Baffled by my Zenith carburetors - Help...!

[Luis A.]

OK, so I won't go to BMW Vintage Jail if I adjust both screws in either order until it works, WHEW! I'll try that.

I'm using my new timing light for reading the RPM's and cross-referenced it to an older Sears meter and they are both the same.

Man, I love Heathkit (I put together one of their ham radio transceivers in '78), wish they were still around.

 

[hans3]

1.) You mentioned you adjusted timing, but timing will certainly influence the idle RPM. Especially if you have a vacuum retard connection. Earlier versions of the Blue Book aren't that clear about disconnecting and plugging vacuum lines when setting timing at 1700 RPM. There's also a way to fine tune idle timing (once the 1700 is set) by turning a screw on the dist. vacuum diaphragm.

2.) Per another responder’s post, I'm amazed that wet sanding the carb deck with fine grit actually got it true. I used a steel file on mine and couldn't make much progress.

 

[hans3]

...
The original spacer- it had paper gaskets on both sides.

Per the attached diagram, the paper gaskets on spacer #53 are not visible. Were they originally glued to this spacer?

 

[61porsche]

Yes, they were/ do become glued.

When cleaning, you can notice the paper. Probably a cause or contributing factor to air leaks when they get wet as moisture can seep in the paper. It's hard to notice, but it is the actual seal with the phenolic spacer to both sides or both sealing surfaces since the final finish is not as say the head gasket sealing surface.

It's something that older Porsches suffer from as well with solex and zenith. ( I just happen to have an engine apart for service now after 35 years and carefully scraped all of that paper off the manifolds and carbs.)

I don't have a problem with using the newer gaskets; just torque and retorque after initial setting since they compress more than the original and seem to set better with a retorque.

 

[Luis A.]

1.) There's also a way to fine tune idle timing (once the 1700 is set) by turning a screw on the dist. vacuum diaphragm.

So what should the proper advance be at idle speed and how would I set that timing, since the "ball" on the flywheel is the only timing mark?

Where exactly is the screw on the diaphragm. I've never noticed it.

 

[Luis A.]

After adjusting the carbs to the best of my ability following all the sage advice on the board, we promptly went on a 1,000 mile circumnavigation (almost) of Lake Michigan as part of an event with the BMWCCA. The car performed very, very well and I had plenty of seat time to further diagnose any remaining flaws in my tuning.

There still is slight and short-lived hesitation in the transition from throttle plate just a bit open to opening it fully. It was noticeable if at highway speeds I would completely lift off, then slowly start pressing down on the accelerator and it would happen after pressing down on the pedal just a little (maybe less than 1cm) but not as soon as I pressed. So it happens I think as the tiny openings, above the main idle opening, in the barrel start getting exposed. I cleaned those and I know they were flowing well (test with compressed air after cleaning). And I think my idle screw is turned out past 2 turns which I thinks makes it pretty rich. Both accelerator pumps were working well when I put the carbs back together but I didn't measure the output in cc, lacking that cute little curvy cup. Could this be it?

Other than that, the throttle response is phenomenal. It responds strongly and WOT application is a real trip with all swooshing air sounds emanating from up front. :-D

Speaking of sounds, the original fan clutch and red fan are working just fine and I can clearly hear when the fan is engaging and disengaging almost as if an electric fan that is turning on and off. You can clearly hear the fan sound (with windows open).

Does anyone have a photo of the phenolic spacer with the o-ring installed on the idle mix hole? Is it two o-rings one on each side of the spacer?

 

[61porsche]

Try this:

Hood open, door open observe the linkage at each carb when it moves by pressing the accelerator.

The rear carb has an adjustment wheel to fine tune each carb to react the same by each linkage.

What happens normally is you're working on the carbs and watching from the wrong end- the rear carb.

What really happens is the thottle is controlled in the front to rear so the bar needs to be balanced by using that thumbwheel. ( Sort of like balancing a pencil on your finger) Just make sure they both move at the same time.


Tip- always note what rpm with a symtom. That tells you what part or subsystem needs attention.

As to the cup/ measuring vial- a kids eyedropper, cut off, sealed, and a short piece of wire pierced through the end to hold it works fine. The important part is were they equal first and a stream; not a dribble.

 

[whalem6]

Zenith

Have you tried spraying some start fluid at the back of the carb, where the throttle linkage enters the carb? The linkage can sometimes wabble and cause a vac, leak.
Tom

 

[hans3]

So what should the proper advance be at idle speed and how would I set that timing, since the "ball" on the flywheel is the only timing mark?

Where exactly is the screw on the diaphragm. I've never noticed it.

It's model dependent. For a 2800 USA with vacuum advance+retard it's 0-5 deg ATDC @ ~900 rpm. There is a TDC mark on that flywheel. There's a small screw + locknut on flatside of dist. diaphragm; it's used for adjusting vacuum retard. If no retard feature is present, then only the 1700 rpm timing is deemed relevant.