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Old 08-02-2005, 12:37 AM
djd djd is offline
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Join Date: Apr 2005
Posts: 54
Zenith part needed and advice

Hi guys,
I'm getting into the carbs on a '67 250S automatic with the dual Zenith carbs. The PO must have been "fixing things". The front carb had only one "Mixing Tube" labeled 4N. The rear carb has two tubes - one is a 4N and the other is a 4S. Shouldn't both fronts be 4S and both rears 4N?

If so, I need a second 4S for the front. Anyone have one available?? Who is a good source for rebuild kits? (Any chance the mixing tubes would be in a kit? Ha!)

Also which jet should go where? My Chilton manual is less then clear as to which jet is which. "Idle Fuel Jet", Idle Air Jet", "Air Correction Jet", "Main Jet", ...which is which and what number goes where?

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Old 08-02-2005, 08:35 PM
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Join Date: Dec 2000
Location: Long Island, NY
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The zenith rebuild manual is still available from mercedes. See the classic center's portion of the web site. It should be up on the web too.

The photo quality is poor, but the instructions are fine. If this is your first time, I'd purchase a few spare assembly gaskets for the upper and mid-level of the body. They're a few $ each. I presume fastlane can supply them. The first time I did a zenith setup I went through three sets of those gaskets.

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Old 08-03-2005, 12:10 PM
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Join Date: Mar 2004
Posts: 758

The air jets are the large cylindrical jets that fit into the carb top. You removed them to get at the mixing tubes. Smaller number in primary and larger number in secondary. If your air jets are flat and fit flush to the casting, not the large cylindrical stand-up style, they may have been replaced at some time. Either will work, but I think the tall style is more efficient and less likely to plug with the crud that collects on the carb top.

The main jets are the larger of the three in the fuel bowl. Again, smaller number in primary and larger number in secondary.

The last jet is the transition jet. It is the smallest of the bunch and can go only one place, the small threaded hole next to the secondary metering jet.

If you need mixing tubes (emulsion tubes) then you may need to scrounge some bone-yard carbs to salvage parts. Not a bad idea regardless...

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Old 08-03-2005, 05:55 PM
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Join Date: Oct 1999
Location: Tulsa, OK USA
Posts: 139
Each carb will have mixing tubes of different sizes. My carbs had 4Sin the primaries and 4N in the secondaries. Note - the "S" DOES NOT stand for Secondary.

I will look when I go home and I just might have a 4S mixing tube in my spares box.

I will let you know.

Dan Taylor/ Tulsa, OK MBCA
'84 300D/'90 Jaguar XJ6/XJ40
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Old 08-03-2005, 11:08 PM
djd djd is offline
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Join Date: Apr 2005
Posts: 54
Thank you for the replies. It's starting to make a little more sense to me. I found a link to information on Zeniths in the BMW web site and this helped clear up the definitions.

I have a lathe, so I made up the missing Mixing Tube by copying dimensions from the existing one. Once I got into it, it went pretty fast. However, I'm not clear which tube goes where. My Chilton book says the 4S goes in the front carb and the 4N goes in the rear. Is this wrong? Should each carb have one 4N and one 4S?

Also what jet numbers should I have in the front carb and what numbers in the rear? The PO (the same one that forgot to install a mixing tube) may have mixed things up.
Thanks again for the help.

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