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post #1 of (permalink) Old 03-14-2001, 05:18 PM
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vac adv connection ?

1988 258 BBD Wrangler --the vacuum hose schematics at www.off-road.com/jeep/tech/258bbd/vacuhose.htm indicate that the vac adv hose is connected to " M " around the carb area , which is probably a brass fitting threaded into the intake manifold below the throttle linkage. My understanding is that the vac adv should have a ported vac source , not manifold vacuum . If this be true , where is the correct ported vac hookup on the Carter BBD ? The only ported vac terminal that I can find on the Carter BBD is halfway up the carb facing the valve cover . The schematics show that only the CTO and 4 inch vac switch assembly are hooked into the port halfway up the carb facing the valve cover . Can anyone explain the correct connections ? Currently , I seem to be running AOK at manifold vac , but my readings of past postings tell me that this is the wrong setup . Who can identify the OEM setup? Is the schematic incorrect or did I miss something very basic ?

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post #2 of (permalink) Old 03-14-2001, 08:33 PM
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Re: vac adv connection ?

By the end of that 4 page vac advance posting it was determined that certain ignition systems required manifold vac for the distributor. Your factory schematic is correct-use manifold. I have since changed mine back because after a few days running ported, the Jeep acted up ALOT.

'88 Sahara, 4.2, auto.
post #3 of (permalink) Old 03-15-2001, 12:19 AM
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Re: vac adv connection ?

Thank you for replying , Stooby. Are you saying that the OEM for the 1988 Wrangler 258 BBD was definitely setup as manifold vacuum source to the dist vac advance ? I was beginning to wonder if I should change the current setup of the manifold vacuum feed to the dist vacuum advance to change it to ported vacuum feed which is widely discussed in previous posts. Such a change would probably necessitate that I also compensate for the expected decrease in the supplied idle vacuum as I change from high manifold vacuum at idle to low ported vacuum at idle . I planned to rotate the distributor so that it would get a more advanced spark at idle to compensate for the change / decrease in vacuum level at idle. Is there any site /source that can document what the OEM setup had been ? This answer has eluded me for months. I can not find specifics for this anywhere.

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post #4 of (permalink) Old 03-15-2001, 02:40 AM
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Re: vac adv connection ?

I switched mine to ported after reading the first page of replies on that post. It seemed to run great for a few days and then all hell broke loose. The idle got really screwy, constantly diesled when I tried to shut it off, loss of power and bad milage. I was surprised because it seemed to run so well for the first rew days. Then a few more people added to that post and depending on the ignition and distributor used, manifold was called for. I also havef an 88 with the 258 and it came from the factory with manifold vacuum to the distibutor and now it has it again.(the more I thought about it, I remembered the vac advance on my old 350 was manifold aswell). Good luck.

'88 Sahara, 4.2, auto.
post #5 of (permalink) Old 03-15-2001, 08:19 AM
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Re: vac adv connection ?

I appreciate your explanation of the dist vac adv OEM setup to manifold vacuum . I don't understand why there has been such mass confusion in this topic. I thought that by now , a Jeep guru / engineer would step forward and set the record straight. I am still reading as much as I can to learn what is best . I felt that the Jeep engineers would be well versed upon which setup is appropriate for the vehicles that they design and I (a novice) would tend to favor their OEM setup. I wonder if a switch to ported vacuum with a corresponding dist timing adjustment is an enhancement to the OEM setup . Any comments ?

post #6 of (permalink) Old 03-15-2001, 09:13 AM
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Re: vac adv connection ?

The ported source is actually better for foot-to-the-floor acceleration. It does not supply enough vacuum for idleing. Therefore the spark happens later and more heat is transferred to the engine causing it to get hot. A few set-ups used a CTO(coolant temp. overide) switch(actually a valve) to supply ported vacuum at normal temps and manifold vacuum to advance the timing if the engine gets hot. Some switches, I think the Corvettes, will supply manifold vacuum for a cold engine to help start and warm up. Ford had a duel diaphram distributor that was the best of both worlds. My $.02136478.

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post #7 of (permalink) Old 03-15-2001, 09:18 AM
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Re: vac adv connection ?

Originally vac. adv. was hooked-up to ported (carb) vacuum, but to get better emission control, they added systems that advance during deceleration which needs manifold vacuum. Unless these systems have been removed you want to follow the schematic. There is probably a lot more to it than this, feel free to study the various systems and post the urls that give the best explainations.

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