[BMWCSRegistry] Re: Uni-Synchronizing Zenith Carbs (long)

Tue May 1 16:00:27 EDT 2007

Because of the center mount screw for the air cleaner, and because the choke butterflies stick up above the throat of the Zeniths, you can't use a Uni-syn without an adaptor.

I do the odd bit of fiberglass work (make surfboards in some off hours) so I made an adaptor from some scraps of cloth.  Basically it's a cylindrical section that fits over the carb where the air cleaner 0-ring sits, about 3 inches tall.  Over that I glassed on a dome shaped piece, with a 1-1/2 inch hole at top center.  Adaptor on carb, uni-syn on adaptor, and syncronizing proceeds.  With a little care and proper sequence (see below), you can adjust airflow in those carbs to a gnat's ass.  Whether the engine then runs okay will depend on a lot of other things, but airflow will be equal.

Others have used the proper sized plastic food container, with a hole punched in the lid or bottom, for the adaptor.  I could never find one that was satisfactorily stiff, so I did the glassing job.

HOWEVER you can do this much more easily using about six or eight feet of clear plastic tubing that costs maybe ten cents a foot.  Attach each end of your home-made manometer (that's the technical term for this device) to each carb vacuum port and hold the tubing up in an "M" shape, with the low center portion of the M filled with auto trans fluid, or colored water - something easily visible.  Start engine and observe the height difference on either side of the colored fluid.  Adjust.

Comment/caution: my experience also includes using the factory synchronizing tool, a box with two hoses that connect to the carb vacuum ports, with a little meter on the box.  The factory box is terribly sensitive, and useless for anything above idle speed.  So your experience with the manometer may not get you anything better than a perfectly adjusted idle.  I dunno.  Although I have the tubing I usually just use my hood and unisyn.

Tidbit:  adjustments is made in three steps:  first at idle with the linkage completely disconnected, second at idle with the linkage connected, and third at about 2500 rpm.

Tidbit the second:  there's a screw in the center of the linkage that will hold a particular speed for you.  Use it in the third step but don't forget to back if off again when you're done.

Tidbit third:  if you reassemble and the idle is way high but the idle speed screws aren't touching the linkage, make sure the rod coming up from the bellcrank on the side of the block isn't too long. DAMHIK.

T 4th:  Motor's gonna be good and warm for synchornizing, as it needs to be.  You'll have adjusted thte valve lash the day before, when the engine was stone cold, and set points gap and timing (static at TDC is good enough) the day before you pulled the carbs.  Have a towel to lay over the valve cover so you don't pan-fry your hands.  Use a short straight screwdriver on the idle speed and mixture screws.

T5: When you reassemble, don't mess with the fast idle screw inside the choke housing, you'll regret that.  But if you do anyway, after all the adjustments are done to a warm engine, take off the choke housing and set the fast idle speed screw to the highest point on the cam.  Engine speed should be about 1700 I think.  This is the cold (fast) idle speed and though you will spend very little time on choke with cold idle, you can synch the carbs under that condition too, even with the engine warm.

T6:  DO NOT OVERTIGHTEN the screws that hold the carb castings together.  If you do, you will likely contribute to warping these castings, and the carb may leak internally and/or externally, contributing to that gas smell that all coupe owners know and love.  I have a spare set of carbs whose bodies are pretty warped, and I know mine aren't real straight either.

Maybe you can R&R the carbs and not need synchronizing.  I doubt it.

With all this advice, my car still runs like shite, partly cuz the cold idle setting is all to hell and I have to go back in (T5, above).  But by the time I get down the hill I live on, things are mostly warmed up, and I'm getting onto a downhill section of "freeway" that's moving at 15 mph anyway.  I just coast.

I'll probably think of more stuff later.  Makes you think that Ljet or Motronic isn't such a bad idea at all.

Charlie 

-----Original Message-----
From: spanlab at entelnet.bo
To: bmwcsregistry at idb.ded.forest.net
Sent: Mon, 30 Apr 2007 5:25 AM
Subject: [BMWCSRegistry] Synchronizing Zenith Carbs

Hello Folks, 

I will be removing my zenith carbs from my 3.0CSA 73 for cleaning. I am concerned that after I put them 
back on they will be out of sync. Must I purchase that funky setup with the dual caps & needle meter to 
sync them. I have a sync meter for my Porsche? 

Thanks, Enrique 

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