Zenith Secondary Troubleshooting Please

[andyleonard]

I have a new-to-me 3.0 automatic with "rebuilt" Zeniths. The car runs and idles just fine but I cannot get the secondaries to open for love nor money. Nor will it rev over 4200rpms either static or under load. Both secondary diaphragms are good, newish, and free. They both pass the "hold vacuum with finger over little hole" test and secondary butterflies are free... so I'm becoming convinced what I have is a lack of vacuum in the carb body to pull the diaphragms. In both carbs.

Question: Is there an additional source of vacuum for the diaphragms other than the teeny weeny holes in the side of venturis? Is there somewhere I can't see where an incorrect gasket or other leak could deplete that particular vacuum signal and cause the secondaries to remain closed? On both carbs. The teeny holes are open and air passes through to the diaphragm area but cleaning the passages completely requires drilling plugs...

Question #2: Has anyone converted the Zeniths to mechanical secondaries? I've done the 2 barrel Solex on the 2002 with good success but never these.

 

[Thomas76]

Don't know anything about beemer automatics, but I think on old Ford's automatic transmission had something to do with secondaries on the carb?

 

[andyleonard]

Don't know anything about beemer automatics, but I think on old Ford's automatic transmission had something to do with secondaries on the carb?

I don't know much about the BW automatics either, except how much they weigh and how hard they are to get rid of after you swap in a 5 speed. And how 3 speeds just isn't enough. But if there's a connection between the carbs and the trans, it must be down low in the linkage somewhere because there's no cable or vacuum I can see up top.

I did look at the Zenith throttle plate and figured out that with 4" of tie wire you can take the secondary off the diaphragm (which is operated with a vacuum orifice smaller than an idle jet), wire it to the linkage and operate it with your foot. The appropriate progressive ramp is already there, so I'm guessing the vacuum doesn't suddenly snap the secondaries open when you want them but pulls the diaphragm back and ready so that when you push your foot past half-way the ramp opens the secondary. So you're not changing the progression of throttle application ...you just now have the ability to do it with no vacuum or a bad diaphragm.

Let's try that and see what happens.

 

[deQuincey]

I have a new-to-me 3.0 automatic with "rebuilt" Zeniths. The car runs and idles just fine but I cannot get the secondaries to open for love nor money. Nor will it rev over 4200rpms either static or under load. Both secondary diaphragms are good, newish, and free. They both pass the "hold vacuum with finger over little hole" test and secondary butterflies are free... so I'm becoming convinced what I have is a lack of vacuum in the carb body to pull the diaphragms. In both carbs.

Question: Is there an additional source of vacuum for the diaphragms other than the teeny weeny holes in the side of venturis? Is there somewhere I can't see where an incorrect gasket or other leak could deplete that particular vacuum signal and cause the secondaries to remain closed? On both carbs. The teeny holes are open and air passes through to the diaphragm area but cleaning the passages completely requires drilling plugs...

Question #2: Has anyone converted the Zeniths to mechanical secondaries? I've done the 2 barrel Solex on the 2002 with good success but never these.

it will be extremely difficult for me to discuss this
there is no need of additional vacuum, cleaning is easy and no drilling is needed
or you did something wrong, or your gasket is incorrect
in order to free the secondaries there is a need of release a mechanical blockage, there is a precharge in one of the lever screwstops
make sure that you do every little job on those zeniths

 

[Dick Steinkamp]

IMHO, the engine with no load (static) should easily rev past 4,500 and beyond (quickly) on just the primaries. II suggest you look at timing, throttle linkage hanging up before the throttle plates are vertical, valve adjustment, compression, vacuum leak.

Also, the vacuum operating the secondaries is not how we normally think about engine vacuum. We usually talk manifold vacuum which could never operate carb secondaries since it is strongest at idle or light throttle cruise and goes to zero with a heavy load on the engine. The opposite of what we need to operate the secondaries. That little pin hole in the venturi is what is producing the vacuum to operate the secondaries.

Here's a good explanation of the difference...

http://corvette-restoration.com/wp-content/uploads/technical_papers/vacuum_explained.pdf

 

[wilies13]

As Dick said already:
MHO, the engine with no load (static) should easily rev past 4,500 and beyond (quickly) on just the primaries.
Willy´s view: I suggest you look epecially at timing/ignition.

The secondaries will open only when 2 conditions are fullfilled in parallel:
a) vaccum as described before (correctly)
b) primaries are fully open (realized by some mechanical stuff)
I fear, that it´s not possible to bring secondaries to "open" without full load >1 sec

The 3rd thing I can mention:
Since when do you have the problem ? Since the rebuild carbs or even before ?
If it ´s correlated to the rebuild carbs, maybe there was even a mistake when rebuilding.....
Both carbs same effect ?

Have a lot of fun and success !

