Newer Direct Connection Distributors - Mechanical Advance Limiter

[PurpleBeeper]

This is probably old news to everyone except me. I bought a new DC distributor last year & I finally am working on my timing & fuel curves. I bought a FBO mechanical advance limiter plate, but when I gutted my distributor, I found that this newer one has that feature already built in. Great engineering Ma' Mopar!

 

[biomedtechguy]

So there are multiple square holes in the disc under there?

 

[HEMI-ITIS]

In my old one just tacked it to lock it out,,,,,,,,

 

[Darthomas]

I used to fit spacers into the slot where the weights swing out.
This became necessary after real leaded premium gas disappeared in the early 80s. I limited mechanic advance to about 10 degrees, and picked up 12 in vacuum.
The stops made full advance seem more smooth and fixed in timing than when just letting the weights run out to the springs' limits. The engine was more ragged with the springs only, much smoother with the solid advance stops.

 

[biomedtechguy]

This is the FBO part. See pic. These are the instructions:
http://www.4secondsflat.com/J685S instructions.htm
I was curious what the difference between the Mopar unit and this is relative to mech advance limiting features.

 

[PurpleBeeper]

So there are multiple square holes in the disc under there?

Nope. On the newer Direct Connection distributor, you loosen two star head screws (screwdriver in one & other is half hidden behind one of the advance springs) and then you can twist the center plate (the screws will slide in those slots). The screws/slots are the mechanical advance limiters, so you basically have the FBO plate built right into the distributor. The distributor came with the screws where they're shown in the picture... all the way at one end. So I loosen the screws, slid the plate about 1/8" and tightened the screws back down to reduce my total mechanical advance. I don't need the FBO plate I bought at all. The only downside is that there aren't any "numbers" on the distributor slots, so you have to "ballpark" how much you're limiting the advance & try it. My first guess of 1/8" seems to be pretty close. With 15 degrees initial, I've got 35 total (no vacuum).

One thing I noticed about the FBO plate was that the least I could limit the mechanical advance to was 18-degrees. My cam isn't all that radical and I'm fine with limiting the mechanical advance only to a 20-degree limit and I couldn't do that with the FBO plate.

 

[biomedtechguy]

I see. Thanks. After I installed the FBO system, I made adjustments to the 6 bbl carbs with major improvements at idle. The outboard carbs were supplying way too much fuel at idle. I am idling at about 850-900 rpm now, pulling 10" of vacuum. Next is to bring the rpm up a bit to see if I get 11" of vacuum because now that I have the carbs "dialed in" my vacuum advance isn't functioning at idle. It was (before carb tweak) adding the 12° of advance it was supposed to. I may try tweaking the vacuum can allen adjuster too.

 

[Darthomas]

I see. Thanks. After I installed the FBO system, I made adjustments to the 6 bbl carbs with major improvements at idle. The outboard carbs were supplying way too much fuel at idle. I am idling at about 850-900 rpm now, pulling 10" of vacuum. Next is to bring the rpm up a bit to see if I get 11" of vacuum because now that I have the carbs "dialed in" my vacuum advance isn't functioning at idle. It was (before carb tweak) adding the 12° of advance it was supposed to. I may try tweaking the vacuum can allen adjuster too.

It's some work, but if you'd like more vacuum at idle, get a set of offset cam keys. Use the 5 degree key and advance the cam timing. Much better drivability, torque, and helps mileage.
Just have to keep that in mind when figuring out total advance.
This makes the engine a "more efficient air pump" at lower rpms.
I also figured I would be ahead even as the timing chain stretched.
If you are primarily racing, this cuts into top end output, then you wouldn't want to do this.

 

[biomedtechguy]

Manual brakes, my choice even though I switched out from stock drums to discs up front. It's easier for me to feel how much brake I'm applying that way. I'm ok w/the vac, but I want my distributor to work as designed, as it did when I installed it April 28th. I'm going to tweak it today. Thanks for the suggestion, though.

 

[Darthomas]

Spring in vacuum advance can should be set so that no advance comes from it at idle, or very little. The vacuum advance is supposed to be pulling the full amount of advance when at part throttle and steady load. As soon as you go deeper into the throttle, it should back off timing so you don't have knocking. I like to hear just 1 tiny ping as I go into the gas, then I know I'm at maximum efficiency.

 

[biomedtechguy]

Interesting viewpoint darthomas..different from my understanding. What I got from Don's explanation is more advance during inefficient burning and lean fuel cycle allows for more complete burning of the air fuel mix because it is "lit" earlier. Anyway, using the recommended constant manifold vacuum source this did (and should again if I can figure it out) provide more advance at idle/closed throttle, and less vacuum advance as you open the throttle.

