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Center mount carb theory
Posted by: rrp0403 ()
Date: January 02, 2018 09:28AM

Hello all,

I've been doing some reading on the center mount single carb option, and it seems clear that a holley 390 is still a bit too big for my corvair 140. The other problem is that there is a gap in throttle response due to the long runners. I believe this is due to there being too much and not enough fuel.

I haven't done any testing of the following theory and I was hoping to get some feed back. I have a 4 runner manifold that accepts a 2 barrel carb. I have a 4 barrel carb. There is an adapter plate that I could buy that would allow a 4 barrel carb be mounted to the 2 barrel manifold. This set up would restrict air flow compared to having the 4 barrel manifold.

Would this air restriction help the acceleration gap? It seems that guys use the 50cc accelerator pump to give more fuel to compensate for the excessive air, but by reducing the air flow I think that would provide the same solution by different means. Maybe the 50cc accel pump would benefit this set up even more?

Thoughts?

Re: Center mount carb theory
Posted by: Allan Lacki ()
Date: January 02, 2018 10:30AM

A 4-runner 2-bbl center-mount intake manifold? Did anybody ever make such a thing?

I recall that the old IECO 4-runner manifolds were drilled for Carter AFB 4-bbl carbs. You'd probably need an adapter plate to mount a Holley 390 on it.

Getting back to theory, the IECO 4-bbl units had a splitter in the plenum that separated the right-hand barrels from the left-hand barrels That essentially turned the single 4-bbl carb into two 2-bbl carbs, just like a dual-plane manifold on a V8. Theoretically that, along with the fact that the 4-bbl has progressive secondaries, should eliminate some of the bogging.

I have one of those IECO 4-bbl center-mount manifolds in the basement gathering dust, but I never installed it. Can anybody out there comment on whether the IECO manifolds work better than the knock-offs that didn't have the splitter in the plenum?

Re: Center mount carb theory
Posted by: MattNall ()
Date: January 02, 2018 10:48AM

Here's the best carb for your Manifold.... no problems.. adjustable accel pump and easy jetting changes... I ran one for over 20 years....not a problem!



Fomoco 2100

[www.ebay.com]







MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo




Edited 1 time(s). Last edit at 01/02/2018 12:06PM by MattNall.

Re: Center mount carb theory
Posted by: vairmech ()
Date: January 02, 2018 05:17PM

There are lots of choices, I ran a SBC Quadrajet that I got ALMOST all of the hesitation out of it. I have run Holley 390's with almost as good a results.
One of the things I do is make sure the center divider in the center adapter is all the way to the base of the carb. That helps with the low end torque and hesitation.

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

Re: Center mount carb theory
Posted by: joelsplace ()
Date: January 02, 2018 08:58PM

They made 2 different variations of the Quadrajet that were 2 barrel carbs. One that was just the primaries and one that was split the other way with the tiny primary and a big vacuum secondary. The 2 primary Dualjet was very common but I don't know what the other was original on. The only one I've seen was in a dumpster full of carbs at the local metal scrap yard. Anyone know? I think it was also called a Dualjet IIRC. It might be a really good carb as tiny as the one primary is. I guess you could simulate one by blocking a one primary and secondary.

Re: Center mount carb theory
Posted by: vairmech ()
Date: January 03, 2018 04:55AM

joelsplace Wrote:
-------------------------------------------------------
> They made 2 different variations of the Quadrajet
> that were 2 barrel carbs. One that was just the
> primaries and one that was split the other way
> with the tiny primary and a big vacuum secondary.
> The 2 primary Dualjet was very common but I don't
> know what the other was original on. The only one
> I've seen was in a dumpster full of carbs at the
> local metal scrap yard. Anyone know? I think it
> was also called a Dualjet IIRC. It might be a
> really good carb as tiny as the one primary is. I
> guess you could simulate one by blocking a one
> primary and secondary.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


There is no need for the above the SBC full size Quadrajet works very well. While it is a 750 cfm carb it is a true variable secondary venturi. It can be set up quite well to use on the Corvair. It has small primaries which give you the good low throttle response and the ONLY place I ever had hesitation was if I snapped the throttle wide open from idle. Even then it was only as bad as one cylinder missing one beat and then the engine would just go! The reason for the "miss" was the mixture would go lean for that 1/2 second, there is nothing you can do about that but I found a way to minimize that with the adjustments available on the Quadrajet.

