Zenith blues

[Dano]

Zenith blues

Why is it every Zenith carb I have owned is knackered????

Just been trying to tune my motor, got the dwell set, gapped the plugs and set the timing by ear but its running lumpy on idle, I have rebuilt the carb with all new jets and throttle etc but still lumpy and conks out if left to idle on its own, the idle mixture screw has very little effect and looking down the carb I can see fuel seeping slightly from the joint where the accelerator jet sticks out,

There are dozens of threads on here about carbs which I have posted on before but what is the general opinion on the best option?

a) try the Zenith fix (I have never got this to work but this carb is not leaking too badly)

B) replace with a new Zenith £71.09 (from here http://shop.dingocroft.co.uk/acatalog/Carb...rs.html#a266693 are there any other suppliers?)

c) Replace with Weber £75 plus VAT from Paddocks

Your views are appreciated

Thanks

[Jon White]

Strip the zenith. Put a bit of P400 wet and dry paper onto a sheet of glass (perfectly flat surface) and using a little light oil lap both parts of the carb flat. When you start doing it you'll be amazed just how far out they are. Rebuid with a new gasket and new O ring and you'll probably sort it out. I did one for a mate recently and was amazed at how far out of flat it really was!

Jon

[bill van snorkle]

Do as John said. I did mine over ten years ago and haven't had a moments bother with the carb. The castings should have been normalised by heating and cooling at the factory before all the surfaces were machined, but were never done. Normalising stress relieves (relaxes) the castings and once machined they maintain their shape through the normal heating and cooling of regular operation. Zenith carbs unfortunately stress relieve them selves in service so consequently all the machined surfaces end up out of wack with each other.

Bill.

[Bowie69]

For piece of mind, I smear a very thin film of sealant on the o-ring before reassembly, mines been like this for 4 years and not started leaking again.

[Betsy]

I've done this with good results. Make sure you lap the main body, the emulsion block, and the carb top cover. I also set the float height to 34mm which is a couple of mm over the standard, this seemed to smooth the idle a lot.

[Dano]

Well I have flattened all of the mating surfaces, used a new "O" ring, even gave it a b it of sealant, it looks like the joint down the venturi is fairly sealed, there is a slight wetting where there may be fuel, there was a drip from the pipe leading into the centre of the venturi so I tightened the idle mixture down half a turn and this seemed to stop it (can anyone tell me if the idle mixture comes from this pipe?), I managed to get it to idle fairly well at about 850 / 900 rpm but its still a little lumpy then if left it will conk out after 5 min and doesn't want to start, I took it for a spin and got the same thing so checked the float chamber and found no fuel, I used the lift pump to fill the float and made it back the 100 yards to my house

I am guessing that either it is running so rich (and it smells like it is) that its emptying the float or the fuel pump is knackered but the lift pump works ok

I am as sure as I can be that all the jets were replaced like for like although the rebuild kit seems to have different threads on the idle mixture screw (so I still have the old one) and there are some odd jets etc

Any ideas would be appreciated (the Weber is looking better and better every day)

Thanks

[Bowie69]

Sounds like fuel pump to be honest, but also could easily be split/leaking fuel pipes -you shouldn't have an empty float chamber from a short run round the block!

[Dano]

how far do you think it should go on a full float chamber?

pipes are not leaking, I am still thinking carb but I really dont know

[Dano]

Just had a thought, it could be the temp filter I put on the pickup pipe, I used a small bit of foam which I checked for flow and thought would be fine, my tank really has some crud in it and the old gauze was broken and the pipe blocked, I will remove that and clean the tank out and see if that makes a difference, ill do this before buying a new pump (only £10) unbelievably a new pickup pipe is £40!!!

Does anyone know the part number or name of the gauze filter on the end of the pickup pipe, I cannot seem to find in anywhere

Thanks

[SteveRK]

I suspect more than one problem here - sounds like the carb' issues have been improved but the empty float chamber could be due to a problem with the mechanical lift-pump,

Runs o.k at higher RPM (driving) but on idle cannot sustain sufficient flow hence after 5 mins stalls?

