DUI on 258, Port vs Manifold Vac Carter BBD. - JeepForum.com
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post #1 of 23 Old 09-22-2021, 08:38 PM Thread Starter
PrincessJ
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DUI on 258, Port vs Manifold Vac Carter BBD.

Added a DUI on my 258. Ill update my build thread later but I am getting mixed signals with ported vs manifold vaccum and where to hook them up on my BBD. I have it hooked to the manifold currently but it seems the sweet spot when tuned with a vaccum gauge (Near 20 In hg) the idle is a tad high at 900 rpms. It has great driving manners just seems too high idle. I had my non computer 13/18r distributor hooked to manifold when I put the pre feedback BBD in it. What are your thoughts.








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post #2 of 23 Old 09-22-2021, 11:59 PM
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There are only a few connections on the BBD for vacuum. The manifold one is below the throttle butterfly and it seems I use the air cleaner one on the diagram below. As it is the only one with vacuum at idle, if you are getting vacuum at the higher port you have an issue. Manifold is favoured by myself, others disagree, but that is why i do not use the one marked for the distributor.

the timing at idle will then be:

Initial +
vacuum advance

Whilst DUI are advertised as having custom advance curves set on Sun machines, this relates to the mechanical advance curve. The mechanical advance only comes in above idle. No knowing what curve this is unless you measure it yourself.

So what you are seeing is the best idle with 20" Hg vacuum. This used to be a secondary method of setting the timing and is useful if the engine is old. The timing chains can stretch or the fuel it was designed to run on very low octane. You retard to the best idle vaccuum and then advance by 1". Then you check for pinging under load.

But what does this mean for initial advance? have you measured it with the vacuum advance disconnected? When you twist that dizzie you are changing initial advance, nothing else.

The reason I ask is that this is the main determinant/adjustment of total advance under WIDE OPEN THROTTLE. The other one, mechanical advance, is determined by the shape of the advance head.

The advanced method is to check the timing marks, set the initial advance about 8 to 10 deg BTDC, check the mechanical advance amount at 2500 rpm with the vacuum disconnected (may be 35 to 40 deg), hook up a vaccum gauge and run the vehicle down the road without any vacuum advance and then with vacuum advance hooked up.

The reason is that at fast cruising speed about 2500 rpm the vacuum advance should advance the spark about 10 degrees, at idle and with your foot off slowing down it is a lot more but if you only measure it at idle it does not tell you a lot about how it impacts on timing.

What you do is take that vacuum reading, get a small vacuum pump and with the engine idling pull that same vacuum and see what the distributor does and then adjust. You adjust the vacuum can with a small Allen wrench until 10 degrees advance matches your cruising vaccuum, then it will cruise about right and you will probably improve mileage. I say this as many vaccum cans are not producing these results, and cursing the gas mileage will not fix it.

See diagram below (ignore mechanical advance curve, it is not for you) of how much manifold vaccum moves that vacuum can. the sweet spot is around 12" HG for 10 degrees advance but it only works if YOUR rig develops 12" HG at cruise AND the can is set up as per the diagram.

(All degrees are crankshaft degrees, the distributor runs at half speed so halve these for mechanical advance head etc)
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post #3 of 23 Old 09-23-2021, 03:36 AM
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My experience is that manifold vacuum (throttled down to about 6-700) is a sweet spot. However the rule of thumb is that sometimes does not provide the best idle emission performance. I run manifold (stock distro, Howell FI), but have the connection intact for ported if I ever need to switch it.

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post #4 of 23 Old 09-23-2021, 03:52 AM
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My jeep seems to run best on manifold since I switched to a BBD. Try to figure out why it won't idle lower. Something may be hanging up.


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post #5 of 23 Old 09-23-2021, 06:33 AM
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Princess,

I'd leave it connected to manifold vacuum.

Did you try adjusting the idle speed screw to lower the RPM?

Matt
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post #6 of 23 Old 09-23-2021, 01:12 PM Thread Starter
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Quote:
Originally Posted by Matt1981CJ7 View Post
Princess,

I'd leave it connected to manifold vacuum.

Did you try adjusting the idle speed screw to lower the RPM?

Matt
I did, the butterfly is at its resting position with only a tad resistance on the screw. I think my timing may be a tad far advance. I set it 10 btdc (the same as the duraspark setup) with no vaccum advance. Got it to idle at 600 hook up advance it goes to 900. My initial thought is to try to slowly retard the timing from the distributor.

