[john65ss]

I've noticed some carbs have a "duct" welded to the secondary baffle plate that shrouds the passenger side secondary discharge nozzle. What's it there for, and does it help or hurt performance (or have no effect)?

[unruhjonny]

I noticed this last night when I took apart a chevy carb for parts to re-assebmle some more desireable poniac carbs;
The carb I pulled mine off was (iirc) a 7029233;
I figure it's gotta be there for some reason... but it's curious that it's only on the choke side of the carb.

I was going to post up here today, or sign up for cliffs forum to ask this, and a couple other questions.

[zeebo]

pics??

[Cliff R]

Those can be found on both sides, but most often on the right side, and have no effect that we can measure on performance either way. They do make a very good stop for the airflaps, if one is converting a later rear only pull-off truck carb to a front pull-off.....Cliff

[Sun Tuned]

H-O racing used to say it corrected a lean condition in cyl 8 on the 455 super duty...

Got several of them and wondered if it really made any difference.

[unruhjonny]

Originally Posted by Sun Tuned

H-O racing used to say it corrected a lean condition in cyl 8 on the 455 super duty...

Got several of them and wondered if it really made any difference.

interesting!

[67cruiser]

I have one love to sell it???

[Half-Inch Stud]

Thought that was uniquely a Chevy fix for a poor untake distribution.

not familiar with PMD applications needing/using the secondary flowbox diverter valve

[77 TRASHCAN]

It would make sense if the box was on the LH side for the #7 cyl. (Pontiacs) or on the RH side for the #8 cyl,(chevys) each being the furthest away form the carb, possibly creating a lean condition in these, per specific makes.....

Doug Roe still alive??? He'd know....

[Cliff R]

As mentioned those "boxes" have no measurable impact on performance that we can tell. I've had carbs on the dyno and at the track several times that used them, and they run equally as good as a carb not using them.

It's more important, when it comes to this topic, to adjust the secondary throttle plates for full opening, and the airflaps to the ideal angle to provide plenty of cfm for the application, without them rolling over too far and hurting fuel flow from the nozzles.

We've installed adjustable stops on many q-jets and played with the open angle on the dyno and at the track. I can say this for certain, setting them up to open too far is just as bad as not far enough.

I would also NEVER remove the divider in the secondaries, we see this a lot, it is just a bad idea, the engineers really did know what they were doing by putting it there........Cliff

[Half-Inch Stud]

Originally Posted by Cliff R

...I would also NEVER remove the divider in the secondaries, we see this a lot, it is just a bad idea, the engineers really did know what they were doing by putting it there........Cliff

THAANKS: agonized over the purpose for that SEC-divider ( not obvious to it's helpfulness ) and luckly have kept dividers in, but not knowing the benefit.

[zeebo]

cliff, do you have any pics you can post?

[Firebob]

Yes. I'm curious to know how far is too far and how far is just right opening of the sec flaps.

[unruhjonny]

Originally Posted by Firebob

Yes. I'm curious to know how far is too far and how far is just right opening of the sec flaps.

iirc it's detailed in his book

[Sun Tuned]

I honestly don't remember what cyl they said it was....
Memory ain't what it used to be.

I can look it up, not that it matters.

I've seen all sorts of gizmo's over the years. Angled flaps spot welded underneath the air flaps and such.

I always wanted to ask Cliff about them but never remembered to do it when I was talking to him.

[77 TRASHCAN]

Cliff,
How far open, is too far???
Possible measurement???

[Sun Tuned]

Ok looked it up...

The instructions say....

"Replace secondary air baffle with one with a booster Venturi on the left side (7032324) to correct lean #5 and #8 cylinders when using 180 manifolds."

H O racing December 1974 super stock magazine.

[unruhjonny]

Originally Posted by Sun Tuned

Ok looked it up...

The instructions say....

"Replace secondary air baffle with one with a booster Venturi on the left side (7032324) to correct lean #5 and #8 cylinders when using 180 manifolds."

H O racing December 1974 super stock magazine.

HO Racing primarily catered to Pontiac enthusiasts right?

What is a "180" manifold?

[poncho in a can]

180 degree manifold, every other cylinder in the firing order is connected so that in reality, it is like there 2 manifolds in one, each one of which feeds 4 cylinders. Also know as the Dual Plane intake, it is what is used by the factory on the majority of our favorite carbureted vintage vehicles. good example; edelbrock performer rpm. If you look at the intake you can see the 2 layers and the way that the cylinders are split up. The opposite of the dual plane is the single plane, example Edelbrock Victor, all eight cylinders feed off of one common plenum. BTW, the mid eighties Mopar Q-jets used the duct welded to the secondary baffle also.

[Cliff R]

The divider is there for a reason. Look closely at how the fuel nozzles are set in the carburetor, and how the airflaps open. They completely cover the exit holes, creating a low pressure area under the nozzles. The divider plate increases the "signal" due to it's location, as it separates the high velocity incoming air in front of the airflaps from the nozzle exit holes.

"Engineers do things for reasons", and I'm sure they spent a lot of time on the development of the q-jet, and testing nozzle length, location, depth, and flow from the nozzles at various airflap open angles.

Rocky Rotella did some airflow testing some time ago on his flow bench testing secondary flap open angles. I've never done this.

I have added an adjustable stop to the secondaries, and spent a LOT of time on the dyno and at the track to find the optimum angle, maximum angle, etc.

It's interesting when it comes to this topic, is how Edelbrock rated their 1910 q-jet at 850cfm, when it is exactly the same casting as their other later model q-jets rated at 795cfm.

The only difference between those carburetors, is the full open position of the airflaps.

The 1910 opens to apprx 1.240" (nearly as I can remembers without going out and measuring one). This is quite a bit further than the same basic carb they rated at 795cfm. This measurement is from the upper section of the airflap back to the rear edge of the opening as described in my book.

On my own 1977 Pontiac carburetor, I run the open angle at 1.315", and on the dyno and at the track see no improvement going further open than that. The car doesn't really start to slow down any until I reduce the open angle to less than 1.380".

It's also interesting that I see a LOT of carburetors where the stop is bent, or ground completely off to where the airflaps open a LOT further than the numbers above. Makes me wonder if that isn't part of the trouble with them, and why we are getting them in here for custom tuning.....Cliff