I have a 75 F250 with 390 and points ignition. Before it was running on only 7 cylinders and ran pretty decent, even idled decent. I just got done rebuilding the engine and when I start it up, it runs smooth at 1500-2000rpm but at idle around 600 rpm it runs a little rough. Here is what I checked:

-Ignition timing is at about 10 degrees at idle
-Dwell angle is good, points are good condition.
-Carb is the same, nothing changed.
-Vacuum at idle is 15in/Hg but needle bounces (has mild cam)
-I then checked the coil and it was in spec (1.7ohm primary, 8900ohm sec.)
-I checked the coil positive with the key on, engine off, and it was 7-8 volts. Then when it's running its 12 volts. This seems like the opposite that it's suppose to be for points ignition. From what I read it should be 12V with key on, then 6-8 volts when running. I looked under the dash for the pink wire resistor, but I don't see it anywhere. Is it somewhere else on a F250?
-I noticed that the battery voltage bounces around too much at idle, it goes from high 13-15 volts rapidly. It's the same alternator, maybe a couple years old and before it would hold a steady 14.2-14.4 volts at idle with nothing on. I noticed the clamp nut for the wire harness (back of alternator was loose, but I don't think it would cause this. Does the wiring look correct?

I thought I had it the same as before but maybe something is wrong. The GRD I have going to ground just like before, and also have a ground going to the top of the alternator bracket as another engine ground.

 

You have to be careful doing this test with breaker points. The number you see depends on the state of the points (open or closed), which depends on where the distributor shaft stopped the last time the engine was cut off. The measurement is not as simple as it is with electronic ignition.

With the engine running, you will see 6 to 7 volts if you have a cheap meter set to DC. This isn't a real number - it's just what the meter "thinks." With the engine OFF but the key ON, you will see 6 to 7 volts with the points CLOSED, but 12 volts with the points OPEN. Before trying to draw any conclusions, you need to qualify your measurement. I can make some educated guesses that explain the numbers you reported, but you need to report the conditions of the measurement first.

Okay, lots of questions:

1) Does this truck have an ALT light, or an ammeter?
2) What's the deal with the tape on the charge wire?
3) What all happened during the engine swap? Did the front clip come off?

Most importantly: you're trying to troubleshoot a very broad problem (rough idle) by narrowing in on a very specific item that in all likelihood is not even related (primary voltage). What lead you here? What counts as "a little rough?" You vacuum gauge needle is bouncing - have you checked for vacuum leaks? You should start wide and narrow down - not the other way around.

 

If you eliminate other possibilities (vacuum leak, idle mixture, timing, etc) check the spark plug gap. Remember, you have points these require a much narrower gap than plugs with electronic ignition. Gap the plugs at .035". If that's what they're set at and it still idles a little rough, open the gap up to around .038".
Also, what kind of shape are your plug wires in?

 

-Ignition timing is at about 10 degrees at idle
Might try bumping this up to 12 to 14
-Dwell angle is good, points are good condition.
check
-Carb is the same, nothing changed.
Hmm is it adjusted/functioning properly?
Does it have vacuum leaks?
-Vacuum at idle is 15in/Hg but needle bounces (has mild cam)
This is Ok, needle will bounce around a little with a cam.
Would like to see it between 16 and 17 though. Vacuum leaks????

-I then checked the coil and it was in spec (1.7ohm primary, 8900ohm sec.)
-I checked the coil positive with the key on, engine off, and it was 7-8 volts. Then when it's running its 12 volts. This seems like the opposite that it's suppose to be for points ignition. From what I read it should be 12V with key on, then 6-8 volts when running. I looked under the dash for the pink wire resistor, but I don't see it anywhere. Is it somewhere else on a F250?
-I noticed that the battery voltage bounces around too much at idle, it goes from high 13-15 volts rapidly. It's the same alternator, maybe a couple years old and before it would hold a steady 14.2-14.4 volts at idle with nothing on. I noticed the clamp nut for the wire harness (back of alternator was loose, but I don't think it would cause this. Does the wiring look correct?

