Carburator

nico13

nico13

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Hi All,
I am trying to adjust the CO of my Zenith carburettor, but the CO will not get above 1% at the outlet of the front 3 cylinders?
Do I have an air leak or could there be something els wrong in the carburettor?
Does anyone have a suggestion.

Best regards,
Nico
 
Assuming you have all of the obvious vacuum line leaks plugged and have checked the booster hose for invisible leaks, you need to spray some carb cleaner around the carb base gaskets. If the idle goes up, you'll have to pull the carb(s), remove the studs and lap the bowl bottom, throttle plate and the intake manifold until they're flat and regasket. Even when they were new the Zeniths sucked in the paper gasket between the 2 barrels and caused the problem you have.

Another quick test is to block off each secondary with the back of your hand and see what happens. If one of the carbs is feeding idle with air from the secondary, it'll show up. Unfortunately if both base gaskets are bad, this test is worthless.

People that spend a lot of time working on old carbs like to point at Zeniths and laugh. We had a 55 gallon drum in the corner of our shop where we threw them.
 
I am a fan of the much maligned Zenith INAT 35/40. When rebuilt, trued like Andy says (including the top piece), and adjusted to spec, it is a great carb. I've had both the dual Zenith and Webers, and I'd take the (properly tuned) Zeniths anyday.
 
Hi Andy,

A day before I could adjust both carb on a 1/4 turn accrued of the mixer screws with the set up as in the picture. Today I checked the CO again but no result and the idle speed was not smooth.
I did the test with carb cleaner but did not gave a change as such I used break fluid.

Now the front carb increased the idle when by spraying on right hand of the CO screw.

But still you may think that the back carb can be adjust as it has no leak I think.

What do you mean with "the secondary" in the quick test do you mean the inlet of the second stage of each carb. The idle should drop (no air) then that is good??

Great to have this site where people are helping each other.

Best regards,
Nico
 

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Dear Andy,
What do you mean with "the secondary" in the quick test do you mean the inlet of the second stage of each carb. The idle should drop (no air) then that is good??
I have rebuild both of the carburettors.

Best regards,
Nico
 
A day before I could adjust both carb on a 1/4 turn accrued of the mixer screws with the set up as in the picture. Today I checked the CO again but no result and the idle speed was not smooth.
I did the test with carb cleaner but did not gave a change as such I used break fluid.

Now the front carb increased the idle when by spraying on right hand of the CO screw.

But still you may think that the back carb can be adjust as it has no leak I think.

What do you mean with "the secondary" in the quick test do you mean the inlet of the second stage of each carb. The idle should drop (no air) then that is good??
================================

If the carbs were good one day and then not the next, unless you had a big backfire, the poor idle on one carb is probably something else like a split vacuum line and not a carb suddenly changing. If your car has smog fittings and steel lines under the carbs on the manifolds, check for new pinholes and corrosion there.

These carbs are so sensitive that EVERYTHING else must be perfect before the adjustments start, so compression, valve adjustments, plugs, timing, carb balance and linkage, etc. all must be done first. Only then can you find the small problems you are chasing.

Assuming you have everything perfect on a perfect motor and have removed the linkage from both carbs, backed off the choke fast idle cam screws and the idle stops, balanced both carbs and evened the idle mixture and throttle stop screws at some matching adjustment, you can warm up the motor and then start the final tune. Set the idle by adjusting both carb idle screws EVENLY - maybe even check the balance again if you have the hats - until you have an idle around 900-1100 and then try the mixture screws EVENLY.
If everything is perfect, you'll get what you want. If something is different between the 2 carbs (leak, clog, dirt) it will show up now.

Brake cleaner works fine. The fact that the idle went up when you sprayed the base gasket on the front carb is exactly what you are looking for. If you block off the secondary inlet with your hand (the inlet without the choke) and nothing happens, then the carb base gasket is good and all the air for idle is coming in through the primary intake. If the primary barrel is pulling air from the secondary through a torn base gasket, the idle will drop when you block the secondary inlet (or the secondary butterfly is stuck or bent.)

Nothing personal about these carbs, but I've been working on them since they were new and it's not a coincidence that they were only used on a few models of car from one country. If they were as idiot-proof and tunable as a Weber or DellOrto, they'd be a sought-after retrofit for everything everywhere.

