77 CJ5 258 Emissions/Vacuum Help - Page 2 - JeepForum.com
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post #16 of 47 Old 03-28-2020, 02:22 PM
Matt1981CJ7
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Your EGR has been blocked off. It's fine to leave it that way.

The bare minimum for vacuum-related systems is the distributor vacuum advance and PCV systems.

Your PCV valve appears to be hooked up correctly to manifold vacuum. I can't see the air cleaner side of the PCV system in your pics, so I've provided the diagram below.

I would connect the orange hose to the distributor. Everything else can be omitted and/or capped off.

Matt

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post #17 of 47 Old 03-28-2020, 02:38 PM
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I had a couple paragraphs written up, and then the internet dropped and I lost it.

You have an EGR blockoff plate.

We'll do it this way:

1. Put timed port vacuum to distributor. Plug the other timed ports.

2. Full manifold vacuum is where it needs to go. Plug the rest of the full manifold vacuum ports.

3. The hose on the rear of the valve cover goes to your air cleaner assembly. Do NOT put vacuum to it, or it'll pull oil past your rings.

4. You have a thermal choke. There's a hose going to it off the manifold, and there may be one coming off it. I can't tell for sure. If there's a metal hose coming off it with nothing attached, run a hose from that metal hose to the highest port on the carburetor, the one higher than the fuel bowl. This'll provide enough airflow for the choke to heat and do its job. Do NOT put it to full manifold or timed vacuum.

5. Find and plug all other open ports.

The Jeep should start.

If you have an automatic tranny, you'll have a one or two more vacuum hoses to address before the thing will drive right, but this should get you started.

Josh
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post #18 of 47 Old 03-28-2020, 02:40 PM
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Quote:
Originally Posted by Matt1981CJ7 View Post
Your EGR has been blocked off. It's fine to leave it that way.

The bare minimum for vacuum-related systems is the distributor vacuum advance and PCV systems.

Your PCV valve appears to be hooked up correctly to manifold vacuum. I can't see the air cleaner side of the PCV system in your pics, so I've provided the diagram below.

I would connect the orange hose to the distributor. Everything else can be omitted and/or capped off.

Matt
Do not put that to the distributor. That's full manifold vacuum and will give you full advance immediately. You need to put timed port vacuum to the distributor.

Josh
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post #19 of 47 Old 03-28-2020, 02:47 PM
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One or both of these ports is timed. Figure out which one, and put that orange hose to it and then to the distributor.

That'll give you proper timed vacuum.

When you time it, you'll want to disconnect and plug this hose. You don't want the vacuum advance operating at all when you time it.

Josh
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post #20 of 47 Old 03-28-2020, 03:19 PM
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Quote:
Originally Posted by Wabatuckian View Post
Do not put that to the distributor. That's full manifold vacuum and will give you full advance immediately. You need to put timed port vacuum to the distributor.

Josh

We'll have to agree to disagree on that.

Ported vac advance was for emissions purposes, at the expense of idle quality. Manifold vac advance will provide a stronger, cooler, more efficient idle. The vast majority of CJ owners who either Nutter their computerized carbs, or switch carbs, prefer manifold vac advance.


Matt


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post #21 of 47 Old 03-28-2020, 04:11 PM
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Quote:
Originally Posted by Matt1981CJ7 View Post
We'll have to agree to disagree on that.

Ported vac advance was for emissions purposes, at the expense of idle quality. Manifold vac advance will provide a stronger, cooler, more efficient idle. The vast majority of CJ owners who either Nutter their computerized carbs, or switch carbs, prefer manifold vac advance.


Matt
But if you have full manifold vacuum to the distributor, it'll cause the vacuum advance to slam on full at idle.

As you accelerate, you'll actually retard the timing, because vacuum drops under load. If you let off the accelerator, it'll slam on full again.

You want vacuum advance to increase as engine speed increases.

I can't think of a single instance in which applying full manifold vacuum to the vacuum advance would make any sort of sense.

Can you help here?

Vacuum advance is an emissions and performance feature. You don't necessarily need it since you have centrifugal advance. In fact, I ran only centrifugal advance in old CJ for quite a while.

If that's what you want, just leave the vacuum advance unattached.

But providing full vacuum to the vacuum advance will allow the timing to retard under WOT (etc,) and makes no sense.

Unless, of course, I'm missing something. If so, I'm always up for learning.

Regards,
Josh
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post #22 of 47 Old 03-28-2020, 09:10 PM
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Test it for yourself. Hook the same tune up to both manifold and ported vac advance. You will see manifold produces 125-150 more RPM at idle, all else equal. That’s because an engine needs more advance at idle, not less.


Ported vac advance was a bandaid solution to reduce emissions.

Matt
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post #23 of 47 Old 03-29-2020, 06:50 AM
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Quote:
Originally Posted by Matt1981CJ7 View Post
Test it for yourself. Hook the same tune up to both manifold and ported vac advance. You will see manifold produces 125-150 more RPM at idle, all else equal. That’s because an engine needs more advance at idle, not less.