[SteveRK]

Zenith_361V_Carb.pdfcouple of attachments

Zenith_Carb_flow_diagram.pdf

[Dano]

Thanks for the diagrams, I had not seen the second one, its very interesting, there is another very good thread on here with a great document about the zenith, after reading it I decided to fully dismantle and clean the carb, blew it all through with air and reassemble setting the float to 34mm in case this made a difference, I also drained the tank, filtering the petrol, and cleaned it out, I placed a gauze filter at the end (made from a plastic sieve), I checked that the carb floats were not leaking, I cut the fuel line before the carb and installed a universal filter, this way I can see if there is fuel and obviously provides increased filtration

The first thing I noticed was that the new filter didn’t fill fully, this may indicate a fuel lift pump problem

I managed to start the engine with 2/3 choke but it seemed to run very rich, I could not maintain idle and there is more fuel issuing from the emulsion tube than before I dismantled it! I tried to wind in the mixture screw but this had no effect I checked the carb again noting that there was fuel in the float chamber, I swapped over the compensator jet with another I have that has a smaller hole, this had no effect so changed it back

This car used to have an LPG conversion which I believe cracked the original block and head, it may also have damaged the carb beyond repair, also because of the LPG kit the carb pipes may be set up wrong, I have a head breather coming from the rocker cover this meets a “T” piece that goes to the pipe that runs from the oil bath to the carb and the third leg runs to the base of the carb, does anyone know if this is the correct set-up? I disconnected this pipe and blocking with my finger resulted in the engine running smoothly although I couldn’t maintain it as it was a three handed job, I cut a setscrew to length to block the hole to see if that made a difference but couldn’t start it at all after that, reverted to the pipe still cant start it, this is where I called it a day

So after a days work I am still no further along, I guess the next step is to change the fuel lift pump and then probably the carb

[Dano]

just ordered a replacement pump from DLS (looks like it replaces the pump with a bowl) and a new zenith (Indian copy) from Dingcroft

if that doesnt slove it it gets a v8 with No2

[Gazzar]

I had similar problems with my 2286, however my problems were more to do with poor petrol (vehicle lying up too long) and, perhaps more relevantly to you, perished connectors on the vacumn advance.

The reason I am suggesting you check the connecton is that when you blocked the breather vent from the rocker cover you may have increased the vacumn, and thus the advance. Perhaps?

There are two rubber pipe connectors - one on the carb and one on the dizzy. My engine has a metal pipe between the two, although I was told that this is uncommon, and the pipe is usually plastic. Having the dynamic timing right will help.

I also replaced the o- ring on the breather cap, this allowed me to have the vacumn system as standard.

By the way the other pipe on the tee connector went to the servo on my late series 3.

There are two positions on the zenith conrol lever - make sure you have connected throttle cable to correct one.

To check the fuel pump I disconnect the fuel pipe at the carb and get someone to turn the engine. Glugs of fuel indicate that the pump is ok.

Take all precautions necessary for glugs of flamible liquid splashing about!

I suspect you are close to success so peservere!

G.

[Dano]

is anyone able to confirm that this pump is a direct replacement for the pump with a bowl and that no new pipes etc are required

thanks

[Dano]

this is the exact setup I have so it cant be that the pipes are wrong, it must be the carb, dont think its timing as the engine has run, the symptoms keep changing evertime i dismantle the carb, i can only hope a new one sorts it out

[Gazzar]

Just checked on mine: that's the set up.

My earlier post was wrong, there is a seperate line fron the inlet manifold to the servo.

G.

[Dano]

Right just fitted the new fuel pump, I tested the old one and although it was holding a vacuum the new one is performing much better as you would expect, I do wish I have gone for the old type pump though as for some reason I do like the glass sediment trap especially as it splits out water etc, I now have a filter between the pump and the carb which is nice because you can see if fuel is getting to the carb

The new zenith arrived, it came very well packed and looked very good quality, I opted to fit it without taking it apart to check the float, hoping that it would run fine and there would be no chance of damaging or changing anything unnecessarily, if the float needs readjusting I can do that anytime, bolted on and primed with fuel, the engine fired up but I still cant get it to tick over below 1000 and its really lumpy and smelling rich, there is a real burbling from the exhaust, I couldn’t get it to run on the idle circuit (i.e. without the throttle butterfly open) so the idle mixture screw had no effect on the running, I think the carb is performing better than the old one i.e. no fuel comes from the emulsion tube when the throttle is closed (even though the engine stalls just after that) so that is a good sign

What else to check now? Could it be the dizzy, turning the dizzy has a very lazy effect on the running i.e. 15+ deg either way has little effect past that conks out, I would expect a significant change in running for every couple of degrees, the dwell is a constant 57deg with no change surely any wear would be shown in the dwell, could it be I have the wrong condenser or points? (I will check this) it has a new coil, I cant remember if the leads are new but they look it, the dizzy cap could do with a change but I have seen much worse on an engine running perfectly

Or would you suspect valve timing, on other threads I said I had problems timing this engine until I got a new cam pulley with markings on it, could it be the cam even, the valves should be lashed correctly (the tolerance on the rocker cover states 0.10 hot or cold so I am sure that any little change since I lashed during the rebuild it would have no effect) I didn’t check the cam also I didn’t lap the valves in which I should have done at the time

[Dano]

Having slept on the problem and dreamt a few solutions I will start looking for leaks around the manifold, I will try the wd40 trick but I also have just got a vacuum gauge/pump does anyone have any advice for searching for manifold leaks with this?

