I've just completed my first Zenith 40TIN service after specializing in Webers for the last few years and have noted differences in design between the two which I think would be interesting to those using or considering use of these carbs on their 911 engines. Since there seems to be some need for Zenith service I have decided to extend my existing Weber remanufacturing/restoration services business to include these carbs. I am also planning to add Solex 40PIs as well in the near future.
In no particular order:
The main venturis are not interchangeable with Webers in three ways:
The Zenith venturis are 45.5mm tall and Webers are 44.5 mm tall;
The Zenith venturis have a 46mm bottom diameter and a 50mm top diameter where Webers are a constant 46mm;
The waist of the venturi is 13mm down from the top of the Zeniths while the Weber waist is 8mm down, this is important as the height of the waist must be matched to the bottom of the auxiliary venturi to provide correct activation of the main circuit.
The auxiliary venturis in the Zenith are 16mm OD x 10mm at the waist (ID) x 83mm tall where the Weber aux venturis are 14mm OD x 10.5mm at the waist x 51mm tall (OEM) and 107mm tall (906 style). The taller auxiliaries may be of benefit for larger displacement engines to help smooth the transition from progression circuit operation to the main circuit.
The Zenith auxiliary venturis mount in the main throttle bodies with a full circle ring that fits the ID at the top of the throttle body where the Webers drop into slots. The Zenith installation is a more secure mounting design.
The accelerator squirter nozzles have .3mm diameter holes and the Webers are .5mm.
The accelerator squirter nozzles on the Zeniths are made from brass tubing and may be "aimed" into the annulus between the main venturi waist and the OD of the auxiliary venturi whereas the Weber nozzles are selected for squirter nozzle length and squirt angle and are not individually adjustable for "aim".
The dry weights of the carbs with intake air horns are: Zenith = 7lbs 6oz and Weber = 6lbs 6oz; both without inlet fuel fittings. The zeniths were weighed with "High Speed Enrichment" components in place.
The progression circuit air corrector jets are 1.65mm for the Zeniths and 1.10mm for IDA, IDS and IDAP Webers, 1.40mm for IDT and IDTP Webers and 0.80 for 46mm IDA Webers. Note that these adjust the amount of air emulsified with the fuel during progression circuit operation and that the larger the air corrector size the leaner the progression is. Note also that the IDT and IDTP are Weber's emission compliant carbs and their air correctors are larger than the pre emission IDA, etc. air correctors.
The check valves for the Zeniths use glass (or similar) balls where the Webers use stainless steel balls.
The floats in the Zeniths are plastic and are top hinged from the top covers of the carbs where Weber uses brass floats which are pivoted fron the side of the main throttle body. The plastic floats seem more durable than the Webers as the Webers may bend the tabs where the float needle valves rest and upset fuel levels accordingly.
The Zeniths use three individual throttle shafts per throttle body where Weber uses two; one long shaft and one short shaft.
The Zeniths use a shaft to drive three individual accelerator pumps which is mounted parallel to the throttle shaft axis. Since three pumps are used per throttle body one is pump dedicated to a particular cylinder and therefore individually adjustable. The Weber uses one pump per throttle body driven from the long throttle shaft via levers and a connecting rod (typically adjustable). The Zenith accelerator pump shaft is driven from a cam which in integral with the throttle arm of the throttle body and includes a spring-loaded follower cam which is adjustable to prevent binding of the throttle shaft if the adjustments at the individual accelerator pumps is performed incorrectly.
Jets are manufactured by Solex for the Zeniths.
Although availability of jets exist for the Zeniths, emulsion tubes beyond OEM are not, thereby limiting tuning options.
Aftermarket venturis are available from MotorMeister and pcar9119@yahoo.com. Stock venturis are 27.5mm and do not lend to reboring as their wall thickness is insufficient for a meaningful size increase.
The "High Speed Enrichment" system is left in place on this set of Zeniths but has been effectively defeated. The purpose of this system was to provide fuel to the engine when the throttles were closed during deceleration to help reduce emissions.
The Zenith operation manual describes how at high RPMs the accelerator pump nozzles have fuel drawn from them to help enrich high speed fuel delivery. This is typical of the Webers as well.
The Zeniths have four transition ports where the Webers use three. No further speculation regarding this topic at this time except to note the IDA, IDS and IDAP carbs used a different hole size/spacing than the IDT/IDTP variations. Photos of the Weber transition ports are viewable on my web site on the Technical page.
That's enough to think about. I attach a picture of the Zeniths I used to collect this info to help identify items discussed:
Paul Abbott
www.PerformanceOriented.com
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