4.2 with DUI and motorcraft 2100, which vac advance? - Page 2 - JeepForum.com

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post #16 of 20 Old 03-20-2013, 02:10 PM
86cj74.2L
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It's been a while since I looked into that very subject mat but I have some idea that back in 06 or so the CJ ran on manifold vacuum during warmup and then switched to ported via the Vacuum switch.

Like I said its been a long time. But I thought that and the article with all the vacuum advance specs is what talked me into trying manifold vacuum all the time.

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post #17 of 20 Old 03-20-2013, 02:40 PM
Matt1981CJ7
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86,

Yes, AMC was all over the place with their vacuum layouts depending on year and location (ie: California had it's own special layouts for a while).

The combo ported/manifold layouts seem to appear in the early 80's along with the AIR pump system. When they switched to the computer-controlled Pulse Air system, the vacuum layouts begin to show only manifold vac to the dizzy, like the layout I posted.

I've read that ported vac advance was just a bandaid fix to get around new emissions requirements. The engineers figured out that by retarding the timing at idle, combustion temps increased, allowing the AIR pump system to have a greater affect on emissions, at the expense of performance.

Matt
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post #18 of 20 Old 03-21-2013, 08:25 AM
CJ Chet
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I got tired of all the arguments so I just called Davis and asked them. They said, MANIFOLD! I've tried it both ways. I have a Weber 38 and No emission crap. The only vacuum lines I have are advance and brake booster, and to be honest it seems to run just fine BOTH ways!

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post #19 of 20 Old 03-21-2013, 09:02 AM
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Quote:
Originally Posted by CJ Chet View Post
I got tired of all the arguments so I just called Davis and asked them. They said, MANIFOLD!
There you have it, straight from horses mouth.

I, too, have tried it both ways on my 258 and 360. In both cases, I noticed a smoother idle, better cold starts, better low end throttle response, and cooler idle temperatures with manifold.

Hope this helps,

Matt
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post #20 of 20 Old 03-21-2013, 09:26 AM Thread Starter
drespecki
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Once I have the new charcoal canister setup, I will try the manifold vacuum nipple just above the EGR valve. Does it matter that it is shared with the power brake booster?

It only hurts while you're breathing..
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