Zenith carburettor

[Max911]

Good afternoon Gentlemen,

A kind help in understanding.

On my 911 2,2 T my 1970(ex USA model) there is on right carb the "solenoid valve" part 911.605.101.00 connected to his switch on left carb, operated by accelerator linkage.

What is the correct function?I read on a manual "enricher for overrun"...

My car is perfectly set in terms of carbs and idling apart for some popping in overrun on higer revs (above 4000) with the switch above disconnected, but not affecting the drive at all.

When the switch is connected and operating the car return to idle slowerly or not completely, start to run higher idle when cold but no other differences...

Is it something that could stay NOT operating?

Consider also that my zenith have no valve 911.100.987.00 . A carb specialist told me that was for lowering emissions but not fundamental to the correct carb setting.

Thanks as usual
Max


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[911hillclimber]

You need a response from Gary71 on here, Zenith expert in a real world.
He is bound to be along soon.

[Gary71]

I’m no expert!

My technique with the enrichment circuit on my car was to remove it and plug all the ports.

Just let the carbs work as carbs rather than some hopeless attempt at emissions control.

Details on the site in the link in my signature

[bjmullan]

I'm with Gary, had my Zenith rebuild by Fenn Lane Motorsports last year and all of the US emissions stuff blocked off.

[Max911]

Thank you so much Gentlemen!
I agree with you!
Let the carbs breathe as natural as possible!
Now I stop the car and make inoperative the switch immediately


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[Max911]

Dear all

Trying not to duplicate threads and topics

Setting zenith carbs

I read porsche manual, Haynes manual, youtube dedicated topics.

I reached a good set up but not really satisfying.

I followed Haynes and Dempsey procedures but what in my opinion that is not clear enough is how to set carbs according to real reactions

I try to explain: I understood how to go richer or weaker with screws and balance air with a syncronizer....

But: if car is spitting from stack is too rich or too weak? My car reacts not in easy to understand way:
shell I enrich with idle mixture screw or give more air with air venturi by pass?


I'm not a principiant with carbs setting and syncronizing but on my 911 is the first time I'm setting from zero after last setting done from dealer porsche that in months has been going bad with lowering idle and 4/5/6 cylinder become too rich and spark plugs real black

At the moment I get a smooth idle, some stack spitting when accelerating and some engine off after a high revv acceleration

Thank you so much for your kind suggestions
Max


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[fetuhoe]

The vacuum enrichment system on the Zenith Carbs works in conjunction with a speed transducer which allows the solenoid to open and deliver fuel when the engine speed transducer provides the correct signal.

At engine speeds above 1350rpm the speed transducer activates the solenoid which in turn allows a vacuum to operate the auxiliary diaphragm of the mixing valve which then allows additional fuel to be delivered to each cylinder. This circuit only operates in overrun conditions.

At engine speeds below 1350rpm the solenoid should close and the idle circuit should operate in a normal manner.

If you remove the electrical connection from the solenoid then the enrichment circuit should not be able to operate.

There should be a micro switch on the throttle linkage which isolates the vacuum enrichment circuit and prevents it from operating during acceleration.

If you don't have a mixing valve fitted to the carb body it is difficult to see how fuel can be added but if the solenoid is still operating there is the chance you can have an air leak on one cylinder during overrun conditions.

[Max911]

Thank you fetuhoe

In the meanwhile a reached a good set up apart from some but rarer spit and idle dropping.

At the same time in Italy weather changed... From 35 degrees umid to 25-27 dry... I live at sea level but next weekend I will race on mountains nearby 1200/1700 m altitude...

The car at the moment seems a little richer.

I have no auxiliary enrichment system mounted, holes plugged.
I still have solenoid mounted but the switch is not operating.

During last days I read many posts about zenith issues on pelican parts forum and I understood that the right balance is really really delicate...

What I want to try next is to change the small orings on idle mixture screw(changed during carb rebuild...year 2013...although car is regularly on the road only since december 2018...)
I suppose that they can be cooked by hot temperature...do their job anyway until newly molested during a carb set up losing their sealing function and not allowing right setting/blowing air...

I will keep you updated
Thanks
Max


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[fetuhoe]

Setting the idle and balancing Zeniths is no more difficult than a Weber 40IDA3C and in some respects the Zenith can perform very well indeed and can superior to a Weber.

Setting idle is relatively straightforward but it does help to check CO levels,

The car will idle quite well at relatively low CO levels say 3% but could then have a small flat spot at 2-3000 rpm. A CO level at 4.5 -5% at 900rpm will normally produce a smooth transition from the idle to the progression circuit.

I am not sure that the O rings on the idle mixture screws will have a significant impact on idle performance.

Many carbs don't use any seals and the slight leak through the thread is likely to be relatively insignificant and probably quite consistent.

If the O rings deteriorate then leakage may increase but if the carbs have been adjusted to deal with this then it shouldn't be much of an issue.

Any small air leak in this region can be compensated for by using the Air Bypass Screws during synchronisation of the individual barrels.

One of the benefits of the Zenith is that there are 3 accelerator pumps per carb and this can allow accurate adjustment of pump delivery volumes but the small pump jet sized at 30 (0.3mm diameter) is prone to blocking so it is worthwhile to check pump delivery on each cylinder to make sure that there is sufficient volume. If one or more of the pump jets are blocked cleaning them is tricky and they need to be disassembled and blown through.

These is also a high speed enrichment circuit in the Zenith (This is also a feature of a Weber 40IDS but is not present on a 40IDA). This feature is provided using the pump jet for fuel delivery but is activated by using a small brass 'stop' valve in the body of the carb and these small valves can stick and fail to deliver fuel. This means that if either the pump jet is blocked or the stop valve is stuck then no high speed enrichment will occur on that cylinder and at high engine loads individual cylinder can run weak and may tend to overheat.

The other potential issue concerns the cam which operates the accelerator pump. The mechanism that operates this cam has a small pivot which runs on a PTFE sleeve within the assembly and this sleeve is not provided as part of a repair kit and is generally unobtainable.

If the sleeve wears badly the cam alignment may deteriorate and this will delay the operation of the accelerator pump. Even when this sleeve s badly worn the pump will still function but not quite correctly and it can result in a slight hesitation at the start of the acceleration phase.

It is also worth checking the condition of all the angle joints in the throttle/drop links. As these joints wear, they develop a backlash and can make it virtually impossible to balance left and right hand banks.

Zenith carbs are normally provided with a thin insulator between the cylinder head and the intake manifold and these do help to reduce the percolation that can occur when the engine is turned off.

Carb temperatures commonly increase from around 45degC during normal running to around 70degC within 20 minutes of shutdown.

On Webers this commonly causes fuel to evaporate from the reservoir overflow into the cylinder which creates a hot start problem.

The manifold insulators help this problem but we have found that fitting insulators between the carb body and the top of the manifold is much more effective.

[Max911]

Thanks again fetuhoe, your competence is precious and incredible!

All accelerator pump jet have been cleaned with brake cleaner up to get all them correctly squirting, although not checked for fuel quantity per pump.

I will check co level according to your suggestions

Thanks again
To Next updating


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