zenith vs weber

[dede911]

Hi,

I have the intention to backdate my CIS euro SC to carbs.

I know that parts availability of zenith is difficult compared to webers.

What is your experience about build quality and tunability?

For sure the carbs will need a rebuild...

Can a well tuned zenith make the same power as the weber?

Thanks,

André

[1QuickS]

I'm just beginning to add Zeniths to my offered services but one item I know is that the 27.5mm venturis used in the Zeniths will need to be upsized for your application. This is problematic to me especially from all the posts I've seen that say Weber venturis may be used in the Zeniths with a 1mm shim at the bottom to make up for the height difference. This leaves a little info out. Two items to add are of importance to me: 1) the Weber venturi is 46mm and its outside diameter is consistent while the Zenith (Solex) venturi is 50mm at the top and 46mm at the bottom and 2) the location of the bottom of the auxiliarry venturi should correspond to the location of the inner minimum diameter of the main venturi for maximum efficiency of fuel discharge when the main circuit is active - the location of the discharge for the Zenith is 13mm into its corresponding venturi while the Weber distance is 8mm. What this means to me is: 1) the fuel delivery for a Weber venturi in a Zenith body will not be as responsive/efficient as it should be and 2) the bore in the Zenith throttle body is tapered so there is no proper location of the Weber venturi once it is installed, especially since there is no grub screw to secure it like on the Weber.

I expect responses saying that the above is of no value and their Zeniths work great with Weber venturis but as I've just begun learning about Zeniths all I can report is what I see.

The emissions features of the Zeniths need to be disabled to make them better for performance applications. One advantage I can see is that the secondary venturis on the Zeniths are closer in height to those NLA tall secondaries that Webers used on race engines and by everyone wanting better throttle response.

Paul Abbott
Performance Oriented

[Trackrash]

My experience with the Zeniths is they work great on a stock 2.2 T. After I modified my motor up to a 2.5 I had to give up on the Zeniths.
Jets, emulsion tubes and venturis are not available. I used weber venturis shimed around with sheet metal which worked. I used Solex jets. The problem I could not overcome was the flat spot in transition from the idle circuits to the mains. The idle circuits in the Zeniths are designed for the small (27.5 mm) venturis and didn't take to the larger ones.
Maybe others have overcome this problem and will chime in.

[1QuickS]

My guess is the flat spot in transition in Gordon's post above is in part (if not entirely) due to the mis-match of the relative heights between the bottom of the aux venturi and the waist in the main venturi (where the minimum air pressure exists and helps draw the fuel from the aux venturi.)

40mm Webers use 30mm through 38mm venturis successfully without modification of the progression circuit holes in the throttle body although they are certainly not optimized for the different displacement engines they are used on. I would therefore believe that venturi swaps are possible to be performed with correct geometry venturis. I've lately seen another forum post where someone is making custom venturis for Zeniths which would correct many of these issues...probably.

Jets, emulsion tubes and available tuner history are all against tweeking the Zeniths for specific applications.

Shimming the outside diameter of the Weber venturis sounds like a good fix for the tapered bore in the Zeniths but the geometry of the two venturis is still incorrect.

Paul Abbott
Performance Oriented

[dede911]

Paul,

good technical information. This is what I was looking for.

I will dig further in this matter.

André

[James Brown]

Zenith's!I had some vents machined for my 3.0 and re jetted as per Anderson's recommendations and came out very close to perfect. Tweaked them and now they scream (along with the rest of the car) Have not touched them in a year. Very stable, no moving parts, cheap to purchase, rebuild kits available, air cleaners/throttle/manifolds are the same (Dimensions) as the Weber/pmo set-ups. Great looking and made in Germany!
What I'm running now:
Main jets 1.60 solex jets from ebay (alfa 1750)
Idle jets .70 Same ebay guy
Ventures 34mm machined aluminum from pcar9119 on this site or pcar9119@yahoo.com
34mm PMO intake, K&N water shield air cleaners, stock Zenith linkage, pmo fuel regulator.

[1QuickS]

Hi James,

Is your 3.0 cammed different from stock? If so then what is your source for emulsion tubes or did you use the stock Zenith one? Anderson's recommendation for a stock 3.0 with 40mm Webers was a F3 tube. Also, did you adjust the high speed enrichment feature?

I'm working a set of Zeniths now and am learning. By the way, the accelerator pumps have a pretty elaborate parallel actuator rod with spring loaded plain bearing (teflon) cam follower which places a fair bit of radial load on the throttle shaft. At least they have bronze bushings supporting the three throttle shafts which is better than the teflon bushings Weber began using in 1967.

Thanks,

Paul Abbott

[69BahamaYellow]

I have Zenith's in a '71 2.2T and they work great, and are completely stock (including the auxiliary enrichment circuit, which also works). Knowing that ethanol blended gas is all you can get for street use these days, I would not recommend anybody scrapping FI for carbs, as the ethanol will eventually eat the expensive rubber parts in them; that's just my $.02. Assuming you can get viton diaphragms and o-rings for your carbs, then maybe that's a good choice.

