I am suffering from what seems to be the perenial problem of the huge flat spot when coming of tick over .The engine will cut out if not coaxed on to the compensating Jet. I have thoroughly cleaned all jets and am confident there is no detritus left in the body .The tick over is excellent although the adjustment screw is just about fully screwed in .
The jets in the carb are as posted in previous answers . The prgression jet is 50 however in the Pitmans book of the Seven both 3&4 editions they quote the jet should be 100 if this is correct then that will explain what feels like a weaknes on progression .

Can anyone advise or indeed has anyone satisfactorily solved this problem as I have followd all previous postings to date .

Regards
Mike Horsfield

Happy New Year to all our readers

Hi Mike,

According to a 1952 Zenith Carburetter Company published Data Sheet, the progression jet for the 26VA should indeed be 50, the Nippy's 30VEI had a 90 and the Big Sevens 30VM-4 had a 125.

The 26VA's compensating jet should also be 50

Jeff

Hi Mike

As you knnow I have fiddled a fair bit with my own Zenith, and have left it well alone for a while in case I make it worse again!

However, with the help of some diagrams suplied by a fellow forum contributor I now have a better understanding of the Zenith operation, and I now believe most of my troubles are probably due to the progression drillings being partially blocked as they are almost impossible to clean. Based on the diagram and some general theory I think this is probably why a lot of Zeniths do not run well, particularly coming off idle. I have stated in a previous post that possibly ultrasonic cleaning may help, although this is just an untested theory? Many garden machinery repair shops have ultrasonic cleaning tanks that would accomodate the Zenith body....might be worth experimenting with?

I did try turning up the idle speed screw to bring the throttle butterfly past the driling at idle and thus sort of bypass the whole affair! this needed the mixture screw turned almost all the way in to bring the idle speed down again, and did work sort of?
Not a very satisfactory method, but some experimentation with the position of the throttle butterfly whilst fiddling with idle mixture does seem to almost work?

I am waitng for an adaptor to arrive to allow fitment of the SU I have in the garage, which seems to be a route many have taken

Best of luck

Steve.

Hello Mike,

Like Steve, I really do believe that your problem is involved with the progression jet and its body drilling.

The function of this jet (the so called "missing jet" referred to on some web sites) has the function of providing extra mixture as the throttle is opened off idle and the "depression" created by the almost closed throttle disc in the idle setting is reduced by the opening throttle. It is supposed to provide that extra mixture until the compensating jet takes over.

The progression jet obtains fuel mixture from the same source as the idling jet and can also be influenced by over weak idle mixture "setting" which in turn can be influenced by worn/mutilated idle mixture set screw taper and/or seat.

These are a bonzer carburetter when everything is in good fettle, but a fair pig when problems arise.

I would be happy to send you a copy of a sectioned drawing which illustrates the construction if you like.

Regards,

Barry R

Thanks for the response so far guys

I know that the correct progression jet is a 50 but I have tried a set up as follows first the float chamber I have set using the thickest washer supplied this stops any fuel dripping from the carb and allso weakens the mixture to more like it should be .I then fitted an 85 progression jet from an old carb that I had and tried the car on the road .The difference is fantastic the flat spot has almost dissapeared and the car performs better that it has in the short time I have had it .I contacted one of the carb specialists who confirmed that the original should be a 50 and that the carb is probally sucking in air somewere possibly the float chamber warped etc but hey I am tempted to open the progression jet out to 100 as listed in the Pitmans book just to see if that last slight flat spot will dissapear .

Watch this space !

PS When the engine is switched of the carb heats up from the manifold and causes the fuel to expand and a small amount drips from the carb mouth does anyone know of a shield to put between the carb and the manifold or will the float stand a thicker washer still

Mike

Hello Mike,

I am almost reluctant to make the following comments because I really am not in proper possession of facts.

I have read somewhere, reasonably recently (last 18 months or so anyway)some comments which seemed to be suggesting that modern petrols have a different density to older types and that this can have an effect on float levels in carburetters. I though I 'bookmarked' the comments for future reference, but cannot find either the bookmark or the item. I actually thought it was on this forum, but if it was then I certainly can't track it down.

Anyway, I think I would be inclined to try an extra washer under the needle valve, and if you have an expendable Prgression jet, then try a slightly opened up diameter.


There seems to be very little doubt that a number of people are facing difficulties getting these carbs to be drip free.

I ran one for years on a previous car I owned. It was never touched in all the time I had the car, which probably goes some way to explaining why it worked OK.

I also think that the "Pearson" Austin book may give some other figures for jet sizes. I will dig my copy out and see what it says.

regards

Barry R.

Hi mike

Interesting to hear your results .
I think its probably right that with modern fuel a certain amount of experiment and tweaking is required to acheive good results. This seems to be your findings, along with other peoples.

Once I am in possesion of the SU adaptor I may have one last fiddle with the zenith just for old times sake!
I am afraid to touch it just yet with it being such a temperamental beast.

Steve.

Hi Bill

I found some info on an Ammoco site which indicated that modern fuel has ingredients which boil at just over 50 degrees C which were not in the older fuels. This 'boiling' introduces bubbles in to the float bowl which then allows the float to sink lower ( it doesn't float in vapour ) which then allows a higher level of mixture in the float bowl which changes the mixture of air/ petrol. This is one of the theories used to explain the bermuda triangle !! Doom will befall us ( Thought I would get that one in first ! )

There are also other ingrendients in modern fuels that vapourise at a far higher temperature then were found in older fuels, I am not sure what effect these have.