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post #16 of 36 Old 11-20-2012, 09:46 PM
Old4X
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Originally Posted by CJ51973 View Post
Old 4x4, I stand corrected (if this link is correct, probably is). Opposite of everything I had read so far. I am old but never got a good understanding of distributors. I will try switching to manifold and see what it does. Probably Friday so I will update.

http://www.jeepforum.com/forum/f12/m...vacuum-994651/
When you switch from ported to manifold as vac source to the dizzy, your idle speed will need to be readjusted. No big deal. Idle mixture screws should not need adjusting, but sometimes they do.

I have tried both vac sources, and they give different results. With ported, the throttle off idle was twitchy and tended to spin the wheels with throttle opening, not good for off roading. With man vac, power more controllable and better gas mileage.

If manifold vac doesn't fit your driving style, it is easy to swap back.


418 miles on 19 gallons of gas. Can't beat the BBD !
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post #17 of 36 Old 11-21-2012, 03:28 PM Thread Starter
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Much better on the manifold vacuum. Still have a slight hesitation. I have 18 inHG at idle in park (1000 rpm). It drops to 750 rpm and 14 in drive holding the brake. Does this mean I need a 9 inHG power valve Instead of the 7.5 I have?

Anything is possible if you don't know what you are talking about.
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post #18 of 36 Old 11-22-2012, 12:35 AM
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Your PV isn't optimal. You should also be able to get more vacuum than that. Have you used a vacuum gauge to set idle screws and timing?

Who knew where this would go?
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post #19 of 36 Old 11-22-2012, 08:46 AM Thread Starter
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Looks like the power valve is fine. For an auto you are suppose to take the vacuum reading in gear at idle. So my man vac is 14 so 7.5 power valve should be right. I will recheck the carb tune tomorrow and see if I can get more vac. Timing to vacuum? Not familiar with that but I will check and see where I am there too.

Anything is possible if you don't know what you are talking about.
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post #20 of 36 Old 11-23-2012, 01:00 PM Thread Starter
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Set the initial timing to 15 (HEI), manifold vacuum, double check the carb idle screws with a vacuum gauge, fixed! Thanks for the help. Manifold vac is 18 at idle. Someone made the comment that this is low. What's normal?

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post #21 of 36 Old 11-23-2012, 03:29 PM
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Originally Posted by CJ51973 View Post
Set the initial timing to 15 (HEI), manifold vacuum, double check the carb idle screws with a vacuum gauge, fixed! Thanks for the help. Manifold vac is 18 at idle. Someone made the comment that this is low. What's normal?

Normal sea level manifold vac for these engines is 20 - 22" .

418 miles on 19 gallons of gas. Can't beat the BBD !
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post #22 of 36 Old 11-23-2012, 03:58 PM
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Did you try opening up the idle screws to get highest vacuum then lower 1"?

Who knew where this would go?
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post #23 of 36 Old 11-23-2012, 06:35 PM Thread Starter
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Guess the engines a little tired. Runs better now. So I am 4 off

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post #24 of 36 Old 11-24-2012, 12:36 PM
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I would check the diaphragm in the accelerator pump, just pop the four bolts and inspect it. I had a similar issue that started as off idle hesitation and got worse with heat, eventually it led to flat out dying when i hit the gas. I found that there is a little plate on the accelerator diaphragm and it was sagging and sticking, it was pretty easy to see when i had the cover off..
just a suggestion as your problem sounds similar to what mine started out as

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post #25 of 36 Old 02-11-2013, 01:49 PM Thread Starter
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OK, old thread. But I have another question so it keeps the info in one place. I had a 7.5 in hg power valve installed. I bought a 6.5 in hg power valve to help the stumble off idle some. But then I do some reading I will include below. Basically the power valve is closed until the set point is reached. So wouldn't I want a larger number valve to open sooner not a smaller. Manifold vacuum is high when the throttle is closed and drops as opened. Sorry for the book below:

When engine load increases (such as climbing an increasing grade on a hill), and during high-speed operation, the fuel/air mixture must be enriched to increase engine power. The enrichment system supplies additional fuel to meet that demand. The enrichment system responds to changes in manifold vacuum. The enrichment system uses an enrichment valve (commonly called a “power valve”) to open an additional fuel metering path from the fuel bowl to the main fuel wells when manifold.

High manifold vacuum (such as at idle or low load, part throttle operation) pulls the power valve diaphragm against an internal spring and closes the valve so that no fuel can flow through it. At low manifold vacuum (such as at high load or high speed open throttle), the force of the spring overcomes the vacuum to open the power valve and provide additional fuel to the main fuel wells.

The power valve is installed on the bottom of the carburetor, with its valve opening in the bottom of the fuel bowl. When the power valve opens, it allows fuel to flow through restricted orifices in the carburetor main body, into the wells just below the main jets. From there, the fuel is drawn into the main fuel wells where it is added to the fuel drawn through the main jets to enrich the mixture produced by the main metering system. In effect, the power valve temporarily “enlarges” the main jet orifices.

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post #26 of 36 Old 02-12-2013, 08:45 AM
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It all depends on the symptoms to tell you if you are going rich, or going lean.... A hesitation that is followed by bogged down acceleration shows rich while a hesitation followed by a surge shows lean. If you are rich, the # on the PV should go down to delay the addition of fuel; If you are lean the number should go up to add fuel sooner.
As for the PV selection rule.... In application the rules don't always hold, I have around 20" vac at idle, but needed to run a 7.5 PV. Even Holley has said the 1/2 idle vac +/- 1 rule is not exact and only provides a good starting point for tuning... Especially in a non-stock application of the carb.
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post #27 of 36 Old 02-12-2013, 10:39 AM Thread Starter
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I am 18 in hg at idle with a 7.5 power valve. I have always thought my stumble to be lean. I feather the pedal a little and it takes off. It is much better then it was if you read through this thread. It doesn't exist in low range. I have the 6.5 power valve so maybe I will just stick it in there and see. Perhaps I am a little rich. If it gets worse then I will hunt up a 8.5 and try that.

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post #28 of 36 Old 02-12-2013, 03:02 PM
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Thats a good plan.... Most of the time carbs take a bit of trial and error to get the sorted. Just keep track of everything so if you rebuild down the road, you know what parts you need before you pull the carb.
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post #29 of 36 Old 02-13-2013, 06:38 AM Thread Starter
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Holley power valve = Motorcraft power valve, correct?

Anything is possible if you don't know what you are talking about.
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post #30 of 36 Old 02-13-2013, 11:45 AM
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Yes... I used holley branded parts in mine and NAPA had what I needed.
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