Zenith carb question

S2a, 2.25 petrol, Zenith carb, 70k miles.

Running well, but running rich. Started after 5 secs with or without
choke, small amount of black smoke while running, doing about 15mpg.
All the indications are that it's running rich. Adjusted volume
control screw as per manual to no effect, so I decided the carb needed
an overhaul. Got an overhaul kit and did the work over the weekend.

Now it starts first turn and idles sweetly - even first thing with no
choke. Misfires a bit when it's revved, loads of black smoke.
Obviously running very rich. Adjusting the volume control screw makes
no difference whatsoever to the idle speed, from fully closed to right
out (at which point it stalls, not surprisingly. VCS is extremely
stiff and almost impossible to turn (as was the old one).
Substituting the old one makes no difference.

All I did to the carb was to replace the jets and economy device
diaphragm and set the float level to the correct setting (33mm from
the gasket face - it was 28mm which is 5mm out - but this would weaken
the mixture, wouldn't it?). Obviously I've done summat wrong, but
what?

Anyone any ideas? Or should I junk it and get a Weber?

TIA

Rich

Series 2a <cough cough>
The rest

On or around 25 Oct 2004 01:00:45 -0700, [email protected] (Richard
Brookman) enlightened us thusly:

>S2a, 2.25 petrol, Zenith carb, 70k miles.
>
>Running well, but running rich. Started after 5 secs with or without
>choke, small amount of black smoke while running, doing about 15mpg.
>All the indications are that it's running rich. Adjusted volume
>control screw as per manual to no effect, so I decided the carb needed
>an overhaul. Got an overhaul kit and did the work over the weekend.
>
>Now it starts first turn and idles sweetly - even first thing with no
>choke. Misfires a bit when it's revved, loads of black smoke.
>Obviously running very rich. Adjusting the volume control screw makes
>no difference whatsoever to the idle speed, from fully closed to right
>out (at which point it stalls, not surprisingly. VCS is extremely
>stiff and almost impossible to turn (as was the old one).
>Substituting the old one makes no difference.
>
>All I did to the carb was to replace the jets and economy device
>diaphragm and set the float level to the correct setting (33mm from
>the gasket face - it was 28mm which is 5mm out - but this would weaken
>the mixture, wouldn't it?). Obviously I've done summat wrong, but
>what?
>
>Anyone any ideas? Or should I junk it and get a Weber?
>

sounds like you have a blockage somewhere, or a leak letting petrol in where
it shouldn't. The mixture screw should have some effect. I assume you had
the correct main jet to put in?

IIRC, on the zenith, the adjustment is for idle mixture only, not for normal
running, which is defined by the size of the main jet and auxiliary jet if
it has one. 's ages since I played with a zenith, though.

On Monday, in article
<[email protected]>
[email protected] "Austin Shackles" wrote:

> On or around 25 Oct 2004 01:00:45 -0700, [email protected] (Richard
> Brookman) enlightened us thusly:
>
> >S2a, 2.25 petrol, Zenith carb, 70k miles.
> >
> >Running well, but running rich. Started after 5 secs with or without
> >choke, small amount of black smoke while running, doing about 15mpg.
> >All the indications are that it's running rich. Adjusted volume
> >control screw as per manual to no effect, so I decided the carb needed
> >an overhaul. Got an overhaul kit and did the work over the weekend.
> >
> >Now it starts first turn and idles sweetly - even first thing with no
> >choke. Misfires a bit when it's revved, loads of black smoke.
> >Obviously running very rich. Adjusting the volume control screw makes
> >no difference whatsoever to the idle speed, from fully closed to right
> >out (at which point it stalls, not surprisingly. VCS is extremely
> >stiff and almost impossible to turn (as was the old one).
> >Substituting the old one makes no difference.
> >
> >All I did to the carb was to replace the jets and economy device
> >diaphragm and set the float level to the correct setting (33mm from
> >the gasket face - it was 28mm which is 5mm out - but this would weaken
> >the mixture, wouldn't it?). Obviously I've done summat wrong, but
> >what?
> >
> >Anyone any ideas? Or should I junk it and get a Weber?
> >
>
> sounds like you have a blockage somewhere, or a leak letting petrol in where
> it shouldn't. The mixture screw should have some effect. I assume you had
> the correct main jet to put in?
>
> IIRC, on the zenith, the adjustment is for idle mixture only, not for normal
> running, which is defined by the size of the main jet and auxiliary jet if
> it has one. 's ages since I played with a zenith, though.

Two main problems, and both about the sealing between top and bottom.
There's the O-ring which seals around the actual air passage, and
they're also prone to the casting warping. This means that the gasket
doesn't seal, and lets petrol leak into a little hole, and thence into
the air. I think the particular hole is one of those towards the
engine-rear.

I've seen a suggested fix involving plugging the hole. There's a web
page about this on a site run by one of the American Land Rover outfits,
or was it actually in Canada?

--
David G. Bell -- SF Fan, Filker, and Punslinger.

"History shows that the Singularity started when Sir Tim Berners-Lee
was bitten by a radioactive spider."

So Austin Shackles was, like

> On or around 25 Oct 2004 01:00:45 -0700, [email protected]
> (Richard Brookman) enlightened us thusly:
>
>> S2a, 2.25 petrol, Zenith carb, 70k miles.
>>
> sounds like you have a blockage somewhere, or a leak letting petrol
> in where it shouldn't. The mixture screw should have some effect. I
> assume you had the correct main jet to put in?

It came as a kit (unlabelled) but all the jets I replaced were visually
identical to the ones I took out. I just removed every jet one by one and
replaced like for like. There was one jet left over that I didn't use,
however, and a load of spire washers and plain washers that didn't belong at
all... perhaps the kit was wrong.

>
> IIRC, on the zenith, the adjustment is for idle mixture only, not for
> normal running, which is defined by the size of the main jet and
> auxiliary jet if it has one. 's ages since I played with a zenith,
> though.

You're right, although it is important to set the idle mixture correctly as
the carb will draw fuel through the idle circuit at all times, affecting the
mixture vthroughout the range, AIUI. At the moment, screwing in/out the
volume control screw makes *no* difference. Normally, it goes from stalling
(too rich) to maximum idle revs, to stalling (too weak) in about 3 turns.
I'd only just set the idle mixture a week before, so I know it's not working
the same.

I'll swap the main jet back for the old one and see if it helps. Grateful
for my policy of "throw nothing away until you're sure the new stuff works".

Cheers

--

Rich

Nullum Gratuitum Prandium