One of my trucks is a '79 F250 4x4 (400,C6, 3.54 D60 front & rear). The engine was rebuilt by me a few thousand miles ago. The truck is used occasionally, mostly in winter for snowplowing and to keep my recently restored '69 F100 out of the Wisconsin salt. Occasionally the '79 is used in the summer too... Because it does not need to pass any smog testing, most of the smog equipment has been removed or disabled (EGR, cats, vacuum switches, hoses, canister...).

About a year or two ago, the engine started to start and run like it had excessively advanced ignition timing... put a timing light on it and WOW! ~25degrees too far advanced... I checked the centrifugal advance assuming it was stuck from lack of use - it was free and working properly. Jumped timing chain possibility is out - the chain is only a couple thousand miles old and anyways a jumped chain wouldn't cause the timing to ADVANCE. I know the distributor was not moved, the truck sits parked in my back yard. I gave up trying to determine the cause and just rotated the distributor to compensate for the "new" timing it somehow had.

Just a couple days ago (its snow plowing time again...), the engine all of a sudden started acting lazy and sounded like the ignition timing was really late... put a light on it and the timing now was retarded about 25degrees from where it should be... Advanced the distributor and it runs perfect again.

What gives? Distributor gnomes moving the distributor around when I turn my back? No seriously, could it be the duraspark box? The box has the blue strain relief...

 

It's not the box. They either work or they don't. When they don't work, the engine doesn't run at all. When you check the timing, is it up to operating temp with the vacuum advance line plugged? 25 degrees almost sounds like you're checking it with the fast idle running and the advance connected.

 

Yeah, it's very important your truck is running at idle speed in drive, not park. The tach should read about 650RPMs when timing initial..

and is it possible the distributor gear is worn? or the cam gear?

 

Not so. Faulty ignition modules can fail quickly or last for months.

The usual scenario:

1. In the beginning, the module overheats, the engine cuts in and out.

2. The module overheats, engine dies...module cools down, engine restarts.

3. The module overheats, burns out...the engine dies and won't restart.

It's difficult to test ignition modules because heat is the enemy. Off the vehicle, the module is cool, so when having it tested by an autoparts store...it may check out OK. Once back on the vehicle, the problem returns.

The absolute worst module Ford ever made is the one with the RED plastic square (D7AZ-12A199-A = marked: D7AE-12A199-A1B or A2B)...mostly these are seen only in CA. During 1978, 1000's of new and nearly new cars/trucks were stranded on Ford dealers service lots because the module was defective from the get go. The second replacement module was D99 = back ordered Detroit. It took Ford 6 months (!!) to come up with a good replacement module that actually worked. The first time around, the POS replacement was no better than the one it replaced.

In order to fix the problem (and get the PO'd customers cars back to them), junkyards were canvassed by a few enterprising Ford partsmen for the 1977 version, which while still the same part number, was better than what Ford used in 1978.

 

I properly checked the timing.

 

Engine RPM was about that. Centrifugal advance was not coming in (I checked for that).

Distributor drive gear looked fine. Cam was new.

 

Yes you're right. What I meant when I said that the engine won't run at all is that it cuts out or stalls, rather than run sluggishly. I should have been more specific.

 

Very interesting.

The ignition timing was dead-nuts on before this problem. Then all of a sudden it is advanced by about 25 degrees - without touching a thing. Later it retards itself by about the same amount - again without touching anything.

The engine's ignition never cut out. If it did I would have replaced the box.

I somewhat suspected the Duraspark box because I remember reading that it can change the ignition timing for starting... perhaps it thought it was doing that... how much can it affect timing - 25degrees?

Now I will install a different Duraspark box.

 

I personally don't think it could cause your problem and since they are pretty expensive I would hold off on it until you're sure it's not something else.

 

this happened to me also, cruisin down the road and it would start pinging and running hot, shut it off then it was fine, few days later it was gutless, then fine again, it turned out being a bad distributor, (dont know why or even how it happened or could happen, disassembled it, rebuilt it changed out the pickup, the vac advance, a new roll pin on the gear) i popped in a spare duraspark dist and it has been fine ever since

 

That is my general policy - don't change parts aimlessly hoping the symptoms go away I pride myself in that I am one who troubleshoots first to identify the defective/failed parts and then replaces only what needs replacing. Like the difference between a true mechanic and just a "parts replacer"...

To recap, the problem is that the ignition timing is changing, and more specifically, all by itself. First it advanced itself by ~25 degrees about a year ago. I then retarded the timing by rotating the distributor to get the timing back where it should be. I never determined the real cause at that time, just compensated for it. Now, the timing retarded itself (again by ~25 degrees), requiring me to rotate the distributor back to where it was earlier (referencing by the vacuum advance position). Now I want to solve the problem for good.

I do already have a spare Duraspark box - who doesn't drive anywhere without one?

 

NOT on 79's.

The timing is checked with everything hooked up at normal Operating temp
at 650 rpm



You can add me to this philosophy

 

Are you 100% sure your mechanical advance is OK?
My old one broke, I got a junkyard distributor and it had the same issue. The spring that holds it would stick or was worn or broken (couldn't get the cover off) and the advance would stick to a position.
It would freely move, but it's not suppose to, it's suppose to pull itself back to the non-running position. Look into the small hole on the top of the cover under the armature, turn it until you see the two plates that are seperated biut should be touching (or close), use a small screwdriver to seperate them, if they don't move themselves back together then the spring is busted.

It really sounds exactly like what it is.. I know you said you checked it, but this is all I have to offer. After my second broken DS I replaced the whole ignition system with a stupid Chevy HEI (I'm somewhat embarrased about it).

 

When V/A's go bad they "don't" advance the plate causing no advance when required.

To the O/P..have you checked for broken/missing distributor weight springs ?

 

the weight springs are what I was talking about.. they're flat metal objects, so I call them plates :x