Zenith Rebuild - My First Carby Project

[porschetub]

I had my main bodies lightly washed in a engine parts hot tank as the alloy may be delicate to this process,they came up really nice,but I had little or no moisture corrosion ,maybe differant otherwise for really old neglected ones.
I didn't cad plate throttle cams/linkages or other parts as you can't see them in a 914/6 ,happy to spend the money elsewhere,it looks nice however.
My carbs in this situation were junk if I couldn't get the remains of the air bleed screw out...I was lucky,had it been brass and not steel it wouldn't have come out.
Research vent size carefully,if you go too big you will move the power curve right out of driving range and have a big "hole " down low.
I ordered 32mm vents and went back to 30mm for my 2.2T ,still a big improvement over stock 27.5mm,reason being I wanted to keep low end torque.
Stoddard Cars in the early days did a lot of R&D with these carbs and reached substantial gains in power by machining out stock vents and upping mains and idles to suit,they fitted the sports 2 pipe outlet exhaust also,they sold is as a sport kit to many happy buyers.
Funny I sort of feel that Porsche chocked back the early 2.2t as the the "E" wasn't that far away powerwise....maybe a marketing exercise ???? who really knows.

Pic is stock vent and 32mm like night and day really,cheers.

[frankc]

Awesome write-up - thanks very much for posting this.

I have started the rebuild of the Zeniths for my 2.4, and would like to remove the floats so that I can soak the top section in carb cleaner. There are two screws for each float retainer. One screw is easily accessible, the other is not so easily accessible due to it being under the float. What have folks done to get to that second screw?

Thanks

[chris_seven]

Originally Posted by porschetub

Stoddard Cars in the early days did a lot of R&D with these carbs and reached substantial gains in power by machining out stock vents and upping mains and idles to suit

The die castings used for the manufacture of standard venturis means that the wall thickness is around 1.5mm so it would be difficult to re-machine to an oversize.

Also the shape of the standard venturi is also very poor and this must also restrict flow which means to achieve good mid-band and WOT performance venturis are often larger than ideally needed which has a negative impact at low throttle opening.

In the UK there has been an company called Superflow manufacturing a range of chokes to suit Weber 40DCOE carbs for the last 20 years and the benefits they provide is well understood.

They have an optimised ‘hour glass’ entry which is parabolic in nature and much better that a typical ‘as cast’ component used in a standard carburettor.

The improved shape effectively improves the flow for a given air speed by reducing the Reynolds Number which increases the pressure drop and hence the ability of the metering system to operate effectively.

In real terms good venturi shape means that the carb will operate more effectively at low engine speeds and provide an improved power band.

The impact of using good quality venturis should not be underestimated.

[rodoredas]

Hi frankc,

in post #24 you can see an elbowed flat piece of metal in one of the pix which I used initially with not much success.

In a recent repair of the floats I just used a newly purchased very long thin screw driver which worked fine for me, but like you was stumped a bit the first time I did this.

cheers
Shane

[frankc]

Originally Posted by rodoredas

in post #24 you can see an elbowed flat piece of metal in one of the pix which I used initially with not much success.

In a recent repair of the floats I just used a newly purchased very long thin screw driver which worked fine for me, but like you was stumped a bit the first time I did this.

Thanks Shane. I also ended up finding a long thin screwdriver (with round rather than rectangular shaft) that allowed me to remove that buried screw.

Now I have another question - regarding the accelerator pumps. Upon disassembly, I noticed two differences in the center pump mechanism vs. the two outers.

First, as you can see in the photo below, the chambers of the two outer accelerator pumps have three bosses as part of the casting whose purpose, I assume, is to retain the diaphragm spring in the center. However, I noticed that the chamber for the center pump does not have these bosses (or whatever the proper term is). I have only disassembled left carb so far, so I haven't yet confirmed if the right carb is the same, but I found this odd and wondered if your carbs were the same (and why the center pump would be different).




