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Unread 09-06-2013, 05:56 PM   #1
phil8825
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Manifold vacuum

Gents,

Got to playing with the cj today .. Decided to replace a couple of things like the radiator, clutch linkage and heater core. Afterwards I decided to take a closer look at my carb idle mixture setting, wanted to fine tune it with a vacuum gauge.

Pull out the vacuum gauge, hook it up to the manifold vacuum on the carb, start the engine, gauge reads 0 .. I figured something is not right here. Must be a bad gauge, but when I hooked it up directly to the intake manifold it showed steady at 15.

Im currently running and AMC 360, edelbrock performer intake and carb (1400).

I have not tested for any vacuum leaks yet, but I went ahead and disconnected the dizzy and the ported vacuum works just fine, increase with increase in throttle.

any thoughts?? would a vac leak at the carb base cause manifold vacuum at the carb to show absolute 0, no response on the gauge whatsoever, seems kind of odd to me.

Especially since the engine runs really well, no hick ups, stumbles, plenty of juice etc..

Any advice or input would be greatly appreciated

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Unread 09-07-2013, 06:29 AM   #2
cruiser54
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That's not intake manifold at that port. That's "ported" vacuum. Vacuum will be present when you open the throttle. Wrong place to have the gauge hooked up for what you're doing. Have to have it connected to a port BELOW the throttle plate.
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Unread 09-07-2013, 06:36 AM   #3
phil8825
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There are two ports on the carb, ported vacuum which I understand is on the passenger side ( higher of the two) and i though the one on the drivers side ( lower of the two) is supposed to be full time/manifold vacuum. I had the gauge hooked up to the lower of the two.
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Unread 09-07-2013, 06:38 AM   #4
cruiser54
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Tee into the intake manifold somewhere.
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Unread 09-07-2013, 06:43 AM   #5
phil8825
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Did that and was solid at 15. Didn't really change much when I was turning the idle mix in or out though

I was just under the impression that the manifold vacuum port on the carb should show something at idle. I did also hook it up to the ported side and that side seemed to work, at least it responded to throttle application like its supposed to.
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Unread 09-07-2013, 06:50 AM   #6
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Quote:
Originally Posted by phil8825 View Post
Did that and was solid at 15. Didn't really change much when I was turning the idle mix in or out though

I was just under the impression that the manifold vacuum port on the carb should show something at idle. I did also hook it up to the ported side and that side seemed to work, at least it responded to throttle application like its supposed to.
That was my understanding also.

Vacuum seems a bit low. Are you running the stock distributor?
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Unread 09-07-2013, 06:55 AM   #7
phil8825
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Yeah, one of those remanufactured napa units is on there. Engine only has 1000 miles on it or so and runs really well, idles well around 800-850.

Unfortunately I don't know exactly what size cam is in there, certainly is not stock though.. Sure does not sound like it at least

Another item I found I was not very pleased with is that my timing at idle is jumping back and forth between 12-15

I
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Unread 09-07-2013, 07:50 AM   #8
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Not an HEI distributor or anything though, right?

If it were me, with a stock dizzy, I would have about 12 to 15* initial advance with the vac line hooked to ported vacuum.
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Unread 09-07-2013, 10:56 AM   #9
phil8825
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Stock dizzy, no hei..

As for timing, that's exactly what I have, and the dizzy runs off of ported vacuum
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Unread 09-07-2013, 11:54 AM   #10
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Quote:
Originally Posted by phil8825 View Post
Stock dizzy, no hei..

As for timing, that's exactly what I have, and the dizzy runs off of ported vacuum
Good. I think originally they ran off of manifold vacuum. I'm wondering about that distributor bouncing like that. I'd be real curious to see what your total advance is at 3000 RPM with no ported vacuum. Also the stability of the timing.
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Unread 09-07-2013, 12:08 PM   #11
phil8825
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Well the last time I checked I only looked at it at idle with the ported line disconnected. I don't own a timing light so I did this when I had the car at a shop and borrowed one. Thats when I saw it bounce back and forth 12-15.

I did a little reading and it seems there are many things which could cause this. Like worn timing chains, distributor weights etc.

I should really buy a timing light, but the price range varies quite a bit.

Timing aside, what I still don't get is why the manifold vacuum port on the carb ( lower one ) shows 0

As soon as I get my hands on a timing light, I will report back.
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Unread 09-07-2013, 01:30 PM   #12
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Quote:
Originally Posted by phil8825 View Post
Well the last time I checked I only looked at it at idle with the ported line disconnected. I don't own a timing light so I did this when I had the car at a shop and borrowed one. Thats when I saw it bounce back and forth 12-15.

I did a little reading and it seems there are many things which could cause this. Like worn timing chains, distributor weights etc.

I should really buy a timing light, but the price range varies quite a bit.

Timing aside, what I still don't get is why the manifold vacuum port on the carb ( lower one ) shows 0

As soon as I get my hands on a timing light, I will report back.
Maybe not the original carb and that lower port is blocked or something? Doesn't really matter.

Rebuilt distributor, right?

Ever considering returning it and going HEI?

http://www.ebay.com/bhp/amc-hei-distributor
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Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

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Refresh*Connections*Before*Replacing*Components
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Unread 09-07-2013, 01:38 PM   #13
phil8825
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I certainly have, im terrified after all the shredded gear stories..

Although it would certainly be cleaner than what I got going on now. Spacing also seems very tight with the power steering pump right there.

Yeah its not an original carb, got one of those edelbrock performer series 600cfm sitting on there. Hasn't given me any problems
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Unread 09-07-2013, 02:21 PM   #14
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Quote:
Originally Posted by phil8825 View Post
I certainly have, im terrified after all the shredded gear stories..

Although it would certainly be cleaner than what I got going on now. Spacing also seems very tight with the power steering pump right there.

Yeah its not an original carb, got one of those edelbrock performer series 600cfm sitting on there. Hasn't given me any problems
OK, Edelbrock. How is it Jetted? Any idea?

There are other alternatives to the HEI. A Ford setup. The stock ignition system is barely better than points. I have used an HEI dizzy from a Chevy with the AMC V8 gear on it with good success.
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Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

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Unread 09-07-2013, 03:01 PM   #15
phil8825
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Ok, so went out and bought a timing light... At idle ( ported vac disconnected and plugged) its holding at 13. 1500 RPM @ 20, around 3000 its at 15-16.. Is it supposed to increase and then decrease like that.. I have no idea :-)

The eddy carb is a 1400 carb, so i gotta look at what it comes with in terms of jets. I didn't change anything.
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