Zenith Secret Revealed

deQuincey

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From coupeking in an earlier thread: "Primary secret is NOT to overtighten the vertical screw that goes at the very top of the carb and into the air cleaner assembly. Overtightening the nut holding the aircleaner in place can separate the middle layers of the carb and create internal air leaks which are difficult to trace."
hummmm, i must check this,...:?:
 

61porsche

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Secret

to the air cleaner stud is that if you over tighten it'll strip out. Weakest link on the carb.


Secret number 2- after you've performed secret 1 check the throttle plates. Half the ones I see are loose or ill fitting. Just use a light and check. Adjust so it fits the bore best- No light.

Secret number 3- secondary plate should be closed- not letting air in or very little. The biggest problem with balancing isn't the screw that you can get to... it's the one under that NO one should have messed with but did. Test by putting the palm of your hand on that side and closing with the engine running. No way will you ever balance if this isn't equal and minimal. No amount of adjustment at the idle will fix this. The factory set these. ( I suspect with a flow bench.) Mercedes actually would stamp each carb- I've never seen it on BMW other than the aluminum tag.


Secret 4- the four screws holding on the throttle plate- use Loctite blue on them. Heat,, 40 years old, etc. - they will come loose and that's the big lean vacuum leak. If you suddenly go lean on one carb, then you'll know for sure.

Secret 5-the new plate gaskets aren't all they're supposed to be. Don't over tighten or under tighten. You'll squish the gasket and close the idle circuit. No, you're idle valve didn't give up the ghost.

Secret 6- use both gaskets on the manifold as a cheap heat sink. Or roll your own, but thicker. Mercedes used a heat sink plate.

That's what I know for the throttle plate section.
 

Honolulu

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So this thread comes back from the dead with a little pre-Christmas goose. Good things indeed to think about and tinker with. I know for sure that my secondaries are flowing a little at idle when they should not be, so I have work to do... as usual.

Thanks to all who contributed... if there are more, let's hear them.
 

Honolulu

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On a whim I did a search for "Zenith 35-40 INAT" which many of us have (or had) stuck on the tops of our intake manifolds. I came up with a document I've not seen before, titled "BMW Service Training School - Instruction Material Supplement: ZENITH 35-40 INAT Two-Stage Downdraught Carburettor"

It's 25 pages long with extensive discussion and diagrams. The title spelling could suggest it was made for the Brit market, but the last page says "Bayerischen Motoren Werke AG Service Promotion" so it looks the goods.

It sits (who knew? no one has mentioned it) in the Tech Section of this site...

But having read the beast, it's pretty thick and not easily comprehended though there are a couple good points if closely read. Additionally it treats two different carbs and two variations.
 
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