Setting up choke disc on Zenith carburettor

[Anrw]

Greetings.

I have stripped my carburettor to try and solve rich running problems and I suspect that the brass choke disc may have been incorrectly set up. By this I mean the orientation of the disc relating to the external choke operating crank. The shaft controlling the choke disc has a four faced end that the crank slides on to giving one right and three wrong ways to assemble it. I have searched the forum including John's (heinkeijb) post of 02.06.12 where he states that he used judgement to put it together but I can't find any definitive guidance and I'm little bit unclear about how it functions in order to use logic to work it out. My brass disc has two slots in it - one large and one half its size, plus a round hole (like the one shown in the workshop manual P: 2.28 and 2.30).

Can anyone direct me to any resource that specifically deals with this issue or indeed can offer any guidance?

Many thanks and regards

Nigel

[heinkeljb]

Evening Nigel,

If I remember correctly, I used the resources of the Porchce 356 clubs technical pages on setting up the Zenith cards as they are basically the same carb just with different jets fitted. I am pretty sure I put my carb together as per this exploded view diagram. https://porsche356registry.org/content. ... _id=479163

I don't think I have had any carb choke related issue other than when you get some bit of rubbish getting stuck in the jets and then the engine runs better with the choke cable pulled out!

There are two documents on their web site which were useful, one was about rebuilding the carburetor, the other about tuning it. Their website has undergone some changes since I used it and you now have to create an account to be able to access things. The page above still is accessable with just a click in the link.

If you think about how the choke is supposed to work, i.e. when it is on, it reduces the amount of air going through it so, the fuel/air mixture has more petrol in it. That means it must be using the smallest hole, because when you pull the choke cable it moves the lever towards the right hand side of the vehicle (when viewed from the rear of the vehicle), to cut off the air that passes through the choke section of the carburetor. Therefore, when the choke cable is pushed in, it will move the lever to the left hand side and open the biggest air hole to align with the carburetor castings hole.

So, now the cavete, This is how I understand the carb / choke system to work. I maybe wrong but I think the worst that will happen is that the engine will either not start on a cold day even with the choke on, or it will run badly with too rich a mixture and tend to bog down if you put your foot down quickly.

[ogdenenterprise]

Hi Nigel,

I have a book of all the carburettors fitted to Steyr puch vehicles, which is printed in German.
I am part way through translating it to English, some of it is difficult to translate using google translate
This is one of the pages I have managed to do. I hope it helps

Dave

[Anrw]

Hi John and Dave

As always thank you for your speedy and helpful responses to my post.

For some reason i completely failed to gain access to Ron LaDow's articles on the NDIX on the Porsche 356 Club and forum sites. I kept getting blocked by a request for a $75 membership subscription! However I finished going through the carburetter and found nothing untoward with it. I changed the mixture control screws as a precaution and was going to change the original accelerator pump piston (leather seal type) with a new (rubber seal type) one but felt that the original one seemed better, I checked the float chamber fill valve and gave everything a thorough clean and reassembled with new gaskets. I followed your logical explanation of the choke disc, John, and installed the carburettor. As soon as the the fuel reached the carburettor it fired up with no choke (ambient circa 10 degrees). It ran like a 'bag of old spanners' and is clearly still overfuelling. I guess my set up of the choke disc is still wrong!

Dave's translation of the German carbutrettor book may be the key to it so I'll take off the choke assembly and check it against the illustration that Dave kindly posted. By the way Dave your translation work is greatly appreciated and I will happily buy the fully translated NDIX section of the book when you have completed it,

I will report back when I've had another go.

Thanks again for your help.

Nigel

[heinkeljb]

Have you checked the jet sizes?

I found the page on the 365 clubs site in the Free section listed on this page:-https://porsche356registry.org/content. ... _id=481598
and then clicking on this link on the page:- https://porsche356registry.org/content. ... _id=479163

I am not sure what happened to the link I previously posted, but I tried it before making this post - It didn't work so I have updated it.

