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Zenith 361V "non-atomisation of fuel"


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I am a newbie having recently joined the forum because I have a so far insuperable problem. I thought the thread started by Betsy (Zenith 361V problems) on Dec 6th last was going to provide some sort of answer to my problem but unfortunately it hasn't. I made the decision to start a new thread on this subject because the old one seemed to have died out without any answer to help me. Perhaps someone will inform me if I should have added to the original thread or started a new one as I have opted to do? (This will help me when posting in future).

Now to the real business:- My series 2a has a 361V which is, as far as I can make out, in perfect condition -that is to say it has had ALL its joint faces perfectly done on plate glass with wet & dry ( even double checked with prussian blue in proper engineering practice), has had the "economy" diaphragm and all gaskets replaced, including the venturi O-ring. The float needle makes a perfect seal and the float height set to 34mm (deliberately set slightly higher than recommended to give a lower level in the float bowl). The butterfly spindle is totally unworn and all the jets & air/petrolways seem to be clear - I having checked the latter with a pressure oil can, (easier to check the circuits this way rather than using petrol which goes everywhere before you can tell where it's coming from, or wire which won't turn corners!).

And what do you know! - after all this it still dribbles blobs of unatomised petrol down the venturi on top of the butterfly causing the engine to make sucking/gurgling noises as the neat liquid fuel passes round the butterfly. The effect diminishes somewhat, but not entirely as the revs are lifted from tickover. This, then is the nub of the problem. One question I have is:- Does a "perfect" 361V do this or does it atomise the petrol into a fine mist/spray as I certainly imagine it should. I wouldn't expect much in the way of MPG if it didn't!

I have the facilities in my workshop here to make up and fit an 1 1/4" S.U. as a last resort - I have already done one such conversion for a friend wirh a 2 litre series 1 with great results over the old Solex that it replaced. But all this is a fair old bit of work, and I feel that by going down this road , not only am I losing "originality" with the seies 2a, but I have had to declare defeat and this is what bothers me more! So, to sum up, you guys out there are my last hope to get a solution for the Zenith! Please ask if you think I've omitted something relevant. Hoping to get some helpful advice soon!

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Very nicely described Mr Serious! You have the exact same issue as me.

I too have a Zenith (actually, I have three). One of my Zeniths runs perfectly well and is economical and has no problems. I have one that refuses to allow the car to go uphill, or around sharp left hand bends, and I have one that constantly dribbles fuel out of the emulsion tube at idle or low revs, but runs ok at higher revs.

I have recently tried to fix the one that won't go uphill or round left hand bends by following the same procedure as you. The carb was carefully rebuilt, but the same problem persists.

I think the fuel dribbling is an internal leak, because at low revs the fuel should only be entering the engine via the idle circuit. There should be very little 'wind' going down the venturi at idle, so in theory it should not be sucking petrol out of the emulsion. The fact that it is dripping backs that theory up, it should be atomised in the airflow.

Sorry I don't know the answer, but at least you know your not alone in your quest to fix a Zenith. If you do manage to find a way of tracing the internal leak, please let me know.

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Thanks Betsy for your reply. I did realise you had this problem from your original quest for help back on 6th Dec., but I didn't know if it had been resolved or not. Obviously and unfortunately still not! I haven't tried the vehicle nose up or nose down to see if that makes a difference as in one of your cases but I reckon if I can't get the damned thing to run right on the level in the garage it would be a bit pointless to see if it worked better off the level. I'd still have a problem. I though the tip on another website forum (North California L/R club - I think) to block off the redundant hole in the main body of the carb (a hole that goes basically nowhere! - the one to your left of the two adjacent holes in the back of the carburettor body looking rearwards), would have had some sort of result but it made no difference whatsoever, - so much for that then!

If we don't get help from somewhere, I can see the day of the S.U. coming. but for reasons given in my original statement, I'll hold off a bit longer - hope springs ( but not non-emulsifying carb) eternal!

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I also saw that bit about blanking the redundant hole, but I can't see the point, its just a hole to nowhere. I am convinced the dripping is caused by a leak in the economy device circuit, but I can't find a way of proving the fact.

The carb I have that won't go up steep hills or round left hand bends leaks out of the emulsion tube when tipped back or to the right, but not when tipped forward or to the left.

To find this I used the hand prime lever on the pump to fill the carb until the float shuts off, then lift the carb off the manifold and carefully tip it back. It only needs tipping about 15 degrees before the fuel starts to dribble out. Same if its tipped to the right. This seems to indicate that the fuel level in the chamber may be too high. However I have drastically altered the float level to as much as 40mm and it made little difference.

I have looked closely at the carb top and emulsion block and cannot see where the fuel is getting into the main circuit at such a level that it leaks out. The only conclusion I come to is that carb must be flooding somehow. I have to admit I am stumped!

