Zenith Rebuild - My First Carby Project

[rodoredas]

So, here I am, 2 Zenith 40 Tin carbies purchased a few years ago and I want to strip and rebuild - I know nothing about these what am i doing!! But have the workshop manuals, other documentation and of course all the useful information on this site contributed by many Zenith fans over the years. I'm half way through an engine, completed my Sporto rebuilds a while ago, so this can't be that hard.

I had a couple of years earlier purchased a 3.0L 930/16 from another Pelican which included SSI’s, Dansk muffler and while I could have installed PMO’s I felt I had more to learn by rebuilding the Zeniths and using them on the engine.

I purchased these locally and they appeared in reasonable condition. I would install new Jets, new Venturi, remove the Auxiliary Unit, install new gaskets, O-rings and the like.

I started with these


and ended the project with these, all new jets, gaskets, o-rings, a new float, new 34mm venturi, air filters and other new bits



and this is the story of how I got there!!

Removing the top and the auxiliary unit which I would not replace. Removing the top cover and intake trumpets and lid (shown below) exposes the float chambers, all pretty dirty in here, and a tired worn out gasket. This was the first time I had taken to any Carburettor so I was careful in the disassembly and numbered and labelled each part as it was removed. I’d read that the aux unit is not really needed and intended not to install this once the rebuild was complete. The bolt holes and fuel ports will need to be plugged later



The accelerator jets (injection pipe) are held in place by an o-ring and these too would be replaced. Once the jet is removed there is a small Pin, to control fuel flow I guess, which at first I could not see. That was until I tipped the carby upside down and one fell out!!



The retaining clips holding the booster in came out with a little help from a small screw driver and some gentle pulling of the booster. These clips also would be replaced, one broke during this process



With the boosters and venturi removed, i then took the threaded plugs, Idle mixture control screw, Idle speed adjusting screen and noted the auxiliary fuel inlet that would later be plugged.



...more later.

cheers

[lrodri64]

Subscribed !!!
Thanks.

[rodoredas]

Main jets removed, as I said I'm tagging stuff as I remove it and numbering the parts according to the diagram in the workshop manual. Remember this is all new to me and at this stage of the project i'm unsure of what I will re-use or replace so I am trying to keep everything in order and documented, might be a bit over the top but it's all a learning curve.



Idle mixture and Idle speed stews, looks like some of this has never been removed before, pretty dirty everywhere



Idle Jets


Auxiliary mixture screw, with that unit removed this becomes redundant but is installed in the final build.



Thread bolts for the float level gauge at the base of each float bowl. Have purchased float level gauge to work with that later



Vacuum port gets plugged later



cheers

[rodoredas]

Accerator pumps - there are 3 of these, one for each throttle/venturi combination and they consist of a plunger, diaphragm and spring that is activated by small fingers on a rod that runs the length of the carby. The rod is operated by wheel/cam on the throttle lever. As I understand it and can see by way of experimentation these are activated when the throttle is open suddenly and provide extra fuel via the injection pipe. The chambers behind the diaphragm are filled with fuel fed from the float bowl and channels seen in the body.

Rod and Cam



Pump Cover, diaphragm, these are worn out and will be replaced along with some new springs


Channels in the body are visible and we can see how fuel from the pump is injected into the throat via the injection pipe, and it's dirty in here. Obviously some water in parts of this at some time.

[rodoredas]

Float Bowls - obviously a reservoir for the fuel with hollow screws in the base that provide fuel to the accelerator pump and other jets.
In one bowl there is a auxiliary valve, a tiny screw at the base of one chamber



and larger valves supplying fuel to the other channels in the body


Emulsion Tubes, seen above in position supply air to the fuel being routed to main venturi outlet at the base of the booster/pre atomiser. Each emulsion tube has an air correction jet at the top which controls the air ratio.




Here's all the bits and pieces so far.


Float removal next, and we start to break things!!

cheers

[mcaterino]

Excellent thread, Shane! This is on my to-do list. Mine are working fine, I guess, but could no doubt benefit from some cleaning and little upgrades. Need to get back in there and install the thermal insulators too..

I'll look forward to the rest of your project.

Mike

[Jfporco]

Excellent documentation!
Thank you for the share.
(I'll be rebuilding a set of Zeniths soon)

[rodoredas]

Float removal - well these are a bit tricky, I started with the float level needle valves and 2 threaded plugs that sit on top of these. As usual one threaded plug and needle valve came out easy, the second was, well shall i say a disaster. It would not budge no matter what I tried. Soaked in WD40 and CRC first I burred the edges, these are soft brass bolts! Then I tried bolt removers, and you see the results!!




Oh dear now what, some gentle tapping with a socket extender on the remaining ridge and it finally came out. A new bolt needed obviously but no damage done to the body, except in the process I broke a float bowl! Maybe would have been a good idea to remove them first, next time. New bowl and bolt added to parts list



The threaded plug at the base near the idle mixture screws are difficult to remove as well, but with patience and square drive screw driver and a lever I managed to get them out - 1 fatality, burred bolt head slot! more additions to the parts list!



Removing the float bowls is also rather tricky without special tools, I found that I could not get proper leverage on the screws that hold these in place so manufactured an L shaped screw driver out of flat metal, that did the trick.

Main fuel lines removed as well. I note on the parts diagram there are small brass filters that surround the brass bolt inside the sphere of the fuel line, these are missing and subsequent enquires recommended these are not required.



I've tried unsuccessfully to remove the actual throttle discs from the throttle rod running through the 3 carbies, the screws holding them in place are dirty so I will clean first and see how this go after that now that I this one all in bits.

I am limited on space so will clean this one up, and re-assemble before tackling the other triple throat.

next cleaning.....