 

[HEMI-ITIS]

Spring in vacuum advance can should be set so that no advance comes from it at idle, or very little. The vacuum advance is supposed to be pulling the full amount of advance when at part throttle and steady load. As soon as you go deeper into the throttle, it should back off timing so you don't have knocking. I like to hear just 1 tiny ping as I go into the gas, then I know I'm at maximum efficiency.

That ping you can hear is not the only detonation going on.That ping is damaging and will take it's toll in time.Take out 1* at a time till you can't hear it.

 

[Darthomas]

Well, it also depends on the year of the carb, and whether the dist vacuum port is below the throttle plate when it's closed.
They moved the port for emission reasons to just even/above at some point, so that has to be considered.
In 38 years, I haven't damaged anything from detonation.

 

[biomedtechguy]

I adjusted my idle up after reaching normal operating temperature. It was around 825-840, brought it to 890-910 and voila! Picked up 1 inch of vacuum (went from 10 to 11) and that got the vacuum advance working again (constant manifold vacuum source). Tried adjusting the vac can on the distributor per Don's instructions (3/32nds Allen wrench) in the nipple of the can, but I couldn't get a "bite" so I am at 18-19° vac line off, 28° vac line on (should be adding 12° as it was setup by Don). Took a "test hit" nice and strong smooth 1st and 2nd gear pull (a833 4 speed) and at 35 mph no ping when I floor it in 3rd...I had advanced the initial timing to 22° last weekend, but from a 10 mph roll in 1st, when the outboard carbs opened it stumbled. Took it back to 18-19 initial and repeated, all good. So that's it for now until Don or someone can tell me why the Allen wrench didn't bite when I tried to adjust the vacuum can. Also I still want to check out the timing advance range again as I go up through the rpm range out of gear watching it with my light. Al in all, between the carbs adjustments and the iggy, better by a lot than before

 

[hunt2elk]

So that's it for now until Don or someone can tell me why the Allen wrench didn't bite when I tried to adjust the vacuum can.

I have a couple of cans that take a metric allen wrench instead of the 3/32".

 

[biomedtechguy]

I have a couple of cans that take a metric allen wrench instead of the 3/32".

I tried +/- 1 step in size, each, standard and metric with no luck. Thanks for the reply.

 

[Ratroaster]

Nope. On the newer Direct Connection distributor, you loosen two star head screws (screwdriver in one & other is half hidden behind one of the advance springs) and then you can twist the center plate (the screws will slide in those slots). The screws/slots are the mechanical advance limiters, so you basically have the FBO plate built right into the distributor. The distributor came with the screws where they're shown in the picture... all the way at one end. So I loosen the screws, slid the plate about 1/8" and tightened the screws back down to reduce my total mechanical advance. I don't need the FBO plate I bought at all. The only downside is that there aren't any "numbers" on the distributor slots, so you have to "ballpark" how much you're limiting the advance & try it. My first guess of 1/8" seems to be pretty close. With 15 degrees initial, I've got 35 total (no vacuum).

One thing I noticed about the FBO plate was that the least I could limit the mechanical advance to was 18-degrees. My cam isn't all that radical and I'm fine with limiting the mechanical advance only to a 20-degree limit and I couldn't do that with the FBO plate.

Beeper, did you take a picture of the plate after your adjustment? I have the same
distributor and am trying to adjust it the same way. Attached, I hope, is a picture of mine. If I read you right, after adjustment, the tab in the slot should be back an 1/8. Is this correct? Thanks. Rat Roaster.

 

[72RoadrunnerGTX]

This is probably old news to everyone except me. I bought a new DC distributor last year & I finally am working on my timing & fuel curves. I bought a FBO mechanical advance limiter plate, but when I gutted my distributor, I found that this newer one has that feature already built in. Great engineering Ma' Mopar!

View attachment 338453

Yes, the later original Mopar Performance distributors used a Mallory designed centrifugal advance mechanism, not the OE Chrysler design. The MP spring kit for them included a gauge set for setting the advance limit on those. Pretty sure they are long out of production now, haven’t seen them available for some time. Had to buy Proform copies of it for the last couple of projects.

 

[beanhead]

Beeper, did you take a picture of the plate after your adjustment? I have the same
distributor and am trying to adjust it the same way. Attached, I hope, is a picture of mine. If I read you right, after adjustment, the tab in the slot should be back an 1/8. Is this correct? Thanks. Rat Roaster.

View attachment 977621

Where the tabs end up will differ for your specific needs. Make your adjustment, put the timing light on it, check the full curve and re-adjust if needed (I like to use small drill bits to hold the plate in place while tightening the screws).

 

[HALIFAXHOPS]

MP has been using that style for a while, but think they stopped the design now. Kind of hard to find. You use the Mallory keys to easily set the mechanical advance. I actually like those units. Just FYI