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-   -   4 bbl carb on wrong? (https://www.jeepforum.com/forum/f8/4-bbl-carb-wrong-4434927/)

85Tuxedocj7 06-09-2021 07:27 AM

4 bbl carb on wrong?
 
3 Attachment(s)
I think I found why my engine feels weak.

Below are images of my carb and a photo of the same intake I am running.

the Intake appears to have a divided plenum, when my dad and I (I was about 13) put it together, the carb was turned so that the throttle linkage was facing the fire wall and the electric choke was facing the radiator. I pulled the carb off to clean and rebuild it and noticed the intake ports closest to to the engine were sooty while the other two were shiny as new.

the primaries were over the outside ports, while the secondary's were over the engine-side ports... Dad said it felt like the "four barrel would never open."

Would the carb being turned the wrong direction have seriously stunted the power?

notice how my secondary's are dirty while the primaries are shiny?

Hurricane4 06-09-2021 10:43 AM

What brand/model of carb & intake?

jeepdaddy2000 06-09-2021 07:28 PM

One primary barrel on each side of the plenum.
One secondary barrel on each side of the plenum.

BagusJeep 06-09-2021 08:42 PM

I am surprised it even ran.

JEEPFELLER 06-09-2021 08:55 PM

3 Attachment(s)
I'm a bit lost on the orientation of your carb.

My Holley 390 sits on the 258 with the primaries against the valve cover and the secondary's facing the driver's fender

This being on an Offenhauser intake.

A better description is that it is sitting "Sideways" on the intake for the throttle and such to work out.

Along with this "ODD" configuration is the rearward slant of the engine

This slant messes up the Holley's internal fuel level and the adjustments, as one primary barrel and one secondary barrel is leaning lower on the slanted intake.

Nothing is "EVEN", precise adjustments are almost impossible.

HOWEVER

I had a plastic spacer machined to compensate for the slant---thus making the carb level again

I have a couple of pics of it in the forum

Here's one in this link (crammed with info)

https://www.jeepforum.com/forum/f8/c...258-a-4408347/

In pic 1) notice the air cleaner is "Level" compared to the rear slanting valve cover---thanks to the slanted spacer

Pic 2) compare the valve cover----the spacer is plainly seen in this pic

-----JEEPFELLER

KevinCJ7Jeep 06-09-2021 09:04 PM

Is it my bad eyes or is that a spread bore carb and a square bore intake

gutthans 06-09-2021 09:40 PM

Depending upon the year of the OFFY manifold, I believe the mounting pad area was changed to a 6* slant to compensate for AMC 258 engine tilt (4*- 6*). (Maybe a number of owners machined the pads themselves? but I believe it was a factory modification. A machined spacer (as noted earlier) will do the same on the earlier models.

The dual plane configuration SHOULD be run with the primaries both facing the valve cover, NOT STRADDLING both plenums. The reason is fairly obvious: The fuel charge will travel different distances at different velocities (and I believe it is correct that each side will run slightly leaner than if they both dump into the same plenum portion). They will both wind up in the chamber, however...and without a dyno you may not/won't notice much difference.

And yes, that does look a bit like a GM spread-bore???

KevinCJ7Jeep 06-09-2021 09:49 PM

60 Attachment(s)
Maybe not my eyes then. That would explain why the secondaries aren’t opening. I believe the secondaries are dirty because they could not open and no gas passing thru them and also cleaning them

jeepdaddy2000 06-09-2021 11:49 PM

1 Attachment(s)
Quote:

Originally Posted by KevinCJ7Jeep (Post 41289817)
Is it my bad eyes or is that a spread bore carb and a square bore intake

Square bore.Looks like a Carter AFB
Quote:

The dual plane configuration SHOULD be run with the primaries both facing the valve cover, NOT STRADDLING both plenums.
The barrels ALWAYS straddle the plenum. The plenum separates the planes, which are set up so the runners are as equal in length as possible. Enclosed is a picture of a V8 dual plane manifold. Look closely at the separate planes and the runners they feed. If the carb is orientated so the primaries are both on a single plane, then both barrels will feed only 3 cylinders(6 cyl) or 4 cylinders (V8).

mudbfun 06-10-2021 03:31 AM

Quote:

Originally Posted by gutthans (Post 41289847)

The dual plane configuration SHOULD be run with the primaries both facing the valve cover, NOT STRADDLING both plenums. The reason is fairly obvious: The fuel charge will travel different distances at different velocities (and I believe it is correct that each side will run slightly leaner than if they both dump into the same plenum portion). They will both wind up in the chamber, however...and without a dyno you may not/won't notice much difference.


That is incorrect. The purpose of a dual plane is to divide the work of each primary/secondary barrel and increase the intake velocity. In other words each plane only feeds 3 cylinders. That provides better low end torque. If both primarys are on the same plane then they will both only be feeding half of the engine!



Even if the intake is a square bore and the carb is a spread bore, it wont matter as long as the manifold is big enough to accommodate the larger secondaries.



PS. Loose the spacer if possible. Dyno tests have proven them to be a negative power accessory.

John Strenk 06-10-2021 03:41 AM

Quote:

Originally Posted by jeepdaddy2000 (Post 41289895)
Square bore.Looks like a Carter AFB

The barrels ALWAYS straddle the plenum. The plenum separates the planes, which are set up so the runners are as equal in length as possible. Enclosed is a picture of a V8 dual plane manifold. Look closely at the separate planes and the runners they feed. If the carb is orientated so the primaries are both on a single plane, then both barrels will feed only 3 cylinders(6 cyl) or 4 cylinders (V8).

