Ouch! Just did the timing curve on my '84 last night and there were definitely some red flags:
(FYI: It is retrofitted with a Duraspark II ignition as well))

Base timing: 10°

Mechanical:

0750 - 0
1000 - 0
1250 - 3
1500 - 5
1750 - 7
2000 - 9
2250 - 10
2500 - 10
2750 - 10


Vacuum advance worked fine, but unlike the '81 had a range of 18°, and didn't come in until around 6hg. Of course that's completely adjustable.

But, only 10° of mechanical advance?? Ouch, no wonder it just doesn't have the pep the '81 has. Something, internally, must definitely be wrong with it. Granted, I DID get it out of a JY when I swapped it over, so I should've expected as much. Just never checked it before.
I'll be picking up another distributor this evening.

 

Yeah, that's one of the better walkthroughs I've seen (and where I got the picture above). I think that's the tough part though is that every engine will be different.

But, I also think there's things that can come in to consideration when looking at your own (in this case, the 300).

It peak isn't coming in until 2500 - 3000, that's one thing for a V6 or V8 that revs to 5500 - 6500 RPMs, but is another for a straight six that peaks out around 4500. 2500 seems to late, let alone 3000.

Then, the 300 is a low revving motor. For example on the stock carb motor, peak torque is at 1400 RPMs. In my case, I don't even have any mechanical advance until 1500. What potential am I missing out on?

That's kinda what I was thinking. A lower static would give a better idle, I would think, and then have it increase a greater distance quickly. But as said, it can definitely be an engine by engine basis.
Having EXACT numbers for everyone isn't going to work.

One of my all time favorite movies.

My continual issue with recurve retest is just...how....long... it takes to disassemble a stock DSII distributor. It's not a quick and easy thing.

I tried putting my '81 at 16° initial yesterday and it seemed to do fine. I'll leave it for a few and see how it behaves.
As for the retard feature on a DS, I've heard of it, but never seen one. I know they're labeled on parts sites as 1 or 2 vacuum ports on the vacuum advance. The ones I've seen, though, just have the one for advance.
How does the retard feature work? What activates it?

Definitely a really cool idea! But I don't know if I'd want to mess with that every morning I go out and head off to work, run to the store, etc. for a some extra timing advance. I imagine if it's your toy, though, something like that'd be slick.

 

Re in"Car"nation High Performance - Home Their phone number is listed on that page...give them a call and run it by them.

Also, go through the ignition section in Classic Inlines Tech section. http://www.classicinlines.com/Tech.asp

 

this is an advantage for the points dizzy...i can get to my springs/weights without too much trouble.


The thing i'm talking about is a feature built into the module- while cranking, the module retards spark advance for easier starts. not sure exactly how.

 

From what I've read, the timing is retarded ~8 degrees and is handled by the ignition module. As soon as the key is released to the "run" position, the timing again advances.

 

I'll see if I can get a hold of them, I'd be curious what they think.
As for information on places like Classic Inlines, how does that translate from a small six to a big six? Don't they have higher redlines and such? I would also think their power curves would be rather different. Dunno though.

On the DSII dizzy, you have to pry the reluctor off, which can be the difficult part, because there's nothing to pry against. I've bent up the cover plate several times trying to do this. It's something I only like to do ONE time and want to be completely done with it after that.

I remember hearing that now about the ignition module itself. Pretty cool feature, but I'd be curious how it does it, since generally you'd have to physically rotate the distributor to change timing (which the vacuum advance does).
Maybe it just sends the signal a fraction late when it's cranking?

 

When the timing should be all in depends on the cam, engine, vehicle, gearing, etc. There's no specific advantage to advancing it sooner - an engine only needs as much timing as it needs. For a truck 3,000 or 3,500 rpm is safe. If it's hauling big loads closer to 3,500. The only way to know if sooner is safe and better is to try it.