[SteveRK]

By disconnecting the breather pipe from the inlet manifold (under the carb mating face) what effect does this have when either the pipe end is blocked, or blocking the fitting where it is screwed into the manifold?

This may help in establishing whether the engine is lacking air or fuel.

Part ERC2875 will likely contain a spring and ball bearing valve that opens at idle (maximum vacuum) and closes as the throttle opens, fumes are then drawn through to the engine via ERC2871 a larger diameter pipe.

I therefore am assuming ERC2870 pipe is connected to the rocker cover?

Someone also has previously mentioned the vacuum connection for the brake servo - remove and plug this connection on the manifold along with any other vacuum take off points to isolate vacuum leaks allowing an alternative path for air to enter the engine than via the carb'

[Dano]

By disconnecting the breather pipe from the inlet manifold (under the carb mating face) what effect does this have when either the pipe end is blocked, or blocking the fitting where it is screwed into the manifold?

This may help in establishing whether the engine is lacking air or fuel.

Part ERC2875 will likely contain a spring and ball bearing valve that opens at idle (maximum vacuum) and closes as the throttle opens, fumes are then drawn through to the engine via ERC2871 a larger diameter pipe.

Since I fitted the new carb I couldn’t get it to idle so I checked for leaks around the manifold and base of the carb and found no leaks,

The fitting ERC2875 on my carb is just a pipe with no spring and ball, I pulled the pipe and blocked it, as I had before, this time with the new carb it idled well immediately, I was convinced that this is the issue i.e. not enough vacuum to make the carb run on the idle circuit, too much air entering through ERC2875,

I had previously taken off a fitting from this pipe (see diagram)

I thought it was a filter but I found now it was a type of valve with a spring and diaphragm, I rigged it up but it had no effect unless I blocked the inlet so I removed it and just blocked the hole with a bolt, immediately the engine idles perfectly at 800 rpm with the idle speed screw all the way out (i.e. it wont idle any slower as the carb seems set up well) the idle mixture screw still has no effect… but that doesn’t seem to matter, idles well drives fine,

I rigged the pipe work from the rocker back to the main air pipe removing the “T” piece and all seems well,

I don’t like to bodge things normally but it seems fine, I can’t foresee any issues, if anyone knows how this should be set up please let me know

[SteveRK]

I think I follow your modification in that you have removed the idle breather pipe from the carb and put a bolt in the hole to seal?

If the throttle butterfly is fully shut then i would expect the engine to stall as it will not be getting any air. Strange.

Might be worth making up an adapter with small bore pipe to replicate the restrcition that the ball valve fitting would have introduced to see whether that works

[Dano]

I think I follow your modification in that you have removed the idle breather pipe from the carb and put a bolt in the hole to seal?

If the throttle butterfly is fully shut then i would expect the engine to stall as it will not be getting any air. Strange.

Might be worth making up an adapter with small bore pipe to replicate the restrcition that the ball valve fitting would have introduced to see whether that works

when on idle, I suspect that with no pressure the valve will remain closed, as you can see from this diagram below, air should enter from 23 when on idle, this hole is above the throttle and I assume any leakage from below the throttle i.e. from ERC2875 would not allow 23 to draw sufficient mixture thus I would guess it would run lean and stall unless the throttle was open to allow fuel from the emulsion tube

Of course I am just guessing here so I have ordered part ERC2875 to see how it is constructed and what affect it has

[Dano]

Part ERC2875 came this morning, there is no ball or spring but the hole is about 1.5mmOD, much smaller that the other fitting I have.

I fitted it and put the pipes back as per the diagram, it fired up and ran fine on idle.

Again the idle mixture screw had no effect other than stalling the engine when removed fully, I don’t want to over tighten it either so it may be that it is not fully in therefore not affecting the mixture but if it runs ok who needs a mixture screw.

It seems I have resolved this issue for the time being, thanks for everyone’s inputs, and I hope this thread will help others as other threads helped me.

[SteveRK]

Good News!

If the idle screw isnt having any effect then looking at the diagram it would suggest that the hole that the end of the pilot screw fits into is blocked. Item 24 appears to show some kind of air bleed that would bypass the pilot screw hole - until the pilot screw is removed venting atmosphere into the carb negating any vacum suction acting on the hole/s at item 24??????????????

Sometimes when pilot screws are wound fully in the very end of the tip breaks off and plugs the hole.

Keep going we aim for perfection on here