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Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: DaveO90s4 ()
Date: December 21, 2018 01:43PM

Hi all,

My 140 engine - +.060 and Otto 20 or 30 cam (now fitted in my 914 Porsche) has modified heads and a pair of triple zenith 40TIN carbs. I have had these rebuilt by a supposed expert. Still not convinced of their skills, other than for their skills at opening my wallet. The car runs great, no bogging, revs nicely. But gets only 14 mpg!! At around 2,500 to 3,500 it is running lambda 0.85 so a bit rich. I have lots of jet options and clearly I have quite some tuning work to do.

But just a thought - has anyone with modified heads such as mine ever reverted 'back' from separate carbs to run a simple single and relatively cheap center carb, such as a rochester q-jet? The thought of not having to balance and synchronise multiple carbs has some appeal! - of course I'd need to have new intake runners fabricated but that should not be too difficult to have done properly.

At this stage I do not want to consider fuel injection. My preference is to drive and enjoy it more than tinker with it - I have too many other cars that require tinkering!

Thoughts / suggestions very welcome.

Cheers

DaveO

Dave O

1975 Porsche 914 with 140 HP Corvair reverse rotation engine, zenith 40 TINS, Otto 20 (or 30?) cam, etc, Australia

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: Gregory_Miller ()
Date: December 21, 2018 02:10PM

As with anything, there are pluses and minuses. Center mount carbs can have icing issues so climate where you live matters. My dad put one on our 68 140 but it may have been too big. Huge flat spot when you stepped into it but would scoot once it got breathing. Icing was an issue where we live and the accelerator cable that replaced the linkage was prone to failure. Your mileage however may vary. Lean up the jets a tick first.

Greg Miller
Eugene, Oregon

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: vairmech ()
Date: December 21, 2018 02:12PM

If you can do any fabricating I can see a 6 runner Q-jet that can bolt on at the heads with an adapter just like for the Zeniths.

BTW, the race car only gets 5 mpg!

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: cnicol ()
Date: December 21, 2018 02:45PM

How the engine will run after conversion depends on your engine's camshaft, ambient temperature, and tuning skills.

Camshaft: 6x1 carburation, assuming there's no common intake plenum, allows a radical camshaft, i.e. 310 duration, while still retaining good low speed torque and idle quality. This is the unique and killer function provided by isolated-runner 6x1 carburation. You get all the power potential of a full race engine while retaining street idle quality. If your engine has a street cam, this point is moot.

Ambient temperature: Atomizing and vaporizing fuel is a chilling process. Run that cold fuel/air mixture down unheated runner tubes from a central carburetor and condensation (fuel on the inside and atmospheric water on the outside) takes place. It takes a very rich idle mixture to compensate for the condensing air/fuel mixture. Place those tubes above the fan intake and you get a variable effect depending on ambient temperature.

If you're good with carburetors, a 4-barrel carb, particularly a Q-Jet can be tuned for excellent results. Most "tuners" find themselves challenged trying to make a center 4-barrel carb work properly.

Craig N. Coeur d'Alene ID.
66 Black Monza 4dr, 4.2L V8 49k
61 Seamist Jade Rampside 140 PG
60 Monza coupe (sold, sniff sniff)
66 Sprint Corsa convt - First car! Re-purchased 43 years later
2+2 gnatsuM 5691

+17 Tons of parts




Edited 1 time(s). Last edit at 12/21/2018 02:46PM by cnicol.

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: steve c goodman ()
Date: December 21, 2018 03:04PM

Dave that is a tight fit between the engine and the
body on the 914. I would suggest some jet swapping
with the Zenith carbs before deciding anything.

The gearbox in the 914 is low geared until 4th/5th
so revs are up and speed is only moderate. Not the
best for economy.

It is easy for me to be half way around the world
and say "don't quit yet", sorry.

best wishes, Steve
Rear Engine Spec. Inc. Golden, Colo.

1962 spyder 3.0L turbo---1965 Crown V8
1967 monza 110/4---1968 monza 110/4
1971 amante gt 110/4
CORSA/RMC/PPCC

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: joelsplace ()
Date: December 21, 2018 08:27PM

I'm with Steve. It seems like since you have this setup all installed and running it would be fastest and easiest to dial in the jetting on what you have.
A Quadrajet is a great carburetor but it's way too big at 750cfm.

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: DaveO90s4 ()
Date: December 21, 2018 08:44PM

Hi Gents, thanks all for your sage replies. What you all say makes sense. I think I am just feeling a bit daunted at the thought of setting up the carbs properly. Strange that I should feel that way - I have all the required tools, I have a decent workshop, I managed to install the Corvair engine into the 914 on my own, etc etc. I will knuckle down to doing the right job right over the Christmas break and will report back anon.

Re carb icing with long inlet runners. I’d probably be ok where I live. Today it is 35 Celsius, yesterday was 41 Celsius. In winter to stay warm in my 928 I simply close the sunroof!! Too darned hot living in Queensland, that’s for sure.

Merry Christmas to you and yours

DaveO

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: 63turbo ()
Date: December 21, 2018 10:39PM

If you are only getting 14mpg, you will probably gain quite a bit of power by jetting it right. That is pig rich to get that kind of mileage, and might only have to lean the main circuit to fix it. That's a lot easier than going to a six tube manifold and starting over re-jetting with another carb. I bet you would have an easier time re-jetting your current set-up than getting a single carb to run right.

