Ok, for those of you that have stock setup with smog and all, and just want to improve milage using this method here seems to be some better info for us:

Link to article - http://www.classicinlines.com/DSII_swap.asp


Getting better gas mileage with your DuraSpark II
written by FordSix forum member: Mark P

If you have a DuraSpark II distributor, you also have smog-control spark advance curves inside. All of the DuraSpark units came this way, to reduce hydrocarbons. Unfortunately, it also reduces MPG. However... today's gasoline reduces the hydrocarbons for you, even more than the slow spark curve did. So, get a set of the Mr. Gasket #925D distributor springs and modify your distributor as follows: First, remove the lighter of the 2 springs in the DII and replace it with one of the #925D springs. DO NOT replace both springs with the #925D springs (more on that in a moment). Next, there is a support arm that holds the larger spring, on it's non-slotted end. Bend this arm in toward the center of the distributor, just 3/32 of an inch.

This combination replicates the "police interceptor" distributor spark curve for the 200 I-6, according to a local "guru" who helped build these for Ford in the early 1980s. It causes earlier spark advance up to 1200 RPM, which then remains steady until 1800 RPM, when the secondary spring starts its advance. This "plateau" of early advance, limited by the 3/32" inward adjustment of the heavier spring, provides more torque at around-town speeds. This improves your MPG because you don't have to open the throttle as far to get the same speeds you were running before. You'll also notice better off-the-line pep and cold-running performance. Mine also starts a little better when hot, and the plugs stay cleaner. My in-town MPG rose from 16.5 average to 18.1 average over 2 tanks of gas, winter driving to and from work. I haven't tested the highway MPG yet, but I don't think it will improve from the 23+ I've been getting. I'll re-post if it does.

If you make the mistake of replacing BOTH springs in your DII with the #925D springs, you will get a SLOWER spark curve unless you extensively modify one of them to provide a "slot", like the heavier spring already has. But I found, even after that exercise, that the engine "hit the wall" at about 2200 RPM and would not accelerate well up hills. Adding in the heavier spring, but delaying its effect until 2000 RPM, did the trick: now it loves to rev, even on regular gas (my compression is about 9.4:1). The maximum advance is reached at 2600 RPM on my engine with this mod: before, the max advance was at 3200 RPM.


To summarize:

Replace the medium spring with a light spring from Mr.Gasket #925D,
Bend the support arm that holds the heavy spring inward 3/32 of an inch.

 

Ill repost the link because its the best info on tuning duraspark ignition that ive come accross.

http://www.classicinlines.com/tech.asp#Ignition

Also a very good article on carb adjustments using a vacuum gage.

http://www.classicinlines.com/Vacuum.asp

I think this is my cure

 

I kid you not its very very nice now!! This setup actually has the lighter spring doing all the work at low rpm, unlike having 2 light springs working together, and then the heavy spring kicks in later at higher rpm. I also restore the gap back to the default 16L.

I need to replace some vacuum lines and hook the smog crap backup, adjust the carb then some final ignition timing, but the throttle feels very good all the way threw the rpm range the truck sounds health.

 

Congratulations!

I'm glad it's where you think it should be.

Thanks also for sharing the link. That's informative...

 

I might have to do this as another cheap mod after I do the blinker one!