Luckily, it is not too expensive to get a brand new Zenith here in the UK. All 3 of my carbs were bought new and lasted about a year before going bad. The current one fitted has done a trip to Morocco and back, plus a trip to France and several long trips in the UK, so far its lasted far longer than its predecessor.

The one common advice lots of people give is replace the O ring, but I am struggling to find a source of spare O rings. My local parts supplier has none, and his supply has dried up.

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Hi,Betsy, thanks for your second reply, (but where are the experts with the answer?!)

As regards the "economy" device, I have even tried removing the cap and diaphragm altogether with the engine running but without significant effect. This device as you probably well know lifts a valve (on centre of diaphragm) under conditions of high vacuum (tickover and light throttle openings on the road) and is thus meant to admit extra air via this valve ( and the associated airway to atmosphere) and weaken the mixture for economy. As our problem is admission of too much fuel I can't see that this has got anything to do with the flooding. However I am open to argument on this one. I even experimented with a weaker spring on top of the diaphragm with a view to making it easier for the engine to draw more air but, again, no noticeable effect. Stalemate on that one!

I wouldn't concern yourself too much with a new O-ring , Betsy, because no O-ring out of a year old carb is going to be perished and besides petrol does not leak up around it - does it? Someday soon, as you have aroused my curiousity, I will fill the carb, remove and tilt it to see if 15 degrees backwards is enough to cause petrol to run out of the venturi tube. Watch this space!

It seems almost unbelievable to me that you have gone through about 3 new(!) carbs and they've all ended up the same - this is almost bizarre!

Can I use this opportunity to appeal to the wider audience - those owning 361V carbed 2 1/4 petrols , to at least eliminate one big unknown for me - i.e:- Does a "normal" running carb exhibit any of these flooding symptoms, maybe as yet un-noticed? If some kind soul would take a moment to remove the air filter connecting pipe and look down the venturi at tickover and blipping the throttle, and observe if the emulsified fuel exiting from the central venturi is in the form of a fine mist/spray or tending to drip in droplet form onto the butterfly, it would at least establish that it in fact should atomise fully or not? Maybe the damned things all do this to some degree and we're chasing something that doesn't exist?! I still live in hope!

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One thing I've found is that you need to be careful not to take too much material off the emulsion block or it doesn't seal to the 'O' ring sufficiently. You will only need to take off material from the seating areas, the two legs which hold the float spindle are effectively in free space so you don't have to take off material to reach their level of warpage which tends to be the lowest points. The last one I did I skimmed 5 thou off and it started like a banana!

I have been wondering about making up a gasket from something more pliable, but of course resistant to petrol, to be a little more tolerant of the warping.

I built my last one with a BritPart kit which had a really carp brass throttle spindle. At £8 though I didn't care too much, I think it's £30 something from Paddocks which seems crazy money for what you get.

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Thanks, Robotman, for your comments. In the flattening process I did ALL the faces; I estimate I had to remove about .003/.005" to restore flatness to the main carb top to body joint and the emulsion block. If, for talks sake, I removed .004" from the main joint and also removed .004" from the emulsion block, this leaves everything in the same relative position to the O-ring - .004" lowers the top but is compensated by the .004 off the emulsion block which raises it up again, so no change in compression of the O-ring - yes? In my case the cover is just held off a nice amount (in my judgement) above the carb body - just enough for the screws to give it a bit of compression , not too little, not too much.

You mention the gasket. I have made one out of the standard .025" petrol & oil resistant gasket paper available in sheets or rolls from any good motor parts factor (also available in .015 thickness). I have been using this for years for all sorts of paper gaskets applications and can vouch for its effectiveness. Check it out Robotman,as it has a small degree of compressibility which could take up a few thou.,and does well in this carb instance. I can even post you a piece if you can't get hold of some.

One other thing I've forgotten to mention so far, is that I have "disabled" the accelerator pump by removing the linkage pin. This takes it out of the equation re flooding etc., and in any case is not really needed if you are light footed like me! I'm embarassed to say this but I once had a passing interest in Reliant 3-wheelers and ran a Zenith carbed one with the pump linkage disconnected permanently with no drawback whatsoever but a benefit in the already good petrol consumption!

Meantime, can I repeat my appeal to one and all for an assessment of the fuel emulsifying of a "good" Zenith 361V?

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I'm not familiar with this model of carb but i wonder if the vacumn is not quite sufficient to draw the fuel properly atomised.

If you were to start investingating on the basis that the venturi 'effect' is not as the carb design requires then start elliminating other sources of air induction to the engine:

Why not try disconnecting any and all vacumn take off connections from the inlet manifold to ensure that all air required for the engine has to be supplied via the carb venturi. If for example a brake servo had a minor perforation, or distributor vacumn advance this may be 'bleeding' off a samll amount of vacumn.

