Zenith 30 VM carb query

[PaulRM]

Can anyone - Keith ? Scott Renner? - who's played with the various Jowett Zenith carbs offer any help here?

I have pair of 30 VM carbs that are for a left hand drive car I'm restoring.

To get this pair to work has been difficult, despite complete strip down and rebuild. Now the Club technical notes provide a useful supplement detailing the function and difference between the various Zeniths fitted to Javelins and Jupiters. However My VMs seem to have extra air holes at the inlet orifice. I am not talking about the progression holes in the barrel near the butterfly, or the hole for the vacuum advance take off. I've attached a photo of one of the carbs showing a cut out slot plus air hole at the base of the carb. Is this standard on the VM? If so its function is not at all detailed in the drawings in the technical notes. This slot is absent in the 30VM-5s - as I also have a spare right hand drive pair and there is no slot.

I want to establish whether the previous owner has mucked about with the carbs by drilling more holes in their barrels. Various other bits , e.g progression jets were missing, and various blanking screws, and I have replaced replaced these.

Many thanks

[Forumadmin]

That looks like a home made vacuum take off. Unless it is a throttle bypass for idling speed. Where does the hole go?

I have never seen that.

[PJGD]

No, it is not a home made modification, it is a standard feature; my 30VM's have that too.

The passage connects to the drilling network that goes up to the long taper air screw. I can't explain what it does.

[PaulRM]

Thank you Phillip, good to have that confirmed.

The car is difficult to start from cold on full choke. Also it will not run below 1600 rpm even when warm without the choke flaps 3/4 closed. However it will start immediately when warm. To me this suggests a large air leak. I do not have a spare set of left hand carbs to swap and test.

Just to mention:
- compression on the engine is very good ( It has new pistons plus rebore).
- leak down test performed and all good.
- valve timing checked
- tappets clearance checked
- ignition timing checked
- healthy spark and correct gaps on correct plugs
-carb set up i.e butterflies setting, enrichment screws, choke flap settings and linkage scrupulously adjusted as per manual
- carb gaskets checked
-floats and float valves checked

(This is a rebuilt engine from a former alleged specialist, who also allegedly rebuilt the carbs. It is not mine but a friend's and I'm doing the work for him. Initially I found a large hole in the internal balance pipe, causing much oil vapour to come out of the exhaust as well as creating a large airleak. I fixed this (as it was in the piece of pipe that runs through the r/hand tappet chest) and checked the pipe and pintle valve setup for correct vacuum using suction and a vacuum gauge set up. All good on the balance pipe now.)

[Forumadmin]

I think you are correct it is a weak mixture on one carb, probably caused by an inlet manifold gasket. When it is running, do both choke flaps make the same difference? Carb bodies can become warped.
I always use a half inch pipe in the ear method for checking for gasket and throttle spindle leaks.
I also use the pipe in the carb intake to listen for differences in intake volume, this can be more revealing than using a balance meter.
I also have a colour tune spark plug to check mixture but looking at the plugs might reveal the mixture setting on each cylinder.

It could be a blocked jet, wrong jet or blocked metering tube or failure of emulsion block gasket. You can use an opposite side spare carb as only the choke would be affected.

[PaulRM]

Thanks for those tips with the pipe and ear. Will give them a go. BTW the choke set up is the same on the left hookers; the butterflies and accelerator linkage and holes in the bore of the carbs are reversed. I thought I could turn the butterfly spindles and linkage simply around, and fit 'right hookers'. You can't. Admittedly you could redesign the throttle cable run and linkage....

[Nick Webster]

I can confirm that the RH drive car 30VM have the same slot. It is the slow running air bleed. The ones I have are schedule C1343A

Nick

[trymes]

I think everyone here is knowledgeable enough on the subject that this is beyond obvious, but I mention just in case (and since I really don't know these engines or carbs well, it's all I've got to add):

This reminds me of the first time I set the carburetor on my Rover P5 3-Litre, after having learned SUs on an MGB and a Riley 1.5. (and yes, I know these aren't SUs, I'll get there). The big difference between the HS4 and H4 carbs on the MG and Riley and the HD8 on the Rover was that the MG and Riley were set to have the throttle plates slightly open at idle. There is no air bleed as is being discussed here, so if you close the throttle fully at idle the car stalls. The Rover was the opposite: the throttle plate is fully closed at idle. The point being that I am fairly certain that these cars should have the throttles fully closed at idle and the slow running screw adjusted to allow the proper amount of air through the drilling in question. (Yeah, I know, you all knew that already, but perhaps double check it's actually closing all the way?).