 

[andyleonard]

Thank you gentlemen. Please standby. Am awaiting parts, specifically new ignition parts (coil, points, condenser) and carb kits. Compression is 170, valves at .012, timing set by light with advance removed, plugs, cap and wires are newish, plenty of fuel. No linkage issues. Manifold vacuum at 16 each side.

Car is new to me so repair history is unclear. Previous owner had health issues and I am certain never went over 45 mph in the last 20 years; he used the car to go to the post office with his dog. So who knows when this started. Or when the carbs were rebuilt.

I have been on the freeway at 75 mph with the carbs like this, but any additional demand or hill/downshift does not trigger the secondaries. It would seem to me that freeway speed plus flooring the throttle would meet the specs for opening them.

I suspect a bad/incorrect gasket in the carbs somewhere as both carbs are behaving the same but I am failing to understand - based on where the vacuum for the secondaries originates and ends up - how an incorrect gasket lower down in the carb would affect the secondaries.

I have succeeded in modifying the carbs for mechanical-only secondaries in a non-destructive fashion... if this next round of repairs fails to operate the diaphragms. I'm also unclear as to why the vacuum secondaries are there in the first place. The popular Weber replacements are mechanical secondaries, yes?

 

[bavbob]

There is a rebuild process outlined in the technical section I believe. I would pull one and follow the process, you may find the issue. I am sure deQ will chime in but.......keep the Zeniths. With care, they are awesome.

 

[Honolulu]

There is a linkage issue that can prevent the secondaries from opening. From dim memory, it has to do with the stepped fast idle plate on the linkage, that's all I remember. A little fiddling with the air cleaners off and you should be able to identify it.

What to do next is a whole 'nother matter - I never found a way to verify the secondaries were opening in the static situation or running down the road, nor could I get them to open while bending over the hood. Didn't want to rev to redline in the garage, and I don't think the secondary venturies might have opened then anyway. Someone correct me if wrong...

I would REALLY not like to have to "go into" Zenith carbs again, given all I've read about their touchiness regarding fuel level in the float bowls and other matters. Surely once you know how, it's not difficult, but neither is flying to the moon. I've read what there is to see about them but I have never had a conversation with an expert, who could reveal the little bits and secrets.

 

[andyleonard]

Well, here are the mechanics of the secondaries opening: Only the primary will open - regardless of what the diaphragm's doing - until the linkage is at the VERY end of it's travel..and then the secondary pops open on the steep ramp.

Having failed so far to find out why the diaphragms on both carbs were not opening at full throttle under load, I'm going to reassemble the carbs with new gaskets and have made the following temporary conversion to mechanical secondaries...and if all is well then I'll clip the tie wire and reengage with the diaphragms...or not.


Both butterflies closed, ball arm at beginning of ramp
,

2/3 of linkage travel complete, primary partially open, secondary closed

Last 1/3 of travel finishes opening primary and VERY last bit on steep ramp opens the secondary

This ball is normally hooked to the vacuum diaphragm and now disconnected, operates with pedal only. Diaphragm can be mounted and simply disconnected from ball.
Secondary would ONLY operate previously with BOTH throttle fully applied AND vacuum diaphragm fully charged with vacuum generated by air moving past a pinhole in the side of each barrel and pulling up on this ball.

 

[deQuincey]

it will be extremely difficult for me to discuss this
there is no need of additional vacuum, cleaning is easy and no drilling is needed
or you did something wrong, or your gasket is incorrect
in order to free the secondaries there is a need of release a mechanical blockage, there is a precharge in one of the lever screwstops
make sure that you do every little job on those zeniths

i explained it here

 

[andyleonard]

I *think* where deQ is headed with the "free the secondaries" and "mechanical blockage" is not to the steep ramp at the end of the linkage travel which would not - if the main throttle linkage were out of adjustment - even TRY to open the secondaries ... but to the fabulous little piece of engineering that tries to make sure that the secondary butterfly is completely closed/no leaks ... but not too tight so it binds or sticks in the bore. There's a set screw on one end and a cool little spring-loaded ball on the other so you can set it JUST right. And not stick when the venturi-driven vacuum coming through it's tiny bleeder holes tries to pull the diaphragm and open the secondary.
If we ever needed a reminder about just how long the engineers worked on these carbs battling creeping smog laws (no coincidence Zeniths were on big BMWs and MBZ right up to FI) other than the astonishing overkill of having hot water, vacuum and electricity simultaneously controlling cold fast idle settings, this might be it. Big bucks and a lot of effort went into these carbs.


Spring-loaded ball is under the tie-wire and other set screw's at the right

 

[andyleonard]

OK I gave up on the diaphragms and wired the secondaries as above. New coil, points, condenser, both carbs ultrasonic-ed and carbs dipped with high pressure air after. Found nothing off with the gaskets, all sections of carbs were flat, lowered fuel level just for drill, dunno. I didn't find what I was looking for...unless it was foreign matter baked in the carb bodies
The car runs perfectly and secondaries are quite obvious when they open up. I like it.