Mechanical advance should start right off idle. How much are you leaving on the table? Maybe not much. The vacuum advance is probably making up for a lot of it. I don't remember at what "/hg mine kicks in but getting vacuum advance all the way down to 1" seems off. The other truck at ~6" sounds right.

I must have gotten lucky because the reluctor pops off my dizzy pretty easily. Maybe it's the big screwdriver and small pry bar. Steel wool the shaft and reluctor then dab with a little grease the first time and it's a little easier the next time. And easier still the time after that.

MSD makes a spark retard/advance dial kit for the dash that lets you change the timing on the fly. I don't know if it only works with an MSD box but, man, that would be pretty handy on a truck when you're hauling a load up a hill with a tank full of crappy gas.

 

The material in the Classic Inlines Tech section is not necessarily application specific but there is good information on the principles of ignition and timing and how to apply them.

 

Well, last night I got another distributor for my '84 since I found out the mechanical advance was only advancing 10° max. After I installed it, I mapped out the advance curve over the RPMs. It had noticeably more power with the new distributor. I'm curious how its gas mileage will improve around town and on the interstate.

However, I found this really interesting! Since my '84's been converted to a Duraspark II distributor, I asked for a distributor for an '81.
The distributor in my '81 is from Autozone and the one in my '84 is from Schucks/O'Reilly.

Here's the advance curve for the '81 (same as on the previous page):

RPM - Advance
0750 - 0
1000 - 0
1250 - 0
1500 - 3
1750 - 9
2000 - 12
2250 - 16
2500 - 18
2750 - 18
3000 - 18

Vacuum (hg):
12 - 24
11 - 24
10 - 24
9 - 22
8 - 20
7 - 18
6 - 15
5 - 13
4 - 12
3 - 10
2 - 3
1 - 1.5
0 - 0

Now, here's the advance curve for the '84 (from the new distributor that's now in it):

RPM - Advance
0750 - 0
1000 - 0
1250 - 1
1500 - 8
1750 - 12
2000 - 16
2250 - 16
2500 - 17
2750 - 19
3000 - 21
3250 - 23
3500 - 24

Vacuum (hg):
14 - 17
13 - 16
12 - 15
11 - 15
10 - 15
9 - 13
8 - 11
7 - 9
6 - 7
5 - 5
4 - 2
3 - 0
2 - 0
1 - 0
0 - 0


They're completely different. Both were ordered and installed within the last year, and both are distributors for a "1981 Ford Bronco 300 I6".

The one in the '81 has 18° of mechanical advance that all comes in by 2500, with an additional 24° vacuum advance that starts coming in immediately, AND quickly.

The one in the '84 has 24° of mechanical advance that takes 3500 RPMs to all come in, plus a smooth gradual 17° of vacuum advance.

Two people that ordered two of the same distributors for the same engine could get completely different timing curves, performance, gas mileage, etc. etc. How would anyone know what they're going to get? Why are they so varied?


One thing I did notice that was interesting on the new distributor is that it's quite clear when the light spring gives out and the heavy spring starts. The consistent 16° mechanical advance from 2000 to 2500 RPMs shows that. Then, it starts climbing again as the heavy spring gives, from 2500 to 3500 RPMs.

 

The engines/drivetrains came from Ford with different calibrations, e.g. 49-state low-altitude, high-altitude, California emissions, with or without different options, etc.

I just confirmed what you said and looked up a dizzy on AZ's site for a 1981 6-banger, the differences are:

- New or reman
- Single or dual vacuum advance

There are literally hundreds of calibrations in the FoMoCo 1980/89 Light Truck Parts Manual, my guess is the aftermarket vendors want to simplify things and provide something that "will work" so they can get your money instead of the dealer.

JMO.

 

It should definitely pull more vacuum and get better mileage. Locking out at 10 degrees had to be killing you at highway rpm.

It should be close as it sits. Try just bending the spring tension tab through the breaker plate to get the advance to kick in closer to idle first. It saves prying off the reluctor for now and may be all it needs.