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ZENITH TWIN 35-40 carb question

Started by peter_h_killara, 17-06-2026 04:42 PM. 14 replies. 348 views.

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17-06-2026, 04:42 PM

peter_h_killara Member

Join Date: Feb 2018
Location: Sydney NSW
Posts: 214

More help needed please.

Each of my Zenith carbs (35/40) has 2 butterfly flaps towards the base deep down in the carburetor body. Picture attached.

Now I've read that the butterfly flaps should open and close freely when the throttle is depressed. So far so good.

Question is should both flaps on each carb open and close when the throttle is depressed or just one?

In my picture attached it shows (engine cold) throttle depressed and only 1 butterfly flap opening. The other one is staying shut.

Is this correct or are they both meant to move simultaneously?

Note: I have yet to clean the mechanism. Was thinking ATF might be the go.

Cheers, Peter.

17-06-2026 05:32 PM

Hussein_N_Sefton Member

Join Date: Mar 2015
Location: Sefton NSW
Posts: 239
Car: 1968 W108 250S - daily

Hi,

Im running Zenith 35/40 carbs on my 250S and they work correctly.

Yes, with the engine cold there should normally be a small free gap between the spring and the lever on both the carb valve spring and the vacuum control lever. They should not be preloaded at rest.

Your starting point in the picture looks close to correct. Final adjustment should be done with the carbs installed, engine warm, and balanced according to factory procedure.

Hope this helps..

17-06-2026, 06:18 PM

Vasilios_Billy

Lifetime Benzer

Join Date: Mar 2008
Location: Campbelltown NSW
Posts: 1,699
Club: M.A.L.A.K.A.S.
Car: 2020 AMG GT C

Peter - yes, what your seeing is normal.

Zenith 35/40 INAT is progressive. small 35mm primary opens first for idle / cruise. larger 40mm secondary stays shut until about 75% throttle, then the side linkage pulls it open.

Cold engine and choke on will make it look like only one flap works. warm engine, choke off, pedal to floor, second butterfly should snap open.

Both flaps should move smooth by hand. test the secondary linkage with engine off.

ATF is ok on linkage pivots after penetrating oil. dont soak the carb body. varnish on throttle plate edges needs carb cleaner, not ATF..

18-06-2026, 09:18 AM

Hugo_Darlinghurst Senior Member

★ Gold

Join Date: Feb 2010
Location: Darlinghurst NSW
Posts: 524
Club: Mercedes Classic Drivers Australia (MCDA) - chapter committee
Car: 1968 W108 280SE 5.0 Coupe

To add to Billy's reference - the Zenith 35/40 INAT was fitted to the W108 280SE 3.5, W109 300SEL 3.5, and W111 280SE 3.5 Coupe / Cabriolet 1969-1972. The 35/40 designation refers to the primary venturi diameter (35mm) and secondary venturi diameter (40mm). The progressive design is documented in the Mercedes-Benz workshop manual section 07.1-216 "Vergaser Zenith 35/40 INAT - Schwimmerkammer und Drosselklappenmechanismus."

The mechanical secondary engagement point is factory-specified at 70% +/- 5% primary throttle position. If yours is engaging earlier or later, the secondary linkage rod may need adjustment - the rod is held by a clevis pin and a brass nut at the top of the linkage. Adjust in 1mm increments and recheck.

Peter - if you can post a photo of the linkage rod from the side of the carburettor, we can confirm whether yours needs adjustment or is correctly set.

18-06-2026, 11:42 PM

Wilfried_Stuttgart Adelaide 1973-1998 / Stuttgart since

Join Date: May 2012
Location: Stuttgart, Germany
Posts: 1,418
Club: M.A.L.A.K.A.S.
Car: 1985 W124 300E

As it is a merc... yes... the Zenith 35/40 INAT is a progressive design and Billy's explanation is correct 🤭

One thing to add Peter - the SECONDARY butterfly is OFTEN seized on cars that have sat for many years... the primary butterfly opens because it must open every day... the secondary butterfly only opens when you drive WOT which most owners don't do... so the secondary butterfly shaft can develop varnish around the bushing and the butterfly stops opening even when the linkage pulls it.

Easy test: remove the air filter, with engine off, depress the throttle pedal to the floor while watching the secondary butterfly. If the LINKAGE moves but the BUTTERFLY does not move, the butterfly shaft is seized in the bushing... 😋

ATF cleaning will fix this if it is not too far gone... if the butterfly is welded shut from years of varnish you may need to remove the secondary butterfly screws and the butterfly plate and clean it seperately... do not lose the screws they are unique to this carburettor and not available...

19-06-2026, 03:18 PM

peter_h_killara Member

Join Date: Feb 2018
Location: Sydney NSW
Posts: 214

Billy, Hugo, Wilfried - thank you all. That makes complete sense now and I feel daft for not knowing.

Tested with engine warm and choke off as Billy suggested. Pedal to the floor and YES - the secondary butterfly snapped open at about three-quarters travel exactly as described. So both my carbs are working correctly, I just wasn't pushing the pedal far enough during the cold test.

Will still clean the linkages and shafts with ATF as preventative since both carbs are getting close to 55 years old.

Marking solved. This forum is gold.

20-06-2026, 02:14 AM

Boonie_W108 Senior Member

Join Date: Feb 2014
Location: Marrickville NSW
Posts: 1,418
Club: M.A.L.A.K.A.S.
Car: '67 W108 250SE

BOOKMARKING THIS.

MINES A 250SE SO DIFFERENT CARBS (SINGLE BARREL SOLEX) BUT LEARNED SOMETHING TODAY ABOUT THE 280SE 3.5. CHEERS BOYS.

Sent from my TeleVoice Pro

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