------------------------------------

Kevin Nash
Friday Harbor Washington
63 Spyder, Daily driver, EFI read about my project here: [corvaircenter.com]
first test start on EFI here:[www.youtube.com]
first official EFI boost test here:[www.youtube.com]
My new fan! [corvaircenter.com]
engine less 62 Spyder
Canadian 64 Monza Parts car



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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: Seth Emerson ()
Date: December 21, 2018 11:28PM

DaveO - I would suggest you look around for a good Porsche 911/914 tuner.Let them chase down any Solex related issues first.

Seth Emerson

Check my new Performance Corvair Web site [www.perfvair.com]

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: j3m ()
Date: December 21, 2018 11:41PM

Dave, I don't think you are gonna see any substantial improvement from the current 14mpg with any switch to a Q-jet on your Corvair set-up.

As the old famous song goes, YOU CAN'T ALWAYS GET WHAT YOU WANT, BUT IF YOU TRY SOMETIMES, YOU GET WHAT YOU NEED.....!!!

If your current twin tripple carb set-up operates near perfectly at all conditions with the only negative being the 14mpg, you may wish to just rock and roll with that and pay the price at the gas pump.

If you really want get both perfection and significantly improved fuel economy, it is my opinion that you will not get both of those at the same time without injection. ......The Ted man drives circles up and down the block.....
..................-AND-.....he can take it out on highway 61 without any problems or needing to pull into the Exxon station every hour and a half............he ain't tangled up in the carbie blues!
Spend your time and effort on what will provide you the best, practical improvement. Going retro backwards might have a cool factor but be prepared for the consequences of retro..........lack of fuel mileage... and barely adequate operation under certain conditions (weather temp changes and altitude changes).
If your car runs great as it is, just be happy and enjoy it. 14 mpg is not going to break your wallet, and if you really need or want more than fourteen.....a 21st century, three letter solution is what you may wish to consider. If you really love to tinker........tinker away.......eventually you might find injection is what you may wish to tinker with and adapt, or just do the one call, thats all, and have it ordered and shipped to you............if that's not economically practical, perhaps you can follow the roadmap and build your own close copy of the great efi system being marketed today.

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: gnvair ()
Date: December 22, 2018 05:59AM

j3m Wrote:
-------------------------------------------------------
> Dave, I don't think you are gonna see any
> substantial improvement from the current 14mpg
> with any switch to a Q-jet on your Corvair
> set-up.
>
> As the old famous song goes, YOU CAN'T ALWAYS GET
> WHAT YOU WANT, BUT IF YOU TRY SOMETIMES, YOU GET
> WHAT YOU NEED.....!!!
>
> If your current twin tripple carb set-up operates
> near perfectly at all conditions with the only
> negative being the 14mpg, you may wish to just
> rock and roll with that and pay the price at the
> gas pump.
>
> If you really want get both perfection and
> significantly improved fuel economy, it is my
> opinion that you will not get both of those at the
> same time without injection. ......The Ted man
> drives circles up and down the block.....
> ..................-AND-.....he can take it out on
> highway 61 without any problems or needing to pull
> into the Exxon station every hour and a
> half............he ain't tangled up in the carbie
> blues!
> Spend your time and effort on what will
> provide you the best, practical improvement.
> Going retro backwards might have a cool factor but
> be prepared for the consequences of
> retro..........lack of fuel mileage... and barely
> adequate operation under certain conditions
> (weather temp changes and altitude changes).
> If your car runs great as it is, just be happy and
> enjoy it. 14 mpg is not going to break your
> wallet, and if you really need or want more than
> fourteen.....a 21st century, three letter solution
> is what you may wish to consider. If you really
> love to tinker........tinker away.......eventually
> you might find injection is what you may wish to
> tinker with and adapt, or just do the one call,
> thats all, and have it ordered and shipped to
> you............if that's not economically
> practical, perhaps you can follow the roadmap and
> build your own close copy of the great efi system
> being marketed today.
Once again you don't know what you are talking about. GM used Quadrajets on large and small engines. I had a Monte Carlo SS with a Quadrajet and it would get 26 mpg on highway trips. A Corvair wwith one that is set up properly should be able to approach those figures with out fuel injection. Heck! Some guys get that with stock Rochesters

Lee J

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: RCSmith ()
Date: December 22, 2018 11:34AM

You might look into changing emulsion tubes to lean the mixture at cruise rpm
For better mpg.

Ray C. Smith
'66 Corsa


Bergen County ,NJ

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: wv-geo ()
Date: December 22, 2018 11:55AM

RCSmith Wrote:
-------------------------------------------------------
> You might look into changing emulsion tubes to
> lean the mixture at cruise rpm
> For better mpg.

There's a guy on eBay selling custom emulsion tubes for the Zenith, alfa1750:
[www.ebay.com]

He also sells chokes up to 35's and jets.

Dan Stark
Hurricane, WV

1961 Lakewood 500, 80HP, PG
1964 Corvair 500 Coupe, 140HP, 4spd (Christine)
1965 Mustang Coupe, 6cyl, 3spd

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Re: Replacing a pair of triple zeniths with ?? Q-jet??
Posted by: nirvairna ()
Date: December 22, 2018 12:07PM

GM used Quadrajets on large and small engines

they were used on the pontiac overhead cam 6 in 1967, engine displacement was 230 cubic inches. since the Q-jet only opens it's secondaries due to air flow demand, it works fine on a Corvair, has been done many times.

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