Yesterday my car started hesitating badly. Iddle was erratic and low. Well, I went under the hood and discovered a probable cause of my carbs' nightmares. I checked the fuel filter and found that it is was full of orange rust flakes which means a rusty fuel tank...
I replaced the filter but, in order to solve the problem, I will now need to have the fuel tank cleaned. Is this something for a DIYer or should I have the tank rebuilt, considering that it is probably rusty?
I know that some specialized companies can do this faily well. Also, I noticed that my old rubber/fabric braided lines are wet. It is probably a good oportunity to replace them. I suspect that an obstructed fuel return line could be the cause of my carbs running rich.
What do you think?

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280S, 1969
250C, 1972

Maybe if the return line was plugged, yes. But a plugged supply line would make it lean, no?

Get someone to boil the tank for you!

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Well, I have some news. I found fuel dribbling from the center nozzles in the venturi (rear carburator). Also, the fuel return valve was leaking. I had to tight it at little bit to stop the leak. I replaced the fuel filter and cleaned the air filter (oil bath type), but the car is still running very bad (high fuel consumption, hesitation and unstable iddle).
What can I do to fix the problem in my rear carb? Is it a simple task or do I need to overhaul the entire rear carb. I am not very experienced with these things and do not have a tachometer at hand, so I am thinking of having the carbs checked and adjusted by somebody who really knows what he is doing.
I will also have the fuel lines replaced and the tank cleaned.
Any suggestions?
Thanks.

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280S, 1969
250C, 1972

You sure love those Zeniths. I see you have two cars with them. I don't believe you can swap them though. I don't know about other countries, but here the barrel size is different. The 69 has 35/40s, and the 72 has 32/40s. I would not recommend having the tank redone except as a last resort. Mercedes tanks rarely rust, because of the quality of the finish inside. If you boil it you may destroy the finish. Also thefuel outlet system in the bottom of the tank is a complicated system of chambers which may become plugged if you try to coat it with some liquid material. You can remove the trunk mat, the plastic cover in the trunk floor, and the fuel sending unit. With the tank almost empty, you should be able to inspect the bottom of the tank through the hole.

There are two possible reasons your rear carb is slobbering through the main jet. Either the idle jet is plugged or the carbs are way out of sync. Based on the rust you found I'd say plugged idle jet. You might be able to clear it by rmoving the air correction jet and emulsion tube on the primary side and blowing compressed air down the hole. If you have junk in the carbs though, it's best to remove the air horns, sop up all fuel, let the bowl dry, and blow the dust out with air.

Peter

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Auto Zentral Ltd.

A plugged idle jet is easy to diagnose. If you look closely just to the front side of the primary barrel, you will see a small hole. With the engine idling, plug it with your finger or a pencil eraser. If the idle speed drops, the idle circuit is OK. If it doesn't change, then it may be plugged.

Do you have idle solenoid valves - these are little gizmos on the front base of each carb with wires running to them? If so, are they working?

Carb balance can be checked with by holding a 2 foot piece of heater hose next to the carb throat (where it meets the intake manifold) and listening to the sound of the air/fuel going into the intake. You can actually use this method to get the carbs very close in balance.

Another cause of the dribbling gas is a vacuum leak caused by the top plate warping due to overtightening of the screws or because of excess heat. This will cause poor running. If the warp is not too bad it can be corrected.

There are a lot of posts on these subjects and techniques, especially by dantrcav and cth350. More than a few by yours truly.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

I do not have soleniod valves in my carbs. I suspect that the cause of my problems is just junk in my carbs. You should have seen how the old fuel filter was...
With respect to cleaning the tank, I see that there are different opinions on the subject. Some have suggested cleaning and boiling the tank (ctaylor738) and others (Autozen) believe that this is not recommended.
If I had large flakes of rust in my fuel filter, don't you think that the tank is probably also full of rust?
If I have rust inside what should I do? and if I just have junk but no rust?
I think that if I don't clean the tank I will keep having all sorts of problems with my carbs.
You help is appreciated.

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280S, 1969
250C, 1972

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=7912437896&indexURL=0#ebayphotohosting

Kinda neat ebay item (w/ 2 NOS Zeniths). One of us has got to pick this up.

It sure would look nice in my living room.
(Or maybe I could just paint & polish my spare 220S engine!)

Happy Motoring, Mark

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DrDKW

Nice!! $750 is not a bad price for a brand new engine. I wonder whether it looks as well inside as outside...
By the way, what is the orange wire running along the carbs?

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280S, 1969
250C, 1972

280S:

There is one other item you might want to look at. Especially since you have apparently only had this problem since the engine was worked on (along with your disconnected choke rod).

The dribbling fuel can also be caused by a mis-adjusted idle stop screw which will cause the primary throttle plate to open enough that it will draw fuel from the venturi. This can happen when someone (the "shop") attempts to either balance or set the idle speed using the stop screws rather than the star wheel adjustors.

A quick test of this is to disconnect the linkage between the two cabs then back out the idle stop screw on the carb SLOWLY. If this corrects the problem you are only "half way home" and will have to readjust the idle using the star adjustors on each carb. Keep the linkage between the two disconnected.