I thought I had it the same as before but maybe something is wrong. The GRD I have going to ground just like before, and also have a ground going to the top of the alternator bracket as another engine ground.

^This is fmc400s area of expertise.^
Follow his instructions

How are your plugs/plug wires?
Look for vacuum leaks.
Do you know how to adjust your carb?

EDIT
Mike, you beat me in again

 

Okay, so the gap is the same as before at about 0.035". Newer plug wires, maybe 100 miles on them. I installed them before the rebuild.

No the front clip was not taken off during the rebuild. Just pulled the engine, and rebuilt the main core but most external parts are the same if they were new before. I have my Fluke Multimeter that I use so it's not a cheap one.

I didn't think about that with the points being open/closed. Can I test it with the engine off, but key on with the points open and then closed? I would take the cap off and rotate the engine so the points are closed or open.

There is no ammeter on the truck.

The tape is just coming off after I pulled it off, it has heat shrink on it but the last owner seemed to put some tape on it.

I think that the alternator is a pretty large issue, because if there isn't steady voltage then everything running off it will have problems, plus it's a main problem in itself. My main question is if the alternator is hooked up right, because I want to fix that first then go from there.

I already checked for a vacuum leak, idle mixture, etc...and this is pretty much the last things I have. The idle mixture was where it was before, richening or leaning out didn't affect anything.

The idle is smooth as far as rpm (+/- 50 rpm), but the engine moves too much similar to a bad misfire or in one case before on a different car the alternator plug was loose so voltage fluctuated rapidly and caused this.

The vacuum gauge bounces from 14-16in/Hg, so not horrible but also not perfectly steady.

Thanks for the quick responses. Do the alternator connections look correct?

 

I found the alternator connections were in the right places so I cleaned up the connections and that helped a lot. Now it stays around 14.5 volts and the idle seemed to improve. One thing I noticed though is the stator wire goes from the voltage regulator up to a plug where I guess was the factory electric choke. Then the other wire goes back to the alternator. These two wires aren't connected so the stator isn't connected, I tried jumpering it but didn't notice a difference. It was like that before so I guess it works for now.

I regapped the plugs to 0.037", the plugs didn't look abnormal they still look new. Checked for vacuum leaks again at idle with WD-40. And the timing is at 12 degrees now.

I thought the vacuum advance should be from a ported/timed port for these distributors, right? I checked the "timed vacuum" port on my edelbrock carb and at idle it was 10inHg, maybe it has an internal leak? Shouldn't it be 0inHg at idle?

I adjusted timing with it disconnected to 12 degrees, then at idle it was 30+ degrees and would advance as I open the throttle. So I adjusted timing to 12 degrees with it connected and still get timing advance as I open the throttle, so it works that way but that doesn't seem right to me.

Here is a video of it idling, it was at about 700rpm but after revving it up it started to idle at 800rpm and bounce around some more. Dwell angle is about 25 degrees. It has a noticeable "lope" out the exhaust, since it didn't do that before as much I think it's a misfire. As you see there is a nice pop out the carb when I open the throttle rapidly. Slowly, it revs up nicely.

Again the idle mixture screws don't seem to affect anything, so I think it's a bigger issue.

Sorry for all the information but I think it's better to give too much than too little.

Picture_zps14a8e655.mp4 Video by ZCHONDACRX91 | Photobucket

 

Let me just say that I don't know shoot about Ed carbs.
I understand the basics but I for some reason just don't like 'em.
They remind me of Qjets and that says GM (yuk).
However as you see below some things from GM are informative.
Yes it should. Possible internal problem like wrong gasket or bad O ring.
That sounds like you're in manifold pressure.

Possible late timing. There are other reasons but let's not go there.
If it was me I would want to rebuild my carb so I was sure of everything. You should be able to kill your engine by turning the screws in.
Your carb may be a big part of your problem.


Here is an explanation for ported vs manifold vacuum.
I only include it because of your questions about it earlier in your post.
This is by an old GM engineer.
I must confess I like the simplicity of purely mechanical advance and it's what I like to use.
Prolly 'cause I'm a moron

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.



