Vac Connection for Distributor - JeepForum.com
 
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post #1 of 5 Old 05-18-2021, 06:21 AM Thread Starter
Charleston82CJ7
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The vac advance for my distributor was originally run from the 3 port heavy duty CTO switch on the manifold.

I removed it from that port and ran it directly from manifold vac this past weekend, but now it idles roughly. Is there a reason for it to be on the CTO switch, or do I just need to adjust my idle/timing?

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Part 4 Vacuum Diagrams Color.jpg  

Last edited by Charleston82CJ7; 05-18-2021 at 03:50 PM.
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post #2 of 5 Old 05-19-2021, 02:22 AM
BagusJeep
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What a mess it was back in the day, they had discovered that emissions were different if you had no vacuum advance or maximum advance at idle but that the best was to be somewhere in the middle. And that it varied if engine cold or hot.

Of course with the throttle open it makes no difference, it is the same.

Some got the Non Linear Vacuum Regulator NLVR so were in the middle at idle but you just got a CTO valve.

Up to 155F the distributor saw manifold vacuum, at 155F the CTO valve moved and it switched to ported vacuum. When warm you just ran on ported. less sophisticated, more reliable. One of the forum members swears this was to reduce emissions, but it is unclear.

Now you have a heavy duty cooling package so there is another CTO valve that prevents overheating. below 220F it uses ported vacuum, above 220 it uses manifold, which raises the idle speed and prevents boiling during long idles.

If you are not interested in emissions you can do away with it and use a 3' length of rubber tubing from the distributor to the manifold and carb.

I prefer it on manifold as the timing is well advanced at idle, others like it on ported when there is no advance at idle. I think it performs better off idle, others disagree.

I will suggest manifold

NOTE: EDITED AS I ADDD THE HEAVY DUTY COOLING CTO VALVE

BagusJeep lives in Bali with far too many 4x4s:
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
1980 Land Rover Series III 109" troop carrier - ROVER
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post #3 of 5 Old 05-19-2021, 03:48 PM
JEEPFELLER
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BagusJeep

I don't mean to butt in---too much anyway!

I've not heard anyone with knowledge of a Non-Linear valve

Of course I have a still strong 258 w/ Holley 390.

I'm more of an experimenter with stuff, in the quest for better MPGs

I have used both manifold vacuum and ported on the distributor (at present-PORTED)

I have 2 Non-Linear valves which have been on the shelf forever

Tell me dear sir----will hooking one up (I have minimal vacuum lines) maybe make things run better or get better MPGs????

Thanks boss!

---JEEPFELLER
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post #4 of 5 Old 05-20-2021, 03:10 AM
BagusJeep
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Well, this is what it does:
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BagusJeep lives in Bali with far too many 4x4s:
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
1980 Land Rover Series III 109" troop carrier - ROVER
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post #5 of 5 Old 05-20-2021, 03:33 AM
BagusJeep
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It is emissions related but the most interesting fact I can glean is that in a CJ7 258ci it ONLY came with an automatic transmission. And not in California at all. And only in a few years in the early 80s.

It came with FSJ as well up until the late 80s with V8s but it was dropped in a CJ7 when the CEC emission system was added in 1983, after which the CJ distributor was just on manifold vacuum.

The plumbing is through a CTO valve but there is no mention in the manuals of its operation being linked to engine temperature, even though the plumbing suggests it only works up to the switching temperature and then goes fully manifold.

That tells us something, but I am just not sure what it is. Wild arsed guess time!

It only makes a difference around idle, so it is unlikely to make any difference to mileage but I suspect it makes a difference on the smoothness of the takeup when pulling away from idle when cold and when warm it keeps the idle up, which is why it runs on manifold and not ported vacuum. This could be why it is linked with automatic transmissions. Just a wild arsed guess.

It is sold as a "Vacuum regulator" by NAPA apparently if you want to find a spare and fit one to your CTO valve.
Attached Thumbnails
regulated vac advance setup.png  

BagusJeep lives in Bali with far too many 4x4s:
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
1980 Land Rover Series III 109" troop carrier - ROVER
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