Ported vac advance was a bandaid solution to reduce emissions.

Matt
At this point, I have nothing with vacuum advance or I'd have tried it already. I'm after a Wrangler or an '80s Cherokee, so that might change, but probably not for a year or so as I'm focusing on the WJ and the wife's Blazer.

In thinking about it, there might be a reason it works, though it has more to do with different a/f mixtures at different engine loads and less to do with engine speeds.

The distributor would likely benefit from a custom centrifugal curve, too, if you're going to run it this way. My old Mallory Uni-Lite back in the day came with different weights, and an adjustable vacuum advance canister. It'd be fun to have that old CJ5 back today.

I'd still recommend the OP use the timed advance -- for now -- since the specs he'll encounter are for that.

If he sets the initial timing to 5° BTDC and throws full manifold vacuum at it, he's going to end up with 20° or so BTDC, and the Jeep will cease to run at idle.

Therefore, I suggest he stick with original timed port initially, and, after he gets it running, go to full manifold vacuum if that's what he wants.

Regards,
Josh
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post #24 of 47 Old 03-29-2020, 07:15 AM
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Over 20 degrees of advance at idle is perfectly normal, and preferable.

Vac advance is load dependent. It kicks in at times of lowest load and highest vacuum. At low loads the a/f mixture goes lean. A lean mixture needs to be lit earlier for best combustion performance. It has nothing to do with engine RPM. That’s what centrifugal advance is for.

Engines have run on manifold vac advance for decades. If you study the vacuum layouts of CJs over the years, you will see that most used manifold vac advance, or a combo of manifold and ported controlled thru a CTO. The ‘83-‘86 models used straight manifold. See red line below.

Hope this helps,

Matt
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post #25 of 47 Old 03-29-2020, 07:32 AM
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Here's my old 258 running smoothly and happily on manifold vac advance.

Matt



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post #26 of 47 Old 03-29-2020, 07:36 AM
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And here's my rebuilt 360 also running on manifold vac advance.


Matt


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post #27 of 47 Old 03-29-2020, 08:06 AM
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Interesting. I was trained on the feedback systems, earlier TBI and multiport systems.

I ran my 304 at 10-15 BTDC at idle with timed vacuum. It had a Crane Cam, Holley 390cfm square bore with vacuum secondaries, an Edelbrock intake, headers of a make I don't remember, dual Cherrybombs, Mallory ignition, and a few other goodies.

The way I learned vacuum advance was that it was for partial throttle before the power valve (if the carb was equipped) opened.

What you say makes sense, too. I'll go back through the pre-1970s stuff and have a look-see.

My main point about keeping factory specs just to get it running is that he'll run into those specs in any manual he might consult. If we say he needs to set it to 15 deg BTDC, and his Chilton (or whatever is in vogue these days) says 5 deg, then there's going to be confusion there on his part.

Regards,
Josh
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post #28 of 47 Old 03-29-2020, 08:40 AM
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There should be no confusion. Just time the engine to 5-10 degrees initial with the vac advance disconnected and the hose plugged. Then re-connect the vac advance and tune the carb. Easy peasy.

BTW, the only way ported advance serves to reduce emissions, or serves any purpose at all, is when it is combined with some form of air injection system. Engineers discovered that retarding the timing at idle, then injecting air into the exhaust stream, increased the after-burn of excess fuel in the exhaust, thereby reducing emissions. Without the air injection system, ported vac advance serves no purpose at all. In fact, it only makes a weaker, hotter, and less efficient idle.

I created this thread to help debunk the usual myths and misconceptions regarding vac advance. Hope it helps.

https://www.jeepforum.com/forum/f8/m...proof-3171937/

Matt


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post #29 of 47 Old 03-29-2020, 12:14 PM
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Thank you.

This is one reason I've come back to forums (was on Parts Mike Jeep Forums back in the day, then it disappeared.) I know there's always something new to learn, even after I thought I'd covered a particular issue or system. Those are getting harder to find, but this is one of those times.

Josh
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post #30 of 47 Old 03-29-2020, 12:37 PM
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It also looks like the spring to control your heat riser is not connected properly.

961719b0-4f04-4840-8468-4fe0188eaf8f_1585423348967.jpg

I see the spring is on there but it should have one end looped over that pin.

Hooking it up correctly cold help your engine run properly when it's cold

Fortunately it looks like it's in the open position so that's good. The gas from the exhaust is flowing right into the exhaust pipe.
Name:  HeatRiserOPEN.jpg
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When it's cold it should be closed diverting the exhaust to the floor of the intake manifold.
Name:  HeatRiserCLOSED.jpg
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Did anyone mention you can get your whole FSM here: https://oljeep.com/edge_77_tsm.html


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