Trying winding in the idle air screw - it might run rough but again worth checking to see if it improves the atomisation.

The butterfly position is obviously critical in its affect on the venturi/vacumn and may be either opened more than expected (reducing venturi effect) or not enough (reducing venturi effect)

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Thanks, SteveRK for your contribution. Yours is an approach which I never even considered - checking for loss of vacuum elsewhere. The only manifold connection existing on mine is the advance/retard vacuum pipe. I have no brake servo or other outlets. Nevertheless, over the week-end I intend to thoroughly check for leaks everywhere as per your recommendations, including mixture screw adjustments maybe causing a variation in atomisation. It is a small ray of hope! Thanks again.

Meantime, I checked out the carb tilting exercise mentioned by Betsy above and, sure enough, if you fill the float chamber normally full, unbolt the carb, and tilt it back, petrol spills out of the venturi tube at about a 20 degree (estimated) angle, either right or left. It doesn't do this, as might be expected, tilted forwards. Trouble is, I don't know what the significance of this "test" is? One obvious deduction is, that if you drive your Landrover up a 20 degree slope the carb will flood! Can't be right, can it? Bewildering!

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Dont foget to check the inlet manifold mating faces etc.

I assume the float in these carbs is Brass or plastic.

If its made of a hard polestyrene type material i have known this type to become porous (happended with a BMW boxer motorcylce) with age and actually abosrb a small amount of petrol (sinks lower into the fuel) and therefore causes a higher fuel level before shutting the valve.

One other thing - IF the valve clearances are too tight, OR an inlet valve is leaking sufficiently this can cause blow back into the inlet manifold and disrupt vacumn levels - i.e. as the valve vents compression up into the manifold during the compression stroke it will upset the air flow.

Getting complicated but it may be nothing to do with the carb!

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You could have a point there Steve. Of the Zeniths I have, the one that runs well has a black plastic float. The one that dribbles has an expanded foam float. I not done a comparison of actual fuel level in the carb when the valve shuts. I just set the float level as per the book.

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Thanks - Its worth buying a new float perhaps- with my old BMW boxer twin i spent hours trying to get it to run right. I cant rememebr why but i ended up buying a new float and floated both the new and old floats in petrol to compare how they sat. The old float (after a few minutes) settled lower in the fuel!

The initial problem was flooding (leaking out of the overflow) it didnt affect the running but if you are familiar with the boxer twins the carbs (one either side) sit right in front of your feet - embarrasing as i was touring Scotland at the time and my boots stank of petrol and I staying at B&Bs.

I used to wash them with shampoo in the bath!!!

Back to this carb problem:

What was happening was either the dam carb would flood (leak) when set to the factory spec (height etc.) or hold back if I re-adjusted to stop the flooding.

With the Zenith I will guess that the 'overflow' is seeping into the venturi for safety reasons as you wouldnt want it overflowing onto the exhaust manifold!

Go for it!

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Well I have just wasted most of a good saturday afternoon working on my carb problem to no satisfactory conclusion. As you, SteveRK were coming from a different angle I decided to check out various factors which you suggested might be causing a loss of manifold vacuum - a very real possibility. Alas, nothing amiss discovered. I checked for vacuum leaks at all the joints between carb and head with the engine ticking over, using WD40 , and observing and listening- no variation in engine tickover whatsoever. I then removed the advance/retard vacuum pipe and did the "suck & tongue" (!) test on it to ascertain that the diaphragm on the distributor wasn't drawing air. Then put a finger over the pipe stub on the carb - no difference in running.

On the leaking valve/tight tappet theory, I did a compression check and all four cylinders came in pretty close. The engine uses no oil worth talking about, by the way. I also checked the c/b points, etc while the distributor cap was off. Incidentally, I don't know what value it is in the circumstances, but if you put your hand over the carb on tickover there is a massive amount of suction - it nearly takes the skin off your palm!

I fiddled about a bit more with the mixture screw - it does make a worthwhile difference, although the engine will run, albeit not happily, with the screw full in. The engine is now ticking over as sweet as a nut at about 500 revs, I guess. It is like a sewing machine and a pleasure to listen to! No smoke from the exhaust and no un-emulsified blobbing of petrol down on top of the butterfly. LOVELY. Ah but.... Lift the revs a couple of hundred and we're back with the old scenario - gurgling neat unatomised fuel emitting from the emulsion tube. B*****ks!

