As stated I am experimenting with the Zenith still, whilst running the SU project in parrallel.

To elaborate on the Zenith problem....

The idle is fine, BUT if you slowly increase the revs it will graduallly die away until sometimes it stops. Once past this point runs very well and pulls hard. All manifold faces etc have been machined flat so no air leaks. The "bad zone" coresponds to a light load cruise at low speeds i.e around town driving, leading to the jerkiness.

Today I tried another experiment....
I slowly raised the idle by means of the throttle stop screw until I was in the "dead zone". the engine would barely run here, BUT I then screwed in the air bleed screw until it ran OK, which means it is now all the way in, so there is no air bleed at idle at all!
It now idles very well!
It now runs very well!
It now runs at light load/low speed very well!
It doesnt soot its plugs.

I would seem to have bypassed the idle circuit completely! However it runs better than ever, even to the extent of not backfiring on over run.

The point now is why?
It would seem to indicate an air leak somewere?
All flanges are OK, the spindle has beeen done by Burlen, its been cleaned more times than I can remember?

I am a happy but confused bunny

p.s Having now studied the thing in some detail, I can see that the "hidden" jet is just an air bleed at idle to allow air past the throttle butterfly when it is shut, it does not flow fuel as it is in the carb body not in the float chamber were the fuel jets/drillings are.

Steve

Not sure what sort of Seven the Colander is, but if you have a vacuum wiper could there be a leak in the rubber tubing? If electric forget it.

Hello Steve,

With some trepidation I beg to respectfully dissent from you assessmemnt of the function of the "Progression Jet"

The function of the " Progression Jet" according to Mr Arthur W. Judge in his tome on Carburetters ( Chapman and Hall, London 1946)is as follows:
"At the throttle edge there is a further outlet which breaks into the slow running passage. Upon the throttle being opened from the idling position this will give an additional mixture to ensure progressive get away from slow running - this explains the title of " progression jet" for that part"

It is worth noting that both the Idling and Progression sytems admit "mixture" rather than fuel.

It does seem to me that your slow running set up maybe was letting too much air into the system, and starving the progression system of appropriate fuel/air mixture.

Very, very vaguely, I seem to remember something about variations in both length and taper on the business end of Zenith slow running screws, so that might just be worth considering.

Obviously, if the slow running adjuster "seat" in the carby body was scored, that could give rise to trouble in adjustment as could a damaged taper, but I feel certain you would have already checked that.


Regards,

Barry R.

thanks Barry

No need for trepadation I have thick skin!

I Take the point about "mixture" as oppossed to air and fuel seperately. I can see how the jet would allow air in as the throttle opens, which I guess would then draw more "mixture" into the engine as it flows through the carb etc. Being a jet I guess this is another candidate for calibration experiments?
Ref the tapers, I have a stubby taper screw and a long taper screw. The only screw which appeared to work was the stubby type, BUT I take the point about damage to the seat which is possible I suppose?

Seen as its running OK perhaps I should take the hint and just leave it alone?

steve

Steve,
Zemith Carburetters are DEFINITELY one animal with which one should absolutely never try and take to bits to find out why it is running so well.

They are a fantastic carby while they are running OK, and many do faultlessly, for what seems forever. But I have had some aweful experiences with them, and nearly everytime it has been a result of either my own or someone else's ( who I would naturally prefer to blame) fiddling.


I would definitely leave it alone.

One good thing about all this is that your quest for SU info has produced some leads on better jet sizes for me to try.

By the way, a fantastic article on A7 Carburetters by Steve Hodgson in 2006C Buff (Grey) A7CA Magazine, which arrived here today


Regards

Barry R.

Just to add fuel to the fire, my 26v has an extra jet in the rim of the float chamber next to the slow running jet. On most of these carbs that I have seen, there is no separate jet here, only a large drilling with possibly a smaller 'jet sized' hole deeper down. Anybody know what this is? The brass jet in mine has what looks to be 2 0 (looks to be a lot bigger than 20 thou) straddling the screwdriver slot/jet hole stamped on it. The engine is a 1931, and I know that it really should have the updraught carb. Does this make any difference.
Probably should have started a new post here.
Barry, any chance of a copy of your 26v spec sheet?