Second, the center pump nozzle (#26 Injector Tube) is also different (see photo below). The part of the outer two nozzles that is inserted into the body is longer, and Part #27 (Plug) just falls out when the two nozzles are removed. However, the center nozzle is shorter and has a jet screwed into the bottom. In addition, for the center nozzle, there is a sleeve pressed into the hole of the carb body with part #27 below it, meaning that #27 cannot be removed without somehow removing the sleeve. Was your carb also like this?

I would like to ensure that the passage to the pump chamber is clear for the center pump, but I cannot do so unless I remove part #27. If I fill the hole for the nozzle at the top with brake fluid, it does not leak out into the chamber, so I assume that the passage is blocked.

[rodoredas]

Hi Frank, I checked a heap of other pictures I have and I'd say that center pump has had a bad time. Like you say I think it should have the 3 studs to hold the pump spring in the centre, all mine did.

Seems previous owners have messed up the injection tube #26 and #27 as well, whatever you have does not look right. The plug should fall out when you remove the injection tube.

You certainly want to make sure all the passages are clean and fuel can flow from the chamber up the passage and out the injection tube. I'd be trying to get whatever is in the hole out and getting new injection tubes.

can you drill out the sleeve you can see, just start with a very small drill. I'd be taking the carb body to a local Porsche mechanic and getting second opinion before doing anything.

cheers

[chris_seven]

I must say that I have never seen a Zenith with 3 x bosses in the central pump location.

We have restored around 15 pairs of cabs and they have all been identical to those shown in the above photograph.

I have also never seen an accelerator pump jet f the type shown they have always been as the two outer jets and have had the small brass 'stop' valve fitted.

The pump jet also has a one-way valve fitted and it is important that this operates correctly.

Most of the repair kits available have O rings for this jet that are slightly the wrong size and don't seal very well.

[1QuickS]

Regarding post #44. If you measure them you will also find the height of the three ears are different for the outer accelerator pump features. I can only imagine the tabs are used to limit the amount of travel of the diaphragm.

When I service Zeniths I counterbore a spot in the bottoms of all three wells of the accelerator pump chamber to locate the return spring; I do not like the lack of the centering(?) tabs in the middle chamber so I provide one in all three. I also remove the tabs to allow full adjustability of fuel squirt.

[Jose_JGC]

Wow!
Shane, thanks for sharing.Excellent work and documentation, I love the paper comments in the pics. I will definitely keep the link to this thread.
cheers
Jose

[frankc]

Well, this is getting interesting. Regarding the tabs/ears/bosses - since Shane indicated that his carbs had them on all three chambers, but everyone else seems to say that they are not present in the center chamber (like mine), could there be two different castings? Perhaps this could be a difference between Zeniths fitted with the enrichment circuit vs. those without?

And regarding the shorter center pump jet, since everyone is saying all three should be the same, I have a theory on what may have happened. Upon closer inspection I can see that there are gouge marks from pliers on the top hex head part of the jet. My theory is that the jet was stuck in the carb body, and somebody forced it out and snapped it at the O-Ring groove - leaving a portion of the jet in the body. So then they drilled out the piece stuck in the body (and destroyed the glass ball), but I can still see the walls of a thin brass tube remaining - just enough to prevent #27 from coming out.

Then, they jammed a jet from someplace else onto the broken shaft of the #26 jet to hold the O-Ring in place. I can see that this "extension" is not on straight - so i presume it is not an OEM configuration. I will know for sure once I disassemble the right carb and compare. But, regardless, I don't see how the center accelerator pump could possibly work well without the glass ball check valve in the jet, but fortunately Alfa sells replacement pump jets.

The pump jet on the left in the photo below is my (hacked) center jet (McGyver repair ?):

[chris_seven]

These pump jets do break and need replacement but I have never seen a 'bodge' of this type.

The small 'stop valve' that is shown on the other two chokes may still be in place if there is the remains of the old pump jet still in the gallery.

Clearly needs some careful work.

[frankc]

Originally Posted by chris_seven

The small 'stop valve' that is shown on the other two chokes may still be in place if there is the remains of the old pump jet still in the gallery.

