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Unread 02-09-2013, 02:40 PM   #1
fourbtgait
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Vacuum gauge use/readings question

After reading so many threads about manifold vacumm, reading it with a gauge, I googled it, read up on it.
Now I have a couple questions about interpreting it.
258 stock engine, carter bbd, nuttered, just installed msd6al.
8*btdc, 650 rpm.

Gauge reads at idle 18.5 with barely a bounce of the needle.
Which means the following? Compression is good, no vacuum leaks. Stock cam, valves not sticking, no burned valves, valves seated properly, valve guides good, valve springs are good, valve timing is good, timing is good, no intake leaks, head gasket is good, exhaust is not clogged when slowly revved up as it rises, idle mixture is good,.
My main question is that when I rapidly accelorate the engine, vacuum drops to 1 or possibly 0, and upon deceloration, rises to 23. Is this ok for my elevation or symptom of worn rings or diluted or wrong oil?

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Unread 02-09-2013, 04:41 PM   #2
John Strenk
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Yes this is normal.

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As the demand on the engine goes up the vacuum drops. This retards the ignition timing a bit so you don't get any knock. It also allows the mixture needle to rise making the fuel mixture a little richer. 14 to 1 is great for economy but it needs to be richer for more power.
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Unread 02-09-2013, 05:27 PM   #3
fourbtgait
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Quote:
Originally Posted by John Strenk
Yes this is normal.

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As the demand on the engine goes up the vacuum drops. This retards the ignition timing a bit so you don't get any knock. It also allows the mixture needle to rise making the fuel mixture a little richer. 14 to 1 is great for economy but it needs to be richer for more power.
Thanks John. Took me 35 years to learn a vacuum gauge, but then 20 of that I ran diesels.
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Unread 02-09-2013, 05:29 PM   #4
skizriz
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Here's a neat website on what your vacuum readings tell you. Yours sounds like scenario #4

http://www.secondchancegarage.com/public/186.cfm
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Unread 02-09-2013, 05:50 PM   #5
roboto65
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Very cool site great find.
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Unread 02-09-2013, 06:13 PM   #6
fourbtgait
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Quote:
Originally Posted by skizriz
Here's a neat website on what your vacuum readings tell you. Yours sounds like scenario #4

http://www.secondchancegarage.com/public/186.cfm
Thats the site that started me learning several days ago, from another thread.
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Unread 02-09-2013, 06:26 PM   #7
fourbtgait
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Quote:
Originally Posted by skizriz
Here's a neat website on what your vacuum readings tell you. Yours sounds like scenario #4

http://www.secondchancegarage.com/public/186.cfm
I thought so also, pointed it out, but looking at it again, they say on that site the vacuum is 15-17, then drops to 0 with quick acceleration. I think their 15-17 is for sea level, so at my elevation it would be 13-15?.
My vacuum holds steady at 18.5 adjusted for elevation above sea level. If I carry that dame thought, my high reading of 23, would be adjusted up to 25, my low of 1-0 to 3-2.

If I remember from another thread, Matt said his was 16 at 6,000 feet elevation?

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Unread 02-10-2013, 06:11 AM   #8
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Yes, normal vacuum readings will be lower at higher elevations. Again, this is because most vacuum gauges are calibrated at sea level where atmospheric pressure is more. The rule of thumb is normal (sea level) readings will be 1" lower for every 1000' of elevation gain.

Four, congrats on an "old dog learning a new trick". It sounds like you have a healthy vacuum system.

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Unread 02-10-2013, 09:59 AM   #9
fourbtgait
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Matt-
Thanks for the insight. I first became curious of vacuum on your thread between yourself and Shawn, but it seemed so high tech. Reality is it is so simplistic yet provides so much information. I was lucky with my engine that only one reading seemed off.
Next up is learning to set up the dizzy and plot the advance curve like you suggested last September.

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Unread 03-28-2013, 09:29 PM   #10
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So after getting mine back and realizing the shop is unfamiliar with cj's, I decided to double check everything. They set my timing at 10* like I specified but didn't disconnect the vac so base was 0*. Corrected. They turned mix screws out to 2 out corrected to 1 5/8. The throttle plate screw was in too far and showed 3" at idle on port vac. Corrected out to 0". I am now at 21" manifold idle at 1/4 turn in on throttle. When I goose the accelerator my vac drops to almost 0 then quickly returns to 21. Just trying to learn about vacuum diagnoses to make sure everything is tuned perfectly. I am going to check the site above after this but any input is healthy.

The site doesn't show vac info or scenarios anymore?!
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Unread 03-28-2013, 10:45 PM   #11
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I am a little confused with mine I did the other day. I have a exhaust valve issue on number one leakdown test confirms it well that and the air pouring out the exhaust lol. But when did a vacuum test it reads a steady 20 no rhyme or reason for that.
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Unread 03-28-2013, 11:28 PM   #12
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Btw, that link doesn't show the situations. At least not on apple. Anyone have another link?
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Unread 03-28-2013, 11:43 PM   #13
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Quote:
Originally Posted by vinnyabs View Post
Btw, that link doesn't show the situations. At least not on apple. Anyone have another link?
Vinny

The link is working for me on my Mac.

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Unread 03-28-2013, 11:45 PM   #14
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Works on my Linux, computer also.
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Unread 03-29-2013, 02:39 AM   #15
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Quote:
Originally Posted by roboto65 View Post
I am a little confused with mine I did the other day. I have a exhaust valve issue on number one leakdown test confirms it well that and the air pouring out the exhaust lol. But when did a vacuum test it reads a steady 20 no rhyme or reason for that.
I was thinking that the vacuum gauge looks mainly on the intake side of the engine. Depending upon how bad of a leak on the exhaust valve it still will be mostly steady, a V* will probably smooth it out even more.

Usually a tune up kit use to have a vacuum gauge and a compression gauge together. With both you can just about diagnose most things going on inside the engine.
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