Hi fellow benz-heads,

Been awhile since I posted here but I've still got my old fintail and I'm having an issue with the choke pull-off on the rear carb. It's not too big a deal but it does mean that when the car is in mid-fast idle mode it has a bit of a rough time idling at that higher speed because it's loading up a bit on the back cyls. It doesn't stall but I wouldn't try driving off like that either. Luckily the front pull-off is working fine so at least the fast idle speed cam is where it is supposed to be during and after start-up. Once warm enough it's fine of course so it ends up being a minor issue.

For the uninitiated, the zeniths have three idle speeds, very fast idle for when cranking and immediately thereafter when cold, mid-fast idle for once it has started when cold and regular for once it's warmed up enough. In practice the very fast idle, which is spec'd around 2200 rpms is not really seen because the choke pull-off pulls it back to mid-fast idle around 1600 rpms as soon as the engine fires (basically that very fast "idle" is only to have the throttle opened wide enough to start with the choke snapped shut)

All that is done by the front carb, which on my car is operating flawlessly. The rear carb has no idle cam at all and the choke only cares about the flap itself. Currently the pull-off diaphragm appears to be shot because it doesn't do anything and the rear carb choke stays closed (hopefully it's the diaphragm and not the little vacuum passage being plugged). At any rate, how do I get this thing out of there? I can undo the three screws at the top of the pull-off and remove the triangular cap with the adjustable stop but i'll be damned if I can figure out how to get the rod that actually does the "pulling" out. If the round choke cover/heated spring is removed from the side I can see that the business end of the pulloff rod has a flat in it that the choke pivot sits inside. There doesn't seem to be a way to actually remove it past that flat, not enough wiggle room, anyone done this?

Regards,

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1967 230S automatic
Boston, MA

I had to "flex" the arm slightly to get them out of the Zeniths on my 68 W110.

On my car the choke idle adjust screw is removed from the rear carb so fast idle is controlled for both carbs by the front choke.

The diaphragm pops open the choke slightly after starting, once the diaphragm sees vacuum, to allow the car to idle. I found that this was not operating correctly on mine after a carb rebuild. The flat spot on the diaphragm rod was in the wrong place by a large amount, not enough adjustment on cover, the diaphragm was working but never contacting the arm to crack open the choke flap, thus the car would not idle till the choke cracked on warm-up. Maybe the kits were for a different year Zenith INAT or the carb kit manufacturer just made them wrong? I was able to swap out the rods from my old diaphragms, bit of a fiddle, and they worked perfectly.

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1968 230 W110 +3.5 M116 & 4-Speed Manual
1985 300TDT S123
1985 500SEC C126

Wow, thanks pedro, that's really interesting because I have two new carb kits as well and I could swear the area of the rod flat looks a little different from what I'm used to... I'll definitely have to pay attention to this when I install the new pulloff diaphragms. My fast idle is all on the front carb only as well, I think that's the factory set up. I'll try giving the bad pulloff rod just a little extra muscle to get it out. Thanks for the reply!

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1967 230S automatic
Boston, MA

Well, I just went out and futzed with it a little bit and installed my new thermospirals and retainer springs on the nice new freely functioning exhaust flaps. I have to say, these DO make a difference at least on my car in this cool humid day here in new england. My car can come off of choke much sooner than before when the flaps were sitting permanently in the non-heated state. Now that the thermospirals hold the flaps in the heated state while cold it seems the mixture has a much nicer time making it's way to the cylinders!

Anyway, I've postponed the rebuild of the choke pull off and the general carb rebuild until it's a bit sunnier out this weekend...

I think there are so many separate little systems on the zeniths that one or two can fail and not really degrade the running of the car all THAT much until eventually, perhaps years later, another thing finally goes wrong and it becomes REALLY noticeable. That causes you to fix the obviously malfunctioning bit thinking "aha! I've found it!", when in reality that was just one of the many fine tuning features that the carb has available and the real source of the problem is perhaps a different system or something even more basic. At any rate, I really like the zeniths, even in their slightly sub-optimal state mine are currently running better than the webers ever did. At their core they are fairly simple once you see past all the little "add-ons" and odd methods of adjusting balance/idle speed/etc.

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1967 230S automatic
Boston, MA

See your other thread, http://www.peachparts.com/shopforum/vintage-mercedes/367299-w111-m180-zenith-air-corrector-jet-mystery.html

Posted a link to some good tech info.

My heat risers were so rusted and stuck in the partially open position that I finally gave up trying to free them up and slightly bent the plates closed. I made some steel block off plates to prevent heat getting to the carbs. I live in NC so extreme cold is not an issue, I do see it would be better to have functioning risers further north though.

I think the Zeniths are a nice carb. You do read lots of complaints and recommendations to switch to Webers but once you do the homework and understand all the intricacies the carbs can perform very well. My car idles and runs very smoothly, good power, no stumbles, decent fuel economy... it will be even better once I get the mixture a bit more even between the carbs. My father had a 67 230 W110 back in the 70s and when he tuned the carbs you could balance a nickel on end on the valve cover at idle. I am almost there.

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1968 230 W110 +3.5 M116 & 4-Speed Manual
1985 300TDT S123
1985 500SEC C126

I would like to revise my last post in this thread about the thermospirals... It seems that while they DID make an appreciable difference in the cold running of my car, allowing the mixture to be fairly perfect during warm up, they also weren't closing off the way they were supposed to once warm. Major bummer since these things cost me quite a bit from Mercedes-Benz Classic Center.

I also installed the little half-moon shaped retainer springs as well that go over top of them. My flaps turn nice and freely with the original weights in place so they should've worked. The thermospirals worked nicely to keep the flaps open heating the carb bases up but wouldn't shut closed when fully warmed. I noticed that the retainer springs fit VERY tightly so I thought they were restricting the action of the spirals. I tried bending the retainer springs a little so they fit a bit more loosely and this definitely helped. The flaps seemed to move freely once again, however, even then the spirals didn't seem to have enough strength to close off the flaps. I'm not sure if it's weak spirals from the classic center or just a lousy design, either way I've once again removed them so my flaps can stay closed. This means I've had to tweak the chokes back again too. It's a shame these didn't work out, at this point I can't say that I recommend trying to get them to work

Maybe it's just that I am not understanding the original functioning of the heat riser system. I read somewhere that the exhaust itself, the gasses flowing through the manifold are actually supposed to PUSH open the flaps. If this is the case, maybe mine is alright and at warm rest or idle they are supposed to sit basically open as they do when the car is cold and only pushed closed when the engine is revved up enough. I should try putting them back on and revving to 2500-3500rpms and seeing if they get pushed closed (or "not heated"). The whole thing does seem kind of odd, I'd hoped that the thermospirals themselves would be enough to close off the flaps, which would prevent roasting the bases of the carbs and boiling the fuel in the bowls/etc.

Meh,

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1967 230S automatic
Boston, MA