[BMWCSRegistry] Trouble in Zenith-land

[knowtree at aloha.com]

cgshawaii at netscape.net wrote:
> Synched the carbs last weekend using my UniSyn and a homemade adaptor. 

I have had no luck with a UniSyn. Maybe back in the 60s they were made with
adequate precision, but the one I bought last year looked like the mold
plug was hand carved from a bar of soap by a blind, one-armed man missing a
thumb. I prefer to listen to the sound of the air hissing, using a two foot
length of rubber hose. Fuel line is abount right, large enough to carry the
sound but not so large it interfers with the air flow. Blow the spiders out
before you stick it in your ear.

> Idle and about 2000 rpm okay. 

See comments below on warpage.

> Tried out a bit of 1/8 inch tubing connected to the vacuum ports, 
> with some ATF.

What are you trying to accomplish here? I have never worked on this type of
carb. Something to do with secondary activation?

> Noticed the rear carb secondary drawing quite a bit of air at 2500, 
> and that the synch previously set goes all to heck around 2500. 

That would indicate a difference is linkage setup between the two.
Secondary opening is based on throttle position. 
 
> The car has been running hideous rich, exhaust reeks, 
> so something's wrong. 
> 
> I came on the comment that the Zenith castings warp if tightened too
> much. I suspect that's what I've done. 

This sounds like a myth. Carb housings can warp but I believe the cause is
uneven heat and uneven torque on the hold-down bolts. Overtightening will
either break a stud or crack the flange.

The usual result of warpage is an air leak at the mounting flange. This
results in an overly lean mixture. Typically it will be impossible to get
the idle speed low enough, because as the throttle is adjusted to close,
the fuel supply is diminished and the mixture becomes too lean to burn.
This condition has little impact at higher throttle settings, where the
bulk of the charge is coming through the throat and receiving gas. I do not
believe a warp would result in a rich condition.

The simple way to check for a flange leak is to squirt oil aound the joint.
If the idle drops and/or smooths out momentarily you have a leak. Sometimes
you can see the oil being sucked into the joint.

> I have a "spare" pair of carbs from a 2800 sedan
> that have warped castings. 

You can practice on those :-)

> 
> Now what to do? 
> 
> 1. Disassemble, try to straighten/sand the castings flat, 
> then reinstall? Dunno if the warp can be ground out or 
> if it's worth it. 

I think you have a linkage problem and something else causing a rich
mixture. These carbs have air jets and other bits that control mixture, and
these can get plugged over time. I would do a complete tear-down and re-build.

> 4. Suck it in and buy 32/36s from somewhere, or if lucky, 38/38s, 
> and linkage? I surely distrust used 32/36s since they wear badly 
> at the throttle shafts - won't hold steady idle then. 

Like I said, too lean. The air sneaking in at the shaft does not pick up
gas from the venturi. But it takes a lot of wear for this to be a problem
.. another myth, perhaps?

> New seem about $250 per. 

And you can get them jetted to match your application. 

You need to match this solution to the over-all nature of the car and your
goals. If you are trying to make the car really nice and reliable, buy new
Webers. If you like fooling around with stuff, do one of the conversions or
swaps you mentioned. I would start with a re-build, with thorough cleaning
and careful adjusting.

Gary Dunn
Honolulu
73 BMW E9 (3.0 CS) 2213583 (rust repair research project)
    http://e9erust.blogspot.com/
74 Lotus Europa TCS 4654R (on the road)
    http://www.flickr.com/photos/garydunn808/