Cheers, Dave

Hi Dave,

I'm sure Barry will shortly ( when the sun rises in the southern hemishere) confirm the following which is listed in an official Zenith spec sheet of the 1950's :

Austin 7 h.p. - 1934 to 1938 - Zenith 26VA
Choke (cast in) - 17
Main Jet - 57
Compensating Jet - 50
Slow Running Jet - 60
Progression Jet - 50
Ventilation to the capacity well - Standard
Capacity Tube (cast in) - 2
Needle and seating (the size stamped on a flat of the hexagon of the seating) - 1.5
Starting - Strangler

Cheers
Jeff

Hello from Tasmania,

First and foremost, let me say that I am no expert on Zenith Carbs, just a poor beggar who has had a need to keep various cars fitted with them more or less operational since the late 1950's. Mind you, compared with the Solex units fitted to the Jaguar Mk2, 2.4 Litre, Zenith carbs are fantastic!

In respect to the specific matter of your "extra jet" I am fairly certain you are takling about the "compensating well" which in some applications is fitted with what looks like a brass jet, and which in other applications is just a fairly large "drilling" in the body. Just to confuse things, Zenith in some of its publications refer to this as the "Capacity Tube" It is this device that "vents" the compensating jet fuel delivery system to the atmosphere, and which is the heart of the mixture correction arrangements which lie at the heart of the Zenith system.

In my experience the usual 26VA fitted to the A7 is specified with the "open" tube without any brass "metering" insert.

The 20 does not refer to 20 thou, but is a system based on "increments" I have a table which explains all that but will have to be after next weekend, as
the book I have it in is at my country retreat.

Again, if it is working, I would leave well enough (no pun intended) alone.

Maybe, if you can identify any markings on the Carby I just may be able to look it up.

Jeff's data seems to be on the ball to me, but I will compare it with the Data Sheet ASAP.

One thing to remeber about all these Zeniths is the fact that while they were made in UK, they have metric fastenings, which can cause some heartache if you try an use UK thread replacements. No so difficult these days I suppose with metric threads being commonplace, but that hasn't always been so.

Another common problem is stripping of the female thread in the body where the fuel inlet "banjo" is fixed. That can often be fixed with a "Helicoil" but can be a bit tricky to do properly, so seeking experience assistance may save botching it up.

On the subject of the Zenith service sheet, as with any material I have here, I am happy to share, and I will make that available. Maybe though, to save a lot of confusion, it might be possible to have Mac facilitate its posting on this site.

I will see if I can contact him first up anyway.

I have just had a bottle of 18yo Macallan Single Malt delivered here by a chap who I have made some stepped studs for. He has definitely overcharged himself, but I am about to try and ensure that I get the mixture right!

Regards to all,

Barry R.

Just an addendum to my previous mutterings.
After one good pull at the Macallam, I had a brainwave!

I remembered that on that fountain of good info the "Speedex" site there is the following:

" A range of jet sizes are available from Zenith specialists, standard sizes are in multiples of ‘5’. You can get in between sizes from carbs for production runs of a standard model. The number relates to the metric size, 75 is 0.075mm, with the range of modellers drills available you may be able to drill out your own — but make sure you re-stamp the sizes!!"

So there is the answer to what the "20" means

Barry R.

Hi Dave and Barry,

The following info from the Zenith spec sheet may be of interest:

"The compensating jet cannot be confused with the main jet as the external diameter of the two jets vary, the main jet being smaller than the compensating jet in diameter. The jets have the same flow calculations and are stamped with the size numbers. The sizes normally follow in stages of "5", although "half size" jets are sometimes used. The greater the number stamped on the jet, the larger is the drilling of the orifice."

"Ventilation to the capacity well - in "V" type carbs the ventilation hole is in the screw over the capacity well. Numbers are stamped on the head of this screw when the size of this hole differs from the standard diameter. In certain carbs where the capacity well is cored to the required size and has no loose tube fitted, the top of the well is open and no screw is incorporated."

"Capacity Tube - this is a separate loose fitting in certain types of carb, but in others the capacity well is cored the correct diameter to give the necessary reserve of fuel for acceleration purposes."

Cheers
Jeff