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Zenith Carburettors

ALC 68A

Donation Time
From the chassis no. my Series 3 GT would have been fitted with the twin Zeniths, but had a Weber 28/36 DCD on a Solex manifold when I bought it. However, it still has the original cast iron single outlet GT exhaust manifold. I found a pair of the correct Zenith 36 WIAs on a S3 manifold, exactly as my car would have had, which have been professionally rebuilt to as-new condition. I plan to fit these over the winter, but am also thinking of fitting a ST type "bunch of bananas" exhaust manifold which I also have, instead of the original. However, the WIAs have different jet sizes from those on the WIPs used with this exhaust manifold on the STs, so I am wondering if the two would be compatible. Can anybody give me any suggestions, please? Many thanks.

Steve
 

twospeed

Donation Time
The WIA has a smaller main jet because it also uses a power valve.The WIP has a bigger jet because of no power valve.
I have used both on my SIII and work nice.
 

twospeed

Donation Time
A little something I did a few years ago.
I modified the main jet to use a NOS insert,now I can change to any size jet I need.
Bottom jet is the modified one and the top is stock.
 

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bernd_st

Bronze Level Sponsor
that´s a great modification! I´m running WIA´s with 30mm Venturis and 142 main jets on a 1600cc. Always considered it runs to rich because almost no choke neccessary anymore. Even though it goes like stink in the upper rev. range. Now I know the error I made not considering the extra valve impact.
Anywaymy above combination didn´t come as standard but believe ideal main jet size should better be somewhere in the region of 130-135. This is because a STD Series III with Gauze air filters & 29mm Venturi was specified with 127 main jet. Certainly need further trial & adjustment...

P.S. Always very interested in Zenith topics...
 

twospeed

Donation Time
that´s a great modification! I´m running WIA´s with 30mm Venturis and 142 main jets on a 1600cc. Always considered it runs to rich because almost no choke neccessary anymore. Even though it goes like stink in the upper rev. range. Now I know the error I made not considering the extra valve impact.
Anywaymy above combination didn´t come as standard but believe ideal main jet size should better be somewhere in the region of 130-135. This is because a STD Series III with Gauze air filters & 29mm Venturi was specified with 127 main jet. Certainly need further trial & adjustment...

P.S. Always very interested in Zenith topics...
All my WIP carbs had a 127 jet
All my wia carbs had a 112 jet
Remember all those old jets have gotten bigger over time from all the fuel going thro them for so many years.
I also modified a Carter Q_jet needle to replace the steel needle for the needle and seat.
Now I almost stopped the carbs from dripping--almost.
Also setting up the WIA power valve was a real fun job. I set mine to open at 10 inches of vacuum, had to cut up a old carb to make a vacuum jig and just kept changing springs. They run real nice now.
 

bernd_st

Bronze Level Sponsor
Key to define the airflow is the venturi size & the airfilter setup. If your WIP(3)`s came with 127 main jet they are definitely from a STD SIII with gauze filters and 29mm venturi.
WIA´s -at least acc. to WSM - only came originally with the SIII GT but then with dry element airfilter & 105 main jet . As far as WSM specifies 112 main jets were only reserved for SII WIP´s . Strange you are having WIA´s with 112´s .
However found many strange things over time on the Zenith´s like different main jet sizes front and rear carb. This might be due to your below bespoken fact,i.e. either widening by age or just a mechanics mistake...
 

ALC 68A

Donation Time
The air filter issue was something that had also occurred to me. As you say, according to the Parts List, the S3 GT had the paper element filter in a large cylindrical canister (basically a Rapier part I think), but I have not seen one of these on a surviving UK car. Therefore I have never been sure whether they were actually fitted, although they could all have just been removed over the years in the search for better performance. They were also specified on Sports for some (presumably dusty) export markets, so I have assumed that they were effectively interchangeable with the metal mesh filters, which are what I intend to use on my WIAs when I fit them to the car.

Regards

Steve
 

RootesRacer

Donation Time
The jetting on the filtered carbs effects more than just the main jets, the high speed bleed orifice is also different and goes with the filtered special jetting.
 

bernd_st

Bronze Level Sponsor
true - even though I have not experimented with high speed orifices yet.
By the way the pump jet is also different which should mainly affect accelleration. Many parameters to be considered to get a perfectly running setup.

To Steve:

Would see no specific reason not to use the wire mesh airfilters. They do work pretty well despite a higher noise level and if Power is what you are looking for they represent the best option...
 
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