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Unread 07-12-2011, 08:15 AM   #1
Matt1981CJ7
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Help with Advance Curve

1981 CJ7 258, Weber 32/36, Holly PR, Team Rush Ignition

The vacuum advance finally shot craps on my original Motorcraft distributor, so I decided to replace the entire dizzy with the NAPA version (Part #48-4691).

Thanks to Jeephammers stellar "How-to" posts, the dizzy install went smoothly.

Below is my current advance curve with the new distributer. I'm curious about the flat spot in the total advance in the 1200-1600 RPM range.

I don't recall seeing other curves like this. Is this normal, or does the timing still need a little tweaking?

TIA,

Matt

advance-curve-1.png  
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Unread 07-12-2011, 08:46 AM   #2
swatson454
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I'm guessing the pink line is the vacuum advance, yes? If so, that's the kind of stuff you get when running ported vacuum. If the blue line is centrifugal, that's one of the reasons people see such a big improvement when they install a D.U.I. That curve above just screams '70s emissions! LOL

Nice job plotting your curve out man!


Shawn

Edit: I should elaborate some more here. Ideally, the vacuum advance curve should be plotted using a vacuum pump. That flat spot is indicative of using the throttle, which is why I said that. Forgive me if I'm mistaken. It's an interesting deal, however, when using a vacuum pump for plotting the vacuum curve and then using ported vacuum for actual use because you don't get actual engine load, you get whatever signal the carb gives you based on throttle position. There can be quite a difference between what you see on the chart and what the engine sees during operation.

You may want to make a separate chart that has vacuum advance only on one axis and the vacuum reading on the other axis, rather than rpm. Then you can test actual driving conditions with a vacuum gauge in the cab hooked into which ever vacuum source you prefer running and get a better idea of what the engine is seeing under actual driving conditions and make any necessary changes.

I hope it helps!
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Unread 07-12-2011, 09:16 AM   #3
Matt1981CJ7
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Thanks, Swatson.

LOL!! Figuring out how to get the right chart out of Excel was the hardest part...

Yes, the pink is total (vacuum and centrifugal). The blue is centrifugal only.

So what do you suggest doing about my oh-so-70's curve...hang some Puka shells on the rear view mirror and call it good?

Matt
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Unread 07-12-2011, 09:31 AM   #4
Matt1981CJ7
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It appears all of the vacuum advance is in between 800-1200, leaving a dead spot until the centrifugal kicks in at 1600.

Should the centrifugal kick in earlier, or the vacuum later, for a good performance curve on a CJ? Perhaps a few turns CCW on the vacuum adjustment......?
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Unread 07-12-2011, 09:33 AM   #5
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I snuck an edit in there Matt, sorry.

Because vacuum advance is so variable, it isn't considered in the "total" advance curve. "Total advance" is initial + full centrifugal. You appear to have roughly 24* of total advance: 11* base + roughly 13* centrifugal.

Ideally, the centrifugal curve would start advancing just off idle and steadily and slowly (with these engines) increase until it reached somewhere near but not usually over 36* total at roughly 3,000 to 3,500 rpm. The vacuum canister would pull maximum vacuum advance by roughly an inch less than manifold vacuum at idle. That means that if your engine pulls 18 inches of manifold vacuum at idle, the vacuum advance is pinned full advance at roughly 17 inches. It then needs to be adjusted so that it provides no more advance when manifold vacuum drops to usually 6 to 8 inches. That will be determined by your centrifugal curve, fuel, engine combo, tire size etc.

It seems like the easiest way to get there with a Motorcraft is to find an advance head that will get you as close to a 24* centrifugal curve as you can get and then adjust the rate of advance with different springs. Luckily, the vacuum canister is adjustable. If that doesn't quite get it, but it most-likely will, I prefer the stick-on hula girl. The Puka shells are nice though! Maybe both

I hope to God above that this thread doesn't blow up!


