-much all it did was get inputs from various engine sensors and then send or not send voltages to various carburetor adjustments. The old carb was unrebuildable, and the car is an orphan with no factory or aftermarket parts support, so I adapted it to take a simpler and better carb. This left the computer with nothing to do, so I took it out, along with miles of vacuum hose and wiring and sensors. But I did a LOT of studying of the factory shop manual before and as I did this, to make sure I wasn't screwing anything up. I would never try such a thing on any newer, more complicated car; I don't know nearly enough, and am probably too old and dumb to learn it. What did Donald Rumsfeld say, "You have to know your knowns and know your unknowns," or something like that.

You undoubtedly know this, Adam, but we all can forget it: you can get in a rush and end up wasting your tuning efforts if you don't take care of basics FIRST!! If your plugs, plug-wires, or distributor cap are several years old, don't even bother testing them, just junk them all at once and get the best new pieces you can, the spiral-wound resister wire and a cap from Standard Products (my favorite, anyway). Pull the distributor and make sure the shaft doesn't wiggle and that the bushings on the advance weights aren't worn and that the vacuum advancer doesn't leak, and that all of it works smoothly. With the dizzy back in place , set initial timing with the blink-light. Put in a new fuel filter. Look in the carb and make sure the accellerator pump gives out a good stream when you twist the throttle linkage. Have somebody floor the throttle (engine off) while you look iin the carb and see that the butterfly(s) open all the way. Check fluids, check thermostat operation. Yadda, yadda.:-blah:-X17

But all that was the EASY prep; now you have to check for vacuum leaks:'(. It's not hard to see whether the diaphragms in the vacuum advancer and the EGR valve leak, but finding leaks in that mass of vacuum hoses and connections and gaskets in a modern car can be pure aggravation. There are various methods (stethescope, propane, carb-cleaner sprayed around the edges of head gaskets and carb gaskets, etc) but it still can be hard to find small or intermittant leaks, especially in the hoses. Or maybe it's just me. Anyway, the last thing, maybe, is to hook a vacuum guage to the engine to make sure it doesn't have something wrong internally, like a sticking valve, or mixture way off, or something blocking the exhaust.

If ya don't prove to yourself, by checking, that the basics are good, you can waste a lot of time in trying to fine-tune the timing and carburetion. I've done this wrong enough times that I've finally got it into my thick skull to do it right, duh.

Somebody, Bronco?, observed he had to turn up his idle speed with ported vacuum. That's what the factory does, too, as part of the idle-emissions reduction (sounds wrong, doesn't it, but remember that it's to keep the cat hot). Working on these old cars, and adding non-original parts, it's often impossible to know where you're at relative to factory specs, so you have to put guages on the engine and work out your own specs.

I see I was maybe mixing up comments by Harte, Bronco, and the Baron, each of whom I have enjoyed yakking with (or at:-X04) in the past; Happy New Year, gents!