[James Brown]

Hey Paul great choice! The emulsion tubes are not available for Zenith's but stock work just fine. Rip that high speed enrichment stuff off and fill the holes with some RTV, done. get the levers close to the accelerator pumps and adjust with the screw by the bearing. this is the real key on these carbs. You can adjust rich/lean on part throttle, WOT, and just off idle conditions. after your close on the jets. Mine are backed off until about 90% open. Cleaned up the AFR alot and only get brown smoke near 7k . My 3.0 is bone stock, just a cat bypass and a Dansk sport muffler. Goes like stink!!!!
True 69yellow, I'm riding this carb thing till it leaks more fuel than I can pump into it, then PMO-ITB's

[pcar9119]

i have one other person who is needing a set of Vents for the zeniths. I was hoping to get a couple more orders before i run them. If anyone else needs a set, let me know. I will be running 36's this time also, It will be my first batch, but i will use the same geometry as the 34's, so they should work great!

[ratpiper71T]

i have one other person who is needing a set of Vents for the zeniths. I was hoping to get a couple more orders before i run them. If anyone else needs a set, let me know. I will be running 36's this time also, It will be my first batch, but i will use the same geometry as the 34's, so they should work great!

If it wouldn't be too rude to ask, what would a set of vents cost? Say 32 or 34 when you begin to do those...

[pcar9119]

I sell the vents for 150.00 shipped in conus. I don't make 32mm, just 34's and 36's. The tooling i'm using is to large to make 32's. Time is real tight for me, thats why i would like to run a few orders at once.

[dede911]

Paul,

Maybe a stupid question, but anyway...

Would it be possible to modify the existing 27 venturis to 34 mm?

What about availability for shafts,discs and bushes (or bearings)?

Any idea about messing with emulsion tubes? (if necessary)

Regards,

André

[neilca]

James,

I was glad to see your post. I have been trying to make Zenith's work on my race car. It is a 2.0l and I am running 32mm chokes. Just recently I put in 160 mains and the engine really loved it. I was hesitant since the original mains are 115. I have bought jets from alfa1750 also, he is a real good guy to deal with. I am going to try the 70 idle jets also.

Andre',

The chokes cannot be bored much past 29mm. The chokes are not solid but a shell. I found it easier to start from scratch. I have a lathe with a taper attachment so It is not too difficult to make with a boring bar. You could also use the compound rest to set the angle.

I haven't touched the emulsion tubes, these are used for partial throttle. We don't normally use partial throttle on the race track, but believe me it happens!

neilca

[1QuickS]

Yes, stock Zenith venturis don't have much wall thickness to modify to larger inside diameters. Best to make from scratch, mindful of where the waist is (13mm down as opposed to 8mm down for Webers).

You might want to change the air correction jet for the Progression circuit. It is 1.6mm for Zeniths and 1.1 for IDAs. The 1.6mm jet will lean the progression circuit richness toward transition.

Paul Abbott
Performance Oriented

[nilknarf]

Hey pcar9119, I need some 34mm venturis- PM on the way shortly.

Curt

[Mukilteo911]

"The problem I could not overcome was the flat spot in transition from the idle circuits to the mains. The idle circuits in the Zeniths are designed for the small (27.5 mm) venturis and didn't take to the larger ones.
Maybe others have overcome this problem and will chime in."

I ran into this too, can anyone confirm that is is the venturi miss match when using the webber parts, I'd like to get back to us gin them and sell my webers. I liked the zeniths on my 2.4, but could not get the flat spot out in the 2k rpm range when i built a high compression 2.5l

[1QuickS]

The Zenith progression circuit was jetted lean and in particular the air correction jet for the progression circuit was humongeous at 160 vs. 110 for the non-smog IDA and 140 for the smogged IDTP which also suffers a lean progression flat spot. I have a modification kit to allow tuning the idle air corrector to help Weber IDTP carbs run better and the kit works on the Zeniths as well. It provides the ability to tune the progression mixture using idle jets as usual and using a tuneable air corrector jet similar to the main circuit and similar to the progression circuit design of IDA and DOCE Webers as well as PMOs.

Paul Abbott
Performance Oriented

[brd]

Have been researching what's need to replace a CIS on an 81 SC engine with carbs... and have also been asking the question of Zenith vs. Webers - but also what is the full package required to do the conversion. So far after reading the different posts here I'm finding:

- Carbs (whether Zeniths of Webers or PMOs)
- Rebuild kits (to start afresh) with appropriate needles, jets, etc for the engine
- (re)Sized venturis to match
- appropriate height stacks?
- insulators?
- Filters everywhere (as per Anderson recommendation)
- Fuel supply pressure reduction (whether via restrictors or lower pressure pumps etc... ideally with pressure guage)
- Recurve dizzy or vacuum delete backdate dizzy ?

Then nice optional additions:
- Backdate heat exchangers and banana

After that, I'm not looking to open the engine and look at high comp pistons and cams etc.
Am I on the right track? have I missed something?
Over here in France, I'm finding Zeniths easier to come by than Webers and with kits available online, it seems a reasonable option. From reading the different bibles and comments, I'll probably try to source Webers first as they appear to be the defacto standard for tuners, with Zeniths then being a viable option rather than first choice.

Thanks in advance for feedback, comments, etc.

[Mukilteo911]

PAUL !! Way cool, where do I buy it? I've got to get the right venturi's too and with your stuff these should rock again on my car.