They are beautifully made, I'll give them that, but WAY too complicated for what they do. Keep in mind that from 1968 on, both MBZ and BMW (a baby company in 1968) were up against loony US smog laws that changed almost every year and the Zeniths (and BMW's infamous "1974 thermal reactor/air injection system") was an attempt to provide "non-injection adjustable injection." Didn't work and everyone went to real injection as soon as possible.
 
Hi Andy,
I did the test with blocking the secondary carb, what append is that my hand was soaked with fuel and the ref went down almost out.
Does this mean that both main inlets are pulling air from the secondary carb??
Does this mean that both carb having a leak in between the carbs?

I have no problems working on these carbs as they are originally used. New carb are too expensive for me.

Best regards,
Nico
 
nico13 said:
Hi Andy,
I did the test with blocking the secondary carb, what append is that my hand was soaked with fuel and the ref went down almost out.
Does this mean that both main inlets are pulling air from the secondary carb??
Does this mean that both carb having a leak in between the carbs?

Hi Nico,

Putting the heel of your hand over the secondary intakes for 2-3 seconds should not put fuel on your hand or affect the idle at all. If you hold your hand there for a long time, it will mess up the carb and idle, but not if you do it briefly.

It sounds like you have a lot of air flow in the secondaries. Try to get the idle down to 1000 to work. It could be many things...are you sure ALL the adjustments are backed off, including the little fast idle screw inside the hot water jacket? Did you take the butterflies out? Please say no. If yes, are you SURE they're closing properly? Is the secondary mechanical linkage working? Push down on the secondary butterfly with a soft tool with the engine running to make sure it's really closed. If all the mechanical stuff is good and there are no vacuum leaks at the hoses, then yes, you probably have bad base gaskets. If you replaced the paper gaskets on the insulator and throttle plate and then cranked down the 4 screws, you may have bowed the already-warped seal face and it may be leaking.

Really the carb/brake spray is the best test. Put the little red wand on and spray a LITTLE juice all around the carb base and try to see where the leak is.
If it's inside the carb, you'll get nothing but usually one side or the other will raise the idle.
Click to expand...
 
deQuincey said:
+1

zeniths are great ! it is only a matter of time and dedication
have you rebuild them ?
Click to expand...

Yes, I rebuilt my pair for the 3.3L five years ago. I took everything apart, cleaned with carb cleaner and compressed air and put it all together again. Dial them in took half-an-hour, runs great and make good power since.

A dual Weber-conversion will always be the quick and dirty fix compared with the more complex but also better Zenith ;-)
 
DerSchwede said:
Yes, I rebuilt my pair for the 3.3L five years ago. I took everything apart, cleaned with carb cleaner and compressed air and put it all together again. Dial them in took half-an-hour, runs great and make good power since.

A dual Weber-conversion will always be the quick and dirty fix compared with the more complex but also better Zenith ;-)
Click to expand...

hi anders, i assumed that in your case, my question was intended for our friend nico
 
Dear Andy,

I did the test again with the carb cleaner there is no change in in the ref.
Does this mean an internal leak?
I will do the test with pushing down the secondary butterfly.

Thank you for the support,
Best regards,
Nico
 
Nico, I see your balancing rig, but have you tried blocking ALL vacuum lines and backing off idle - not idle mixture, but idle set screw - one at a time and letting it run on one carb? If it runs equally on each carb - even at 300-400 rpm, then you can poke the accelerator pump to add a little gas and see if the idle goes up or down. If one carb is too rich at slow idle and the other is too lean and both idle mixture screws are set equally, then you have leaks somewhere.

I'm a little confused that the spray raised the idle in one of your first tests and then didn't the next time.

Try to check everything outside first, as you'll remember that removing the carbs is nasty work.
 
Sorry of not mentioning, i have taken both carbs off the intake and the allen head screws where loosened of the front carb, that was the cause of the leak.
But still the co can not be adjusted and stay around the 1%.
Can i leave the hoses to the air filters open or do they need to be closed too?
There must be a big leak but with both carbs?
Or did i mis something during the restoration of the carbs?

Best regards,
Nico
 
I have turned both big screws in and for the start i have turned the 2 little screws two turns open, the co tester in the exhaust manifold of the front carb.
Co is not adjustable with the mixture screw. Stay at 1% should be 3%@800rpm
Balanced with the idle screws the liquid in the bottles .
It is now 23h13 and need som sleep will check next weekend again.
For now thanks for the support
Best regards,
Nico
 
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