I am aware of the BMW Bing carb flooding problem ( having been a biker all my life , although the only BM I ever owned was a K100) - nice one about the smelly boots! I checked my Zenith double float but found no petrol inside them (as used to happen on occasions with soldered up brass floats). These floats are made of hard black plastic, and neither they or the float needle are leaking - my test for the needle being to pump the hand primer up fully and look for leaks. After doing this a few times, if the feed pipe is pulled off the carb you get covered in pressurised petrol! Also remember that I have set the float(s) to give a lower than recommended petrol level in an effort to curb flooding. Nuff said.

At the risk of becoming a pest, can I again repeat my plea for someone to look down their Zenith 361V venturi and fill me in at to what happens with a "good " carb?


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Stick with it - dont give up!

Interesting observation about raising the idle a couple of hundered RPM and then it starts - I assume the butterfly is being opened very slightly and therefore reducing venturi vacumn slightly.

Does the Zenith have an idle air speed screw to regulate idle speed or is it adjusted by the butterfly?

If so then it might be worth screwing this out and compensating by adjusting the butterfly stop to reduce the gap - or maybe even the opposite to try and see what effect if any this has.

I'm not familiar with how fuel is supplied to the engine with the Zenith carb but are you confident that it is via this tube that is dribbling?

Are you sure this is not an accelerator jet or secondary/main jet that is being subjected to excessive vacumn????

There are normally pin holes in the venturi adjacent the opening side of the butterfly that supply either air or fuel for idle.

I'm getting more confused now!

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Yes, SteveRK, the butterfly is regulated in the idle position by an adjusting screw bearing on the attached throttle linkage. Obviously the screw holds it open just a smidgeon for tickover. It's when I manually open the throttle just a tad more,(don't ask me which is bigger - a smidgeon or a tad!), that the trouble starts. In addition, unrelated to this, the Zenith has the normal air mixture screw for idle mixture setting only, supplying the"pin holes" you referred to above. The carb does not dribble at (my low) tickover as I said prieviously,so I reckon the settings of both screws are close to or at optimum.

I cannot be sure that the accelerator jet, main jet, economy jet, etc are not being subjected to excessive vacuum , all I can categorically say is that no fuel enters from the accelerator jet (pump disconnected as mentioned before). If some jet somewhere somehow is being subjected to excessive vacuum it certainly would cause over fuelling but I'm defeated as to how or which one(s)


In answer to your question about being sure the fuel is entering from the (central) emulsion tube. - yes it dribbles from its tip - fuel enters here and nowhere else, (apart from the miniscule (invisible) amount to sustain tickover from the "pin holes")

Hope this helps your confusion - don't give up and leave this one yet, SteveRK!

I have "guested" at a couple of other L/R forums and this Zenith dribbling problem seems to be quite widespread. I'm contemplating signing on one or two of these if in the end all fails here, although the prospect of starting all this over again is pretty daunting! - any thoughts or recommendations on that, anyone?

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O.K - back to square one then - I cant find any diagrams relating to this model of carcb - what i did find relates to the ford model A !!

Interesting to note (albeit a very simple Zenith carb version) references to Cap and Compensator jets -


May not be relevant at all - red herring.

You have disconnected the accelerator pump circuit? hmm. This normally has a diaphram with counteracting spring and operartes via a mechanical lever connected to the throttle linkage. If the diaphram is holed it may be allowing a through flow of air because the carb side may be connected to vacumn venturi effect and hence fuel being sucked into the venturi by this emulsion tube. This diaphram acts like a plunger to force fuel into the venturi when the throttle is opened.

I think you should look at the diaphram - not easy to detect any leaks as they will have a riveted brass plate in the middle that is almost impossible to visually check if leaking.

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I think thats the 'Economy Valve' that you are referring to.

The Accelerator pump is a piston directly activated by the throttle shaft. It has a simple non-return valve which is easily tested.

I also think something is this economy valve circ uit could be causing this.

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From what i could make out the excess fuel was coming from the brass projection (lower right side) ?

Looks like an overflow drain pipe to me not a jet.

WE NEED A PARTS DIAGRAM FOR THIS CARB!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

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I think the brass projection you are referring to, Steve, is the outlet for the accelerator pump which is inoperative in mine at the moment as the (piston) pump is disconnected. Parts diagrams for this carb are a dime a dozen but, as I have said prieviously, what we are in dire need of is a flow diagram(s) similar to the one posted above for the Model A, but SPECIFIC to the 361V. Meantime, here is a parts diagram showing an exploded view of this carb. - hope it comes out viewable.



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Yes - but having been searching I don't think one exists. :(

The closest i have found is for an acient Zenith fitted to a model T Ford! It may be in principle representative as it links various jets to ambient air pressure and refers to the same types of jets fitted to later Zenith carbs.

What did you think of that article i posted - Page 6 refers to blanking off a hole in the carb body etc. and references to internal distortions causing air leaks etc.

It might be time to try some radical mods to see if they have any affect.


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