Shawn
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Unread 07-12-2011, 09:48 AM   #6
Matt1981CJ7
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Shawn,

Thanks, I will chart the vacuum readings later this evening.

It always comes back to this darn manifold vs. ported vacuum issue. It's my (perhaps incorrect) understanding that the 1979-82 Jeeps were designed to use ported, and many knowledgeable Jeepers seem to recommend sticking with that.

Others swear by manifold, which is probably the way it should be.

I've got mine plumbed per Mcmuds "hybrid" system, which uses manifold only when the CTO is cold.

I think I'm going to give full manifold a try per your instruction above.

Matt
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Unread 07-12-2011, 10:05 AM   #7
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I have a feeling you'll be converted to the dark side! Most guys are, lol.

After recurving my Motorcraft and using manifold vacuum, I found a sweet spot on the canister adjustment at 7 turns out from its seat. Clockwise all the way in and then back out 7 turns. Yours may be different but that worked on my '83.

It comes down to the actual advance curve. Looking at the curve from your stock distributor, no big cap, no stealth module, no e-core coil and no CDI ignition box is going to allow that engine to perform as good as it can. Sure, those things certainly do help but they're not the meat and 'taters of the problem.

One of the worst things you can do, though, is to throw a couple of light springs in there. These engines make a lot of torque at low speeds and they won't tolerate a quick curve. They seem to like a slow and steady rate but it needs to start just off idle and continue all the way up to roughly 36*. They respond REALLY well to a curve change, just don't get aggressive!

It sure made a big difference on mine. I hope it helps!


Shawn
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Unread 07-12-2011, 01:46 PM   #8
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OK, I finished my plowing, so now I can resume my "oh so 70's" curve-fest.

Shawn, when you say "find an advance head that will get you as close to a 24* centrifugal", how does one go about that? I'm assuming they are marked, somehow, and I need to figure out what I've got, now, then go from there? Is a bigger "slot" in the blades what I'm looking for? Do you have a name of a supplier who will know what I'm talking about?

I'm a little iffy about tearing into a brand new distributer, but hey, it's only money and aggravation, right?
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Unread 07-12-2011, 02:13 PM   #9
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I hear you about tearing into a brand new distributor but, aside from money and aggravation, there's some good learning to be had.

You're basically looking for a two-digit number that's stamped near each slot on the advance head. That'll tell you where you are now. Then you'll have to find the one you're looking for and install it.

JeepHammer did some write-ups regarding motorcrafts that have some really good pictures. He and I couldn't disagree more about how to set up an ignition curve so how much of his theory and procedures you listen to is your call. You'd probably have the best luck finding them by going to his profile and checking out his threads. I can't remember if the pics are in the "definitive" thread, something along the lines of "inner workings of your distributor..." thread or both.


Shawn
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Unread 07-12-2011, 03:40 PM   #10
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Thanks again, Shawn.

Yes, I've read the thread, and have seen the pics you are talking about. I remember thinking "this is way over my pay scale" while reading it. I understand a bit more, now, but am still hesitant.

Everything I've read says the centrifugal advance should kick it just above idle. It did so on my original Motorcraft dizzy. So why on earth would a brand new replacement be so late timed, requiring a new head to get it anywhere close?

Dang, this Jeep restoration is testing my patience.....

Matt
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Unread 07-12-2011, 04:45 PM   #11
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I've read about some pretty interesting things coming out of those reman units so who knows. Unfortunately, I don't have the answer. Maybe yours got assembled with some really heavy advance springs. Too bad it's a Motorcraft and you have to yard the whole thing apart to check. I wonder if a few drops of oil down the hole in the shaft under the rotor would help free up a new assembly.

If it runs well, we could use the old saying "if it ain't broke, don't fix it."
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Unread 07-12-2011, 06:21 PM   #12
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That's just it...its runs OK. But, when I hear folks, like Mike Romain, claim their 258 "turns 52 mph in second gear" and "75 mph in 3rd gear", I can't help thinking mine could run a little better. The engine is in great shape, good compression, and it doesn't burn any oil. The plugs have the textbook brownish tan overall color, with the appropriate color change half way down the ground strap.