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post #7 of 23 Old 09-23-2021, 01:28 PM
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Just some info that might help here.

The REDLINE large cap H.E.I. & the DUI H.E.I. have 20-22 degrees advance inside the distributor.
Add the initial timing of 10-12 degrees adds up too 32-34 total advance ("base timing").
This is what you/we are trying to acheive.

When you hook-up manifold vacuum (typically for economy and/or deceleration) with your already high inches of mercury (vacuum) at idle the vacuum canister cranks in more timing than you need. Probably why your idle speed goes to 900 RPM.
Rather than reduce your "base timing", trying to stay at 32-34 total advance, try this:

Take an allen wrench, (1/8"?) insert it into the vacuum canister, connect into it's socket and turn it 1/2 turn counter clockwise, reducing the amount of canister advance. Try this 1/2 turn at a time, don't lose track of where you started from incase you want to turn it back.
UTN

Edit:
You will "probably" only back the screw out 1 full turn, has been my experiance.
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post #8 of 23 Old 09-23-2021, 02:32 PM
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So Princess J,

You disconnect the distributor vacuum, plug the line, time it at the original book RPMs, then reconnect the line.

If you are using manifold vacuum, the RPMs will increase when reconnected.

You probably already know if you try to time it with the vacuum advance connected and the RPMs are higher than the book value for timing,

the mechanical weights within the distributor will be cast outward and you will not accurately hit the desired degree.

Recently I experienced Mr. Jeep was seemingly getting a touch of feeling down. I thought it might be something to do with timing.

I pulled out my timing light and verified he was still on "8", I throttled slowly and found my mark advancing on the scale as it should.

Normally if it don't move, the spring on the mechanical weights has broken

But this would also would have advanced my timing. In the past, I've seen my timing was "wrong" and I changed it back,

with the spring broke the timing is now right (unknowingly not realizing it was broke)

Now it won't advance at all. Watching the timing light/ movement (or lack of) of the marks you finally realize the spring is broke!

Continuing my search, I found the vacuum advance was not working.

I ordered in another distributor and did the Lifetime Warranty swap a few days later.

I found my Lifetime Warranty Distributor of 7/31/96 had been a real trooper for a long time!

Even though this story ain't working for the above, I thought maybe if you detect a problem, maybe using the timing light works to diagnose, not only to set the timing.

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post #9 of 23 Old 09-23-2021, 05:56 PM Thread Starter
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I will play with it when I get home next week and report back.

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post #10 of 23 Old 09-24-2021, 04:49 AM
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Quote:
Originally Posted by PrincessJ View Post
I had my non computer 13/18r distributor hooked to manifold when I put the pre feedback BBD in it. What are your thoughts.

I have the same setup on my 84' model CJ-7. 1980 version 13/38r distributor and a 1980, non-stepper, Carter BBD. One of the best years for non-computered CJ's.

Pictured is the vacuum diagram I use along with 10 initial timing. Works very well for me with this setup.

cj-7-vacuum-routing-post-nutter-bypass.jpg


carter-bbd-1-.jpg

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post #11 of 23 Old 09-24-2021, 05:19 AM Thread Starter
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Quote:
Originally Posted by keith460 View Post
I have the same setup on my 84' model CJ-7. 1980 version 13/38r distributor and a 1980, non-stepper, Carter BBD. One of the best years for non-computered CJ's.

Pictured is the vacuum diagram I use along with 10 initial timing. Works very well for me with this setup.
Much appreciated. I think I should get a 2 way CTO looking at that diagram other than that, I have no issues with running my non feedback BBD even offroad.

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post #12 of 23 Old 09-24-2021, 06:19 AM
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In my experience, connecting the vacuum advance to manifold vacuum, after setting initial timing, should only increase the idle about 150 RPM on a 258. It's odd that yours increases 300 RPM.

I'd try UpTillNow's recommendation to retard the vacuum advance a little.

Matt
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post #13 of 23 Old 09-24-2021, 07:04 AM
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With an old BBD, it's not unreasonable to have quite an air leak through the throttle shafts, is it?


Shawn

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post #14 of 23 Old 09-24-2021, 07:06 AM Thread Starter
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Quote:
Originally Posted by Shawn Watson View Post
With an old BBD, it's not unreasonable to have quite an air leak through the throttle shafts, is it?


Shawn
ill get some starting fluid and inspect for leaks.

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post #15 of 23 Old 09-24-2021, 11:09 AM
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