[rodoredas]

Cleaning - what a saga, something that seems so simple, well at least on Youtube and with chemicals I just can't seem to find here. I could have taken to specialist but wanted the experience myself, so i persevered. Asked the guys at the local vintage car shop for ideas, browsed the net etc.

I started with the manifolds, these were coated in black paint, well not real paint anyway! we suspect now that the job is done they had a coating of POR-15. I soaked the manifolds in a chemical bath at the mechanics for 2 days then attached them with a sandblaster (glass beads) and after 20 minutes hardly moved an inch or two from the surface. This was going to be hard. So more soaking another day or 2, more glass bead blasting, soaked in paint remover. I have my own sand blaster and gave that a go after further soaking, eventually many hours later and slight pitting in one area of the manifold enough of this stuff was removed to allow a new coat of paint. Silver this time.

I soaked the carby bodies in petrol, brushed, wiped with fine steel wool and eventually soda blasted them, washed with carby cleaner. The results while not professional are pleasing and the internals look much better. These shots taken part way through soda blasting show improvement, more to come.



After soda blasting I soaked the body in water, this dissolves the soda in the small fuel channels, but a detailed inspection revealed this was not completely so. I used fine plastic needle from a cleaning brush and the air compressor to ensure all the channels were clean. In a room with a single bright light I found its possible to visually confirm that the channels are clear.

All other bits got a clean but many will be replaced with new pieces. I'm not that happy with the external finish, that nice gold colored zinc finish would have been nice but couldn't find anyone local that was prepared to do it. I'd happily take them apart another day and get that done.

The cleaning took forever, and while that was going on I was ordering all the new bits and pieces i wanted.

New 34mm Venturi, manifold insulation gaskets, 0.5mm injection pipes (not standard 0.3mm) sourced from the UK

I had purchased the rebuild kits sold here, but frankly found them not that useful. The gaskets between the manifold and base simply did not fit and other components, o-rings etc just did not work.

Most if not all other parts came from alfa1750 (ebay), I found him very good and reliable and the stuff I ordered turned up. I would recommend him if you are looking for bits.
There is lots of discussion on jet size so I decided to get various sizes and install the most recommended combination I could find in the many threads on pelican by experts in this field.

parts started to arrive........

Idle Jet
6 x 55,6 x 57
Main Jet
6 x 145, 6 x 162,6 x 165
Air Correction Jet
6 x 170,6 x 175

6 x Idle Mixture screws and springs
6 x Main Jet Holder
6 x air mixture control screws
2 x Needle Valve plug
2 x threaded plug (directly above idle mixture)

A new/old float was sourced from Tom Butler at Rennshop.
The new K&N Air Filter set from Henry Schmidt at Supertec Performance

Stuff started to arrive, and the cleaning was done, time to put this one back together.



but some careful thread tapping needed to be done first to plug various redundant holes!

cheers

[doswald]

Shane, thank you for your detailed and documented experience. I will be undertaking this project in the next few months, and this thread will be a great help. Can you share your source for the 34 mm Venturi, and also contact info for alfa1750 (I checked the eBay store, but couldn't find contact info)?

Thanks.
Doug

[bh912e]

In the fuel bowls there are screws with a glass one way valve. I forget the name, are yours missing the glass ball?

[irl]

Sorry if I missed it but how long did this process take, Great write up!

[kinein]

the link for alfa1750 WEBER PARTS, SOLEX PARTS items in ALFA1750'S CARBURETORS PARTS STORE store on eBay!

[unclebilly]

Those venturis look nice!

[rodoredas]

Thanks Doug, sent details via PM

The scews in the float bowl with the glass valve are #25 on this diagram, mine had the glass ball. #28 is the single enrichment circuit valve.



Thanks UncleBilly, apologies once again for the screw up when you had some for sale.

Took about 6 months or so to get through this, lots of waiting for parts

cheers

[924CarreraGTP]

Thanks for this. It will surely help me tackle my Zeniths pretty soon. I'm also about to order a synchrometer to tune these carbs. The photos are great, and I dig the new filters.

[rodoredas]

Tapped the auxiliary fuel line with a 5mm thread from memory and sealed with grub screw.



Also tapped the auxiliary enrichment port with 8mm and used grub screw on that plus the 2 smaller auxiliary holes



Here's my assembly check list


and all the new pieces ready to go.


Before, during and after of the manifolds, lot of messing around with these


cheers

[bh912e]

Looking great!

[nilknarf]

What a fantastic write-up Shane! You have me wishing I had waited to rebuild my Zeniths until after you had started this great thread!

Curt

[rodoredas]

The new Venturi fit perfectly and are well made, I re-used the emulsion tubes but installed new Air Correction Jets. There is heaps of great technical advice to be found on pelican and after much reading decided to go with 57 Idle, 165 Mains and 175 Air Correction. Interestingly the Air Correction jets removed were 210, with other variants used in the original design, 185 and 195 depending on year from what I can work out.



The Pump Jets went in next with new O-rings, but you have to put the gasket on first!!! So they were removed and installed again later. I used a small dab or rubber grease on the o-ring for easy installation, and you need to make sure the gasket is orientated correctly.



I had installed the PreAtomisers with new clips, the sequence of pix here is not quite right because these should really go in before placing the gasket, which I obviously did, but the picture taking is a bit off, but you get the idea! I broke one of the original clips while doing this, so what followed was another long wait for new ones to arrive.
I found that you have to install the clip and atomiser together holding the clip in place while gently working the atomiser into place.



Back onto the motor while I wait for the clips! You think it took ages to do the carbies, my poor motor is taking ages, waiting on new wiring loom - then and now still.