Yes, I can see the slotted top of the 'stop valve' down in the gallery, but the remains of the broken jet prevent it from being removed. I will be sending this carb to a pro to have the remaining piece of the jet drilled out of the gallery as this is not something I want to attempt with hand tools.

[1QuickS]

Zenith bodies differ in progression hole size/locations but otherwise they are the same.

[rodoredas]

Hey Chris I had another close look at my old pictures and zoomed them up and sure enough I've mislead everyone following the thread regarding the 3 bosses in the centre pump, apologies of that. Mine don't have them either. This is the clearest photo i have, clearly the centre pump does not have any. No idea why and didnt really notice that, good pickup Frank. The more people looking at these the better.



Frank, It really seems your set have seen some bad times and rough work, but lets hope you can get it all sorted.

Great tips Paul, thanks, on centering the spring and machining out the tabs, sounds logical.

cheers

[frankc]

Shane,
Thanks for confirming that the center pump chamber on your carbs is different than the outer chambers, just as everyone else's - that's one mystery solved. But I'm still curious why. I mean, from what I can tell, all three pumps are performing identical tasks, using identical components - so why is the center pump chamber different?

Given that it is a German design, I can't help but think that it is different for a reason .

[1QuickS]

In my work with Weber 40IDA3C, Solex 40PI and Zenith 40TIN I have resolved most design/operation curiosities but this one still eludes me, especially the different height of the vanes in the outer two pump chambers. If the vanes were to be a travel stop to control maximum squirt amount then I would expect them to be equal height. The internal spring is not well located by these vanes so centering the spring does not appear to be the answer, especially since there is no centering feature for the middle spring in its well.

[rodoredas]

Hi Folks

I've had these Zenith's running in the Targa for some time now and I'm now starting to play with different jet sizes to understand and feel the difference they make if performance and acceleration.

My setup was stock 930/16 3.0L engine, 34mm venturies, 165 Mains, 55 Idle, 175 air correction, ignition timing is good, mixture and air bypass all adjusted. Using Synchrometer cylinders were all pulling 5Kg/Hr at 950-1000 RPM, and to me the throttle response felt great, there was no stumbling between 2000-3000 rpm but to me it seemed to be running rich. So I decided to have a play with jets and went to
Mains 140, Idle 50, Air Correction 180 and she was stumbling in the 2000-3000 rpm range after re-tuning to 5kg/hr at 950rpm and lean best mixture, Accelerator jets are 0.5mm (not 0.3mm)

I've now gone Idle 55 (mains 140, air correction 180) and that seems to have alleviated most stumbling but there is still evidence of some.

In doing all this and speaking to guys here some are recommending smaller venturis. The stock ones I have are 28.5, I have 34mm installed. I thought i might install the 28.5 just to see and feel what happens.

I wonder what latest theory is from the Zenith Pelicans on venturi size for the engine config i have. It seemed to me when i did research during this thread that 34 was the way to go. In terms of venturi area the 28.5 provide 637m2 while the 34's give 907m2 - it's a big jump so maybe there is some consideration for slightly smaller venturi - but what size? Targa is mostly street, but track it 3-4 times a year so prefer leaning towards track/street setup rather than street/track if you know what i mean.

what's the latest thinking on this?

carbs and Targa out in the high country.

cheers

[rosie82sc]

Hi Shane,

Just read the entire thread, found it interesting and helpful as I am in the process of rebuilding a 2.7 with zeniths. I will be running bigger cams and pistons so I'm sure there will be plenty of time for test and tune after it's up and running.

Greg

[rodoredas]

Hi Greg

good on ya!. I'm sure you will have a great time rebuilding, tuning and trying different setups. Been running mine for 12 months or so I guess and tinker with different setup all the time and it's great fun.

Currently working through different jet sizes and running taller velocity stacks
cheers

[rosie82sc]

Hi Shane,

I am on a road trip in my SC until next Sunday then plan on placing an order with alfa1750 but would like to have a chat before I do, will PM you with contact details. We will be staying in Melbourne Tuesday night.

Regards
Greg