Anyhow, another thing that bedeviled me with that Rover was that someone had fitted the wrong gasket to the manifold. The reason I couldn't get it to idle properly without slightly opening the throttle plate was that the gasket was blocking the opening that allows airflow through the slow idle circuit! So, do double check that there's no drillings, openings, or what have you inadvertently blocked by a gasket installed backwards or similar, either in the carb or manifold.

Maybe something in all that will prove helpful, probably not. Good luck!

Tom

[Nick Webster]

Tom, Yes you would indeed think that the carbs should be set with the throttle closed and I tend agree that this should be so... except they aren't! I perhaps did not make myself fully clear in my previous post when I described the lowest outlet hole as the slow running air bleed. This bleed of air, set by the needle screw carries over fuel from the slow running jet. Theoretically the engine should run on this supply - or should it? In search of that rare thing - a slow Jowett tick over, I have tried to get an engine to run on just the slow running jet with the throttle fully closed and failed. In the fully closed position the throttle disc covers the progression outlet which - as I understand it - should allow for an instant supply of fuel to avoid a flat spot as the engine picks up. Diagrams of Zenith carbs show the progression outlet quite clearly blocked off by edge of the the throttle disc. Given how thin the discs are, this is nearly impossible to set up. However, setting the disc to cover the progression outlet seems not to matter because Jowett instructions for setting up the carbs says to put a turn on the throttle stop screws, which actually allows the progression jet to be slightly uncovered. As far as I can see this means that in effect there are two fuel supply holes when the engine is idling, although who knows what is actually going on with the pressure (depression) at the two points and it strikes me that the lower point will be doing nothing at all. In the end all the theory is frustrated because at tick over the air and fuel flows are so small that the slightest air leak on the throttle spindles will completely mess up trying to set up the carbs to perfection.

Nick

[trymes]

Nick,

Very interesting, thank you!

Tom

[Forumadmin]

Also remember that wear on the throttle body, as well as on the butterfly, will considerably affect the position of the hole in relation to the butterfly. I have also found that often a new butterfly (as might have occured in this case) completely upsets the closing of throttle since the throttle body has wear ridges on it. So I reverted to the old butterfly on a new spindle with new bushing.

By using a set of micro drills I am able to measure the wear on jets and have found that they too suffer from wear. The shellac (or similar coating) deposited by old petrol can have the opposite affect and partially block holes. Float height can be affected by the easily distorted float chambers and floats can get heavier than they should be due to extra soldering or pin holes. The number of washers under the needle float valve will also affect the float height and hence level in the metering system.

[Nick Webster]

I must be lucky because I have never encountered much wear to the venturi area. In theory it shouldn't happen because the throttle screws should stop too much of a hammer blow. Then again, once it gets difficult to get a slow tickover they get wound away from the spindle lever and then the next step is extra strong springs which only compounds the felony.

Where did you get your micro drills, Keith? I have had some good results with bigger jet sizes, but at the expense of fuel consumption. I only have a small selection of jets and would like to experiment with some more subtle variations. Perhaps there is scope for a King's Langley garage meet... although I confess despite best endeavours I haven't made it to one yet. Perhaps you could consider moving your house further East?

Nick

[alexander]

Hi Nick
I got a number of very small drill from a model railway show.
Alexander

[alexander]

Hi NICK
Model railway show/ shops have drills to very small sizes sold in packs of five.
Alexander

[Keith Clements]

viewtopic.php?p=25370#p25370

Shows all the work I did.


Some of the jets were the wrong size and two were slightly clogged. Their sizes were checked with smooth ends of a set of jewellers' drills. I am not totally sure whether the jet sizes are actually mm sizes or whether they are nominal. Some more research may be necessary here. I will also check the accuracy of my drills next time! They do come from China! The jets were all set to the same spec. for C1334. Choke 23, Main 90, Compensation 50 (one was 70 and one was 50 and slightly blocked), slow 45 (one was 50, one was 45 and was tight on a .45mm drill), Capacity 2.6 , Progression 100, valve 1.5 (one was 1.6), washer 1 ( both 1.4, reduced).