That would be true with an ordinary split plenum setup where one plenum feeds 1/2 the cylinders evenly but the Offey split PLANE manifold is a little different.

[off topic]Actually you want the cylinders to draw air evenly, You do not want 3 cylinders in a row drawing air/fuel and then nothing for a revolution while the other plenum isn't supplying any mixture for a while and then suddenly needs to supply a lot. It cuts the momentum of the air flowing through the carb messing up the mixture. Equal runner manifolds are a little different design than the one pictured. [/off topic]

https://www.whiteowlspeed.com/wp-con...oBackgroud.jpg

https://www.whiteowlspeed.com/wp-con.../09/5999DP.jpg

The plenum is split where each plenum is feeding all the cylinders. the narrower passages are for the primaries to keep the flow moving fast for better throttle response while the large ones are for the secondaries where large amounts of flow are needed.

So you would want you primaries on the smaller side and secondaries on the larger side.


Quote:

Originally Posted by gutthans (Post 41289847)
Depending upon the year of the OFFY manifold, I believe the mounting pad area was changed to a 6* slant to compensate for AMC 258 engine tilt (4*- 6*). (Maybe a number of owners machined the pads themselves? but I believe it was a factory modification. A machined spacer (as noted earlier) will do the same on the earlier models.

The dual plane configuration SHOULD be run with the primaries both facing the valve cover, NOT STRADDLING both plenums. The reason is fairly obvious: The fuel charge will travel different distances at different velocities (and I believe it is correct that each side will run slightly leaner than if they both dump into the same plenum portion). They will both wind up in the chamber, however...and without a dyno you may not/won't notice much difference.

And yes, that does look a bit like a GM spread-bore???

Looking at the pictures above. Looks like one manifold has the narrow passages on top and one manifold has the narrow passages on the bottom.
This would have a BIG effect on how the carburetor is mounted.

If you mixed up the primary and secondaries then you would have probably the same performance as a too tiny 2 barrel and when the secondaries opened up, they would have to squeeze through those narrow passages, ot doing much to increase performance.

If that is a spread bore carb then the secondaries would never open all the way.

The top one pictured above is for 81 and later as it has no provisions for heating the manifold and the thinner mounting pads.
The bottom one is for earlier 258's as it has a flange on the bottom so the exhaust manifold can attach to it so the exhaust can heat the bottom of the manifold and the mounting tabs are longer to bolt up to the stock exhaust manifold.

John Strenk 06-10-2021 04:20 AM

Quote:

Originally Posted by mudbfun (Post 41289941)
....

PS. Loose the spacer if possible. Dyno tests have proven them to be a negative power accessory.

I think that depends upon the manifold design.

On some manifolds it helps with smoothing the airflow out the carb giving it a chance to mix properly.

If the base of the carb is mounted on a manifold that instantly changes direction to the left and next instant to the right, you going to get different mixtures in either direction.

Smooth air flow out is just as important as smooth air flow in for "drivability"

If you only run at WOT then that changes everything also as opposed to driving to the grocery store during the week.

jeepdaddy2000 06-10-2021 05:30 AM

Quote:

Originally Posted by John Strenk (Post 41289945)
That would be true with an ordinary split plenum setup where one plenum feeds 1/2 the cylinders evenly but the Offey split PLANE manifold is a little different.

I stand corrected and owe gutthans an apology.
My depth of knowledge of 4 and 6 barrel intakes is showing:oops:.

John Strenk 06-10-2021 07:26 AM

Quote:

Originally Posted by jeepdaddy2000 (Post 41289957)
I stand corrected and owe gutthans an apology.
My depth of knowledge of 4 and 6 barrel intakes is showing:oops:.

Not a problem.

You should of seen the intake manifold on my Renault driven Lotus Europa.
One barrel of the single 45DCOE fed cylinders 2 and 3 while the other barrel fed cylinders 1 and 4.
SCCA did say you could run any 2bbl carb and still remain in stock classes. :D

Fourtrail 06-10-2021 08:44 AM

1 Attachment(s)
Quote:

Originally Posted by John Strenk (Post 41289945)

https://www.whiteowlspeed.com/wp-con...oBackgroud.jpg

https://www.whiteowlspeed.com/wp-con.../09/5999DP.jpg

The plenum is split where each plenum is feeding all the cylinders. the narrower passages are for the primaries to keep the flow moving fast for better throttle response while the large ones are for the secondaries where large amounts of flow are needed.

So you would want you primaries on the smaller side and secondaries on the larger side.




Wherein lies part of the issue, the Edelbrock carb does have mechanical opening secondaries, but there is a weighted air valve between the venturi and the butterflies in the base plate. If you are not creating enough vacuum on the secondary side of the intake, the air valve will not open and no fuel will flow through. With the larger runners on the secondary side of the offenhauser intake, it is possible that you just aren't getting enough vacuum signal to open the secondary air valve. One option is to drill a small hole in each weight to reduce the mass so the secondary air valve will open/open quicker. Just don't go too much, then you are ordering a new secondary air valve if it opens too quick and causes the motor to stumble.


Image is of the air valve that sits between the venturies and the butterflies.


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