John

[Anrw]

John

Many thanks I have now found and downloaded the Ron LaDow 'Adjusting NDIX Carburettors' article. Very useful thanks.

I haven't checked the jet sizes - just individually blew them through with an airline.

I have now had the choke disc assembly back off the car and found I had installed the disc 90 degrees out. It was reassembled and it started instantly (without any choke on a cold engine). It is much less rich than previously so I need to take it on a good roadtest to fully assess whether I've fixed the problem. I would be interested to know whether people generally actually need the choke on days with an ambient in the 8 to 12 degree range? My engine currently starts fom cold without it in these temperaturs.

I recommend the illustration (see attachment above in Dave's post) as being the definitive way to set up the choke disc / operating lever.

Thanks again

[heinkeljb]

Lurch needs the choke when it's cold, but I can't say what the temperature is at that point as it's not something I have ever thought about. I just use the choke when I think it is cold enough to need it or the engine doesn't start with out it.

On a long road drive I get near if not actually the 30 MPG the book says so I don't think Lurch is running too rich, but maybe I should take it to a Dyno place and get it put through a Dyno test to see how much power it actually makes!

John

[ogdenenterprise]

I have finished translating the carb book with the help of my brother
I said I owe him a drink, his reply was I think you owe me a barrel.
when I scanned the pages some of the spiral edges show on the copies.
Im not very bright when it comes to computers so I have to find a
programme to remove the edge markings so I can print everything.

Dave

[Anrw]

Dave
I'm very happy to have a copy of your tranlated pages - with or without spiral edges. If you can't find a suitable programme that suits how about just scanning the pages and cropping the scans to remove the unwanted spirals, before saving them?

Regards

[Anrw]

I have been trying to get my Haf to run less rich. The problem is not fuel consumption as I haven't done enough road miles to have a handle on how economical it is, but the problem is that it chokes itself up and heavily fouls the plugs making starting difficult. Years of experience of other engines leads me to just replace plugs when they are suffering but that doesn't treat the cause. Early on in this thread I talked of the orientation of the choke disc - this is now solved due to Dave's carburettor book photograph. I have been through the carburettor in detail and replaced every seal, washer, accelerator pump, mixture needles etc and the engine starts instantly hot or cold and quickly runs smoothly. I have also replaced the little sealing ring on top of the pump pressure valve which sits beneath the accelerator pump - the old one was missing! A small amount of choke is required on first starting when the ambient is low but can be quickly put back in.

My sense is that it is running much better and less rich but two issues remain. It is not happy to idle at 750 rpm - it needs to be at about 950 to sustain itself. It runs less rich (according to the smell and visual smoke test) and pick up is good but at tick over it spits out odd 'coughs' of black spots on the workshop floor - it looks like condensation which of course it could be but with such a short exhaust system it doesn't seem so likely. These black spots are not oil by the way - they are either fuel or water. I haven't got the engine really hot as I haven't taken it on a good road test since rebuilding the carburettor but I would be interested to know whether this is a bit of a characteristic of this engine and whether it really should tick over at 750 rpm like the owners manual suggests?

Regards

[heinkeljb]

Have you checked, changed the main & idle jets? They can wear just throught the amount and speed of fuel going through them.

The problem with a lot of haflingers is that they do not charge at 750rpm so most will have higher idle settings to the point where the charge light just goes out.
It take quite a lot of time and effort with adjusting the two idle screws once things are hot, but then that changes when the engine is cold so, you have to decide on the compromise idle speed that suits your ear!

John

[AustHaflinger]

This carb has plastic floats that are made in two halves that are glued together - they can become partially separated allowing them to sink and change the mixture because unregulated fuel passes into the engine. I had this happen to me after a professional carb build where the carb cleaning fluid dissolved the glue on the floats. The engine still ran but was very rich - so check this along with the float levels if they are OK.

As far as idle is concerned - I have never been able to get mine to idle reliably at 750rpm - sometimes it will, sometimes it will not - it is better to go for about 950rpm. Also the idle mixture screws are prone to be damaged if screwed in too tight - so replace these - they are cheap.\

garry