You need to have some way to check the balance between them and the "hose test" works fine.

If backing out the idle stop screw has no affect then set the screw back to its original position.

Just another item when working with Zeinths.

Good Luck

Dan

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Dan Taylor/ Tulsa, OK MBCA
'84 300D/'90 Jaguar XJ6/XJ40

Chuck,
Does the hose have to be exactly 2 feet long? If it is a few inches longer or shorter, does the hose have to be recalibrated?

I'll go along with your advice except the warped air horn. As I recall this would introduce what the Germans called false air. I don't think it would allow the extra fuel. In fact if the air horn is warped, you check it with the method you described. Sticking a pencil tip in the small air hole will cause an increase in RPM.

Dan,
You seem to know these crbs well also. Good point about the idle stop. I had forgotten that.

280S,
Have you tried checking the tank like I suggested yet? Also if you do this on a sunny day, you can harness the power of the sun reflecting bright light into the tank with a mirror.

Peter

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Auto Zentral Ltd.

I chose two feet because that seems to be a good distance for the average American male to stand and reach the carbs without bending and having excess hose flopping around. Or it may be that it's what happened to be laying on the shelf. :-)

I think what happens is that if the plate is warped, additional air enters the carb body and the vacuum/venturi effect of that air sucks gas out through the nozzle rather than pulling it through the idle circuit. I have seen the warping/dripping on a couple of carbs that had good idle circuits and were closely balanced.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

With due respect to all the very sound advice offered by others, I suggest that much of the general "problem" with Zeniths is due to poor application of the basics when adjusting the units. It sounds to me like this may be true in this case. I believe the reference to misadjusted stop screws, while valid as a potential problem area, is symptomatic of a basic failure to balance the units properly. This may have been what was attempted by whomever was foolish enough to have possibly misadjusted the carb in this manner at the outset. That said, I strongly recommend "taking five" before any further adjustments are made and using this down time to thoroughly review the information in a Haynes or Chilton manual regarding basic adjustments. These manuals are essentially regurgitations of the factory information, and are actually quite helpful when studied with a patient eye and an ocassional reference to the engine in the garage.

Each of the previous suggestions made for possible causes of the basic problem is valid and deserves consideration, but I would suggest first a review of the basic settings for float, choke, cleanliness (spray some cleaner into the various orifices and jet passages with the top off while you are checking floats), etc. Once the engine-off adjustments are reasonably close, try to balance the units with a proper Uni-syn tool, which must be used with an adaptor bonnet to get acccurate results.

My attempts at using the old hose-in-ear technique have been miserable failures probably due to my amateur status as a mechanic, so I would anticipate most other amateurs to have similarly poor results. In 30+ years of dealing with the Zeniths on my old crock I found balance to be the most difficult quality to obtain until I gathered the proper tools, and once they were in-hand, those tools made it a non-issue. I then began to spend my time keeping things clean and tight so there were no leaks.

In general, Zeniths can be a PITA without proper tools. I found some of mine at a Solex dealer in Vienna many years ago. I assume there is a state-side supplier who can provide them for the right price. I paid very little, but the previously quoted cost of $800 to set up Zeniths may be a clue as to the true worth of the skills for this almost lost art.

More ramblings from a disorganized mind, for what they are worth...

Good hunting,

230/8

p.s: Sure wish I had some extra $$ laying about for that 250 engine setup, too sweet for words, but beware Ebay, some one is lurking in the bush who will bid the item up to the clouds just before the bid closes, and you can take that to the bank...

230/8 -

Question -

Does your engine have the problem with the heat risers sticking open? This is what causes the top plates on the 130's to warp.

The heat risers stick in the open position and direct hot air from the exhaust manifold up into the intake. The risers are supposed to close as the engine warms, but most often they don't. The resulting heat causes the top plate to warp, vacuum leaks, backfiring, and the dreaded idle drip.

The most cost-effective fix is to jam the risers in the closed position.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

ctaylor738:

I have no carb (low carb?) problems with my old crock...at least none that I consider needing attention. To your question, my Zeniths are fine and seem to be warp-free, but yes, the heat risers have been a perennial problem. I now use this old car very little and only in summer, so to relieve myself from the drudgery of constantly lubing the heat risers to keep them free, I have fixed them in the open position. Now, I use this term advisedly since my definition of open is the normally open running position that does not divert hot gas across the carb base.

The engine runs a bit dodgy for a few minutes when stone-cold, but quickly warms to its task and then is just fine. When the riser would stick the engine became cantankerous on hot days, with the usual dire consequences awaiting at stop lights and when using AC. The stuck full-hot position was what I found over 30 years ago right after I bought it. That is when I first began sorting out the little mechanical problems on the car.

I once obtained replacement bi-metal springs hoping this would ensure rapid opening of the flapper and thus more reliable operation over the long haul. No such luck. The replacements were much too strong and never seemed to relax their grip on the flapper so that it would rest in the "open" straight-through position. Kept the intake manifold mucho hot at all the wrong times. Hence the decision to forego manifold heat.

Hope this answers your question,

Best regards,

230/8