What If I wanted to....
 

I know that the topic of Zenith INAT has been beaten like a cryogenically dead horse on this discussion forum. I am not about to "go there" But I do want to start the discussion on conversion to a single unit carb. For my 72 250. To properly consider a change a comprehensive analysis must take into account the pros and cons.

First off I must ask; what is the advantage of two carbs over one?

Possible benefits for conversion.
1. No sloppy linkage from front to back
2. Only one carb to adjust
3. When the choke sticks, it is all or nothing
4. Cheaper then a dual Weber Conversion (like an Edlebrock for $250)
5. More fuel efficient?
6. Replace with a carb that I can actually find parts for

Possible disadvantages
1. Costs more then doing nothing
2. Finding/Fabricating an intake manifold
3. Finding the right carb for the application (size, jets, etc)
4. Less fuel efficient?
5. Figuring out the linkage to the auto tranny

I know that there are many of you out there may that think I am a fool for even bringing it up. It is not that I don't appreciate the dual zenith INAT that I have, it is just starting to be a real pain to find parts. For example I currently need to resolve the following issues: sloppy linkage bushings due to wear, weak fast idle cam return springs, weak accel. pump on rear carb due to either fault in pump or wear in the housing, two base gaskets missing from the rebuild kit that I bought, and the fact that no body sells the rebuild kit for the 32/40, rather they want you to use the 35/40.

So there it is, please feel free to chime in on the subject. I especially would like to hear from those who could tell me how to calculate the right size for a single carb, and any ideas on intake manifolds that may work.

:stupid:

Contact the manufacturer of the rebuild kit and inform them of the missing parts.

The rebuild kit ze-11k usualy has the parts for the 35, 32, and even parts for the BMW series (Bavaria...)

Can't help with the rest. sorry.

The advantage of a dual carb setup is better air flow at wide throttle, mostly due to equal length runners and fewer turns.

FI is the way to go, not really an option for yours, though.

Changing carbs will require a new intake manifold, and you will have to fabricate one. This will not work as well as the original twin carb setup unless you are a pretty good fluid engineer and have the time (and cash) to get it right, there are NO aftermarket intakes.

Peter

There was a European-market W123 250 that used a single-cam six with a single Solex 4V carb. That engine was designated the M123. I'm not certain that intake manifold will fit the M130 head because, though they look similar, there are differences between the two engines. But if you're determined to convert to a single carb, it's an option worth exploring.

Happy Motoring, Mark

Fluid design
 

Thanks mark, I know that the Euro spec W123 was a higher compression engine, I wonder if compression would have an effect on the "demand" for combustable air mixture.


On the topic of fluid design: How does one go about calculating the demand for combustable air? I am thinking that it is a function of displacment and RPM. for example f(x)= 2500x*6*rpm

And then there would be the issue of mixture, I would assume that the ideal mixture is stated somewhere on the web. I will post more when I find some info.

That was easy see : http://en.wikipedia.org/wiki/Lean_burn

More stuff on ratio
 

AFR

The Air fuel ratio is the most common reference term used for mixtures in internal combustion engines. It is the ratio between the mass of air and the mass of fuel in the fuel-air mix at any given moment.
For gasoline the stochiometric mixture is 14.7:1 at sea level
In Naturally Aspirated engines maximum power is frequently reached at AFR's ranging from 12.5 - 13.3:1

Blue, are you thinking about casting your own intake manifolds? The demand for your engine would be 2.8L * 6000 RPM (I beleive is max) = 16 800 Liters of air / minute.

Forgive me for saying but It does not sound like you're a fluid engineer. ;)

As for the air mixture, when you replace your carbs, you will have to purchase a jetting kit and determine, by trial and error what the best jetting is for you.

Too lean and you'll ping your engine to death, and too rich you'll flood/bog the engine down.

You probably know this but the way to change the jets is to disassemble the carb.

Lots of work. Good luck to you.

Not to nitpick, but remember that each cylinder only intakes once every OTHER revolution. You need to divide that number by two. ;) Also, since the manifold has a vacuum, you can argue that it never fully consumes 8400 liters of air per minute if that air is at 1 atm of pressure. It's probably closer to 5000. Not to say that still isn't a lot - because it is!

about 20 years ago i converted my 3 liter bmw bavaria to a single four barrell after rebuilding the original (junk) solexes at least six times with no success. i bought a manifold from europe for the four barrell solex. a q jet was a perfect fit after relocating two of the mounting bolts 1/4" ea. i had three different q jets on it and the best turned out to be an older one with mechanical secondary actuation. i ordered the jet rods for a 67 pontiac soc six. they worked fine. if you waited to open the secondaries til about 2800 rpm when they opened the engine would just wail up to the redline and want to go way over it. all the books will say that this carb is too big for your engine but a german engine will flow a lot more air and rev higher than the typical american engine of the same vintage and i can assure you it will work fine. i would use a new edelbrock q jet replacement carb with mechanical secondary actuation and jets for the tempest (if available that way).