As far as the Holley, I will only use the mechanical secondary carb or I will convert the vacuum secondary one. When I want all four bbls open I want them open! Again there are a couple of things that need to be set on the 390 to make it work. I used #55 main jets and I blocked off the enrichment diaphragm.

I have driven both of these setups on the street and on the track!

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

Re: Center mount carb theory
Posted by: v8vair ()
Date: January 03, 2018 06:09AM

FYI,One Bone Stock Rochester flows 95 CFM X 4 = 380 cfm

Re: Center mount carb theory
Posted by: MtnVairMike ()
Date: January 05, 2018 07:33AM

The Quadrajet has an often maligned but well engineered vacuum operated secondaries. If the bleed rate is adjusted properly the secondaries come in slower and that really helps out. Mechanical secondaries are for drag racing at WOT, not the street.

Thanks,

Mike

1966 Monza Convertible, 140HP-4 speed, Ermine White

Re: Center mount carb theory
Posted by: Frank DuVal ()
Date: January 05, 2018 10:18AM

All the Quadrajets I have rebuilt had mechanical linkage to the secondary throttle plates. The "vacuum" part was the choke plates of the large secondary venturies. And yes, it is a good carburetor, but misunderstood by the Holley guys!grinning smiley

Quote

One Bone Stock Rochester flows 95 CFM X 4 = 380 cfm

What particular Rochester? Are you saying the Rochester Quadrajet, with unequal size venturies, flows the same air CFM in all four venturies? Or are you talking about the Square bore 4-Jet, with equal size venturies? The 4-jet is 1952 to 1967, whereas the Quadrajet is 1965 to 1990.

[www.carbkitsource.com]

[www.carbkitsource.com]

Frank DuVal

Fredericksburg, VA

Re: Center mount carb theory
Posted by: MattNall ()
Date: January 05, 2018 10:25AM

Frank.....HV's... x4? on a 140 hp





MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

Re: Center mount carb theory
Posted by: joelsplace ()
Date: January 05, 2018 10:36AM

Those aren't choke plates. They are a second set of throttle plates that control the metering rods. Not sure why they were built that way. Interesting topic. They work like motorcycle CV carbs that use a throttle plate and a slide instead of another set of plates. The motorcycle type use the first part of the plate travel to control idle up to about 1/8 throttle like the Quadrajet primaries.

Re: Center mount carb theory
Posted by: Chilly Willy ()
Date: January 05, 2018 01:25PM

Buy this, don’t stack the adapter.
[corvaircenter.com]




The Corvair, Keeping Nader Notorious winking smiley


Will
Northern California

Re: Center mount carb theory
Posted by: Frank DuVal ()
Date: January 05, 2018 03:09PM

Yes, probably not called "choke" plates by Rochester, but they appear to act as such since they make the mixture rich until they open. And yes, they are tied to the metering rods to also adjust mixtures.

AH, "Our" , not "One". Got it, our Rochesters flow 4x..........winking smiley

Frank DuVal

Fredericksburg, VA

Re: Center mount carb theory
Posted by: joelsplace ()
Date: January 05, 2018 05:00PM

How do they make the mixture rich? A choke plate richens the mixture by dropping the air pressure in the venturi and idle circuit making it pull more fuel. No fuel circuits are in play in the secondaries until the top plates pull the metering rods up from what I understand. Adjusting the opening of them has never changed the mixture in my experience, only messing with the metering rods they control does.

Re: Center mount carb theory
Posted by: Frank DuVal ()
Date: January 05, 2018 07:42PM

That's right. The air valves opening raise the metering rods to regulate fuel flow into the secondary venturies.

I "think" of the air vales as making the initial mixture rich because if the air valves were not there, and the secondary throttle opened the metering rods, the mixture would go lean as soon as the secondary throttle was cracked open. The air valve action keeps this leaning from happening, as I understand the process. Maybe not how the Rochester engineer thought of it.confused smiley

Frank DuVal

Fredericksburg, VA

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