Well, I adjusted the vacuum canister out (CCW) 4 full turns, and it eliminated most of the low RPM detonation I was getting. I assume this moved the vacuum advance curve further to the right, making it kick in closer to the centrifugal.

The problem, now, is the Jeep has no "jump" at low speed. It accelerates smoothly, but slowly, when I step on it. I get a noticeable surge in acceleration when the advances kick in, but it's late in the RPM range.

BTW, I tried to tear apart the old Motorcraft dizzy, just to see what's involved in replacing the head in the new one, but I couldn't even get the thing apart. One of the Philips screws on top was so tight, I mangled the head and never did get it loose. Aaaaarrrrgh!!

This whole thing is really starting to frustrate me. A guy should NOT have to tear open a brand new dizzy just to get it close to factory setting.
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Unread 07-12-2011, 07:09 PM   #13
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You might not be willing to spend the money, but buying a distributor from performance distributors would be a great choice.

I had a rough time setting up my curve and gave in to what Shawn was saying about the DUI dizzy.

They set up the distributor to your specs and contains everything inside the cap. You just run a couple of wires to it after you drop it in. I am completely satisfied with my super smooth curve and ease of installation.

http://www.performancedistributors.com/

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Unread 07-12-2011, 07:10 PM   #14
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Quote:
Originally Posted by Matt1881CJ7 View Post
Well, I adjusted the vacuum canister out (CCW) 4 full turns, and it eliminated most of the low RPM detonation I was getting. I assume this moved the vacuum advance curve further to the right, making it kick in closer to the centrifugal.

The problem, now, is the Jeep has no "jump" at low speed. It accelerates smoothly, but slowly, when I step on it. I get a noticeable surge in acceleration when the advances kick in, but it's late in the RPM range
Yeah, that's one of the things you get with smog curves where they try to blend in ported vacuum advance with a really lazy centrifugal curve. It's bandaid smog nazi stuff at its finest, lol. It's really unfortunate that it somehow became "correct."

Ideally, the engine would be accelerated at WOT and you'd adjust the rate and total advance for maximum acceleration. The vacuum advance would then be hooked up to manifold vacuum and set up to provide more advance depending on how light the load is. As you dip further into the throttle and manifold vacuum drops, the vacuum advance adds less and less advance until fully dropping out where you'd rely solely on the centrifugal curve which you already tuned for maximum power. Unfortunately, that set-up won't pass the sniffer test. The engineers pulled out timing big time and switched to ported vacuum to keep the exhaust gas temps up to bring the hydrocarbons down. Too bad the engine doesn't respond well to that sort of thing.

You should see the vacuum advance curve of a California-smogged, BBD! Luckily you have a Weber so it's not too terrible.

Quote:
Originally Posted by Matt1881CJ7 View Post
BTW, I tried to tear apart the old Motorcraft dizzy, just to see what's involved in replacing the head in the new one, but I couldn't even get the thing apart. One of the Philips screws on top was so tight, I mangled the head and never did get it loose. Aaaaarrrrgh!!

This whole thing is really starting to frustrate me. A guy should NOT have to tear open a brand new dizzy just to get it close to factory setting.
I agree. It's really unfortunate the quality of stuff we have to live with sometimes. I'm not saying that anything is wrong with yours but it sure seems odd. I'd take the opportunity that the old dizzy presents and work that screw out so you can get real familiar with how the advance head works and how to get to it. Besides, you might find the advance head you need right there.


Shawn
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Unread 07-12-2011, 07:32 PM   #15
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If you haven't seen it before and feel like wasting an hour, check this thread out http://www.jeepforum.com/forum/f8/timing-1055680/

We managed to get into it (again, lol) but there were some good exchanges about combustion dynamics and the required changes in timing.

I have an 18 year old, Single Malt Scotch that's been calling me for an hour or so. Salute!


Shawn
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