my biggest problem was clearance under the hood. the single carb setup interferred with the mc booster. i ended grinding off the corner of the manifold (i remember thinking while i was grinding it off that i was risking a $300 dollar manifold and thinking "oh well it isnt working as it is"). i also had the manifold milled at an angle at the head match point and shimmed one motor mount and installed a restraining strap to keep the motor from leaning too far.

i used a volvo throttle cable and fabricated several brackets from bar stock. i maybe had 50 hours in making it work before it was all over but when done it ran so much better than ever before it was well worth it.

i was younger then and had a lot of excess energy.

i could have bought a conversion kit from jamb engineering for about $600. that seemed like too much at the time. but it doesnt sound too bad now.

tom w

...

You're right. it takes 2 revolutions for all cylinders to fire. Obviously I'm not a fluid engineer either LOL :silly:

Fluid Engineer I am not
 

As you can tell, the type of "fluid egineering" that I like to do involves a 16oz glass and hops :beer: .

The other option for a manifold would be to fabricate one from tube and plate welded together. Or a unit that bolts on to the stock manifold that joins the two sides together. Either way, yes it would be a lot of work.

Thanks T for the real life experence, I bet that would have been a fun project.

it was fun.

good luck

tom w

Carb Size
 

The following was quoted from ebay carb buyersguide. USE AT OWN RISK

This is a question I get asked a lot. What carburetor would be best for my engine? Whether it be a 1, 2, or 4 barrel, here is a little help for you. There is a formula that will get you very close to the proper cfm for your engine.

The formula goes like this:

cubic displacement divided by 2 (ex: 351=175.5)
maximum rpm divided by 1728 (ex: 6000 rpm=3.47)
multiply those two figuires (ex: 175.5 x 3.47=608.98 cfm)

1 cc = 0.0610237441 cubic in

Hold on we are not done. You now have to factor in volumetric efficiency. A Nascar race engine runs at 85%-90% volumetric efficiency. A normal street car wil be 75-80%, depending on what you have done with your heads and exhaust.
Ex: 608.98 x 75%=456.74cfm


With carburetors, bigger is not better. You will not get very good results with an 850 double pumper on your small block Chevy. If you are after good idle quality, fair fuel mileage, and good performance, stick with this equation. The boys at GM and Ford used this equation for many years and it works! You can not change the laws of physics. I would like to thank Jon Enyeart of Pony Carburetors for teaching me how to properly restore and install a carburetor. He is the Grand Wizard of carburetion.
Kurt Praxl
Kp Carbs

I'm not a fluid engineer either, but I've rebuilt enough zeniths to know how easy it is to get it to be less than perfect.

The kit is indeed, one size fits all. But you can still visit your favorite parts supplier to get a few extras. For instance those base gaskets. Throw away the ones from the kit and buy new bakalite spaces and metal plates. That will keep aways the leaks for a few decades.

For the accerlation pump that seems to leak, did you buy a new one (strongly recommended) or did you attempt to replace just the skirt (You're a guy, what do you know about putting a skirt back on;))?

The linkage parts should be replaced from another carb if they're worn. There is no way around this. You can probably find another set of springs too, but ask the dealer for them (can fast line get dealer parts?)

I long ago stopped cleaning my carb parts myself and began handing them over the a professional shop. They will grumble, but if you ask nice, and promise not to ask for any technical advice, they'll do it and that means a rainbow coating on the potmetal. Be sure to take the time to check for flatness and correct any issues before that level of cleaning, since it can cost you (the toxic waste isn't cheap).

The fun part with the zeniths is getting all the stuff outside the carb in shape. The dashpots, fuel return valves, choke settings, etc. That stuff you handle one sub-system at a time. Each has a bench setting to work with and a way to test it once the car runs.

If you want to switch carbs, there are nice webbers that bolt on once you have an adapter plate. Also you'll find that the unimogs (trucks) had straight 6 motors with a single carb. One of their manifolds would be the start of the conversion.

At present, I don't have any cars with zeniths, but I seem to have one of everything else with a fuel delivery problem. such as it goes. I picked up a set of ford escort throttle bodies way back when as study of how to do TBI. While less efficient than multi-port injection, you can't beat throttle body injection for cost and efficiency, presuming you can get the electronics to do the right thing. One of these days I'll get to that project. There's always an M110E or M114E motor with D-jet. Those are factory supplied at least.

-CTH

the 110 engine has a cross flow head. i think the one cam engine is not. so it is unlikely that the intake will fit very well.

tom w