Remember this is one guys opinion.
I believe there are different requirements for different engines. So I don't believe there is a blanket answer.
But it does give you something to think about.

 

I hear what you're saying, tho can't help but think if I swap over the vacuum line to manifold vacuum it will run like crap. Holley 2 bl. and 64 y block dizzy: Unless ... and would require restring of idle... and mixture and - vacuum advance, correct? aha! Hm... Will have to try this...

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Yeah I may stir up a hornets nest but it's fun to try new stuff.
I had always thought that ported was the way to go but another forum got me questioning myself.
I think it boils down to what parts you have and what era your engine was built to run in.
If you are tuning a smogified motor with older parts, prolly be best to stay ported.
But if you totally rebuild and lose the old system go manifold.

 

Well - did my 64 have EGR? It has a draft tube and regular valve covers. It's basically pre-smogulated.

The carburetor however is a holley 2 bl replacement - and this is what you are getting at then, has provision for spark or, ported timing. Maybe the original setup used manifold vac? This will change the set point for idle and everything else if using manifold vacuum, and also using a vacuum gauge if setting initial??

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Yeah, totally new tune.
And as I said, for me it's just theory right now as I have not done it with old stuff.
Kinda fun tho' to play with eh?
Maybe we should do this in another thread though.
Don't want to throw a bunch of stuff at the OP while he's trying to diagnose a problem.

 

It probably reminds you of GM because they're basically rebranded Carter AFB's, remember them? They were GM's go-to performance carb before the Rochester Q-jet came along. AFB stands for "aluminum four barrel". The one before that was the Carter WCFB, that one stood for "white cast four barrel....end of history lesson, and useless trivia.

 

That was a good read turbohunter, I learned a lot from it. On my 75 F250 it still has points ignition, so it's a late 1974/early 1975. It doesn't have any emissions parts like EGR or catalytic converter. From what you said I should just have full manifold vacuum hooked up to the vacuum advance?

Ported seemed to be for emissions vehicles that needed very little timing at idle. Are different vacuum advance "cans" designed differently for ported vs manifold vacuum, or is it the same for either?

You made a good point about the carburetor, maybe it does need a rebuild. It was idling and running good before the rebuild on 7 low compression cylinders, so maybe it just worked out that way. It makes sense how when I move the idle mixture screws the vacuum or rpm don't change at all. I even turned one screw in almost all the way and that should of leaned it out pretty far.

Also another strange thing is when I was trying the ported and manifold vacuum ports on the carburetor, if I left them open idle didn't change at all. I was expecting a vacuum leak situation, but it didn't seem to matter if they were plugged or not. I don't know what the deal is with the carburetor but I'm going to just pull it off and check it over to start fresh. Once I figure out the carburetor I think it will idle much better.

 

Yeah I think your carb may be most of your problem. At least from my vantage point.
I included the vacuum discussion so that you might get a better feeling for what's been happening as you do your diagnosis.It seemed like you were trying a bunch of different ways to get it running.
I wouldn't try to change things around. I would stay with ported vacuum for now. That's the way your parts are supposed to work.
If it were me I would run it as close to factory as possible and just trouble shoot your parts (like the carb).
I can't remember now if you did, but have you checked to make sure your distributor is advancing?
I know it sounds the opposite of that discussion but if your carb is right it should work.

 

Have you fixed the STA wire on the alternator? In the picture it looks like it's connected by a single wire thread.

Eldebrock carbs (1406 - 1405) have the ported vacuum on the left, manifold on the right of the bigger nipple, but you knew that. Try pulling the mixture screws all the way out - completely out - just watch for the little dealies on the screw ends...any difference?

Yes, vacuum advance cans are the same if you're running ported or manifold vacuum. I won't get in on the poted vs manifold discussion....been there before. I have found, though, it comes down to what you and your motor likes the best. Have to finagle the timing, then carb adjustments no matter which one you use, though.

No excessive play in the throttle plate shafts?

Agree on carb rebuild.... vacuum leak(s) are from somewhere...carb gaskets, loose throttle shafts, internal boogers....
Good luck