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  #1  
Old 01-19-2003, 08:51 AM
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Location: Werribee, Victoria AUSTRALIA
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1972 Merc 250 - Zeniths & Webers

Hi from Down Under,

We've got a 1972 Merc with a 130.923 engine and the dreaded Zenith carbies. The engine has been rebuilt (home project because the cost would have been prohibitive at the Merc dealer and the mechanic who originally started the job was incapable of finishing the job). We're on the home stretch now dealing with the carbies. The Merc has been off the road for the last 2 years. Our only mistake was having yet another mechanic (who was obviously not a Zenith expert) do the final tune. That happened before I found this website and saw the advice about keeping non-experts as far away from Zeniths as possible! The car starts immediately and idles fairly quickly (auto choke) and then suddenly dies. This seems to occur once it has warmed up. It can be restarted but the accelerator needs to be depressed in order to stop it from stalling.

We have put premium unleaded high octane gasoline in the tank (Shell Optimax). When the car was on the road, I used to fill it with leaded petrol, now Australia no longer has leaded petrol but lead replacement petrol and for the last 2 years or so we've had premium unleaded petrol (98 octane) in the market place. We filled the Merc with Shell Optimax and I'm hoping that this isn't contributing to the problem. I remember that my Merc was very fussy on the fuel and that I always filled at the same petrol station. What octane level are people using? Have we done the right thing?

I'm hoping that something has been wrongly adjusted by the mechanic and that we can rectify it by following much of the advice that has come off various threads on this site. I've also got a copy of the Zenith carby manual - via a post on this site which contained the link.

However, I've had carby problems for a number of years and even the dealer has never really got them right (with lots of $$s having been spent). This website has been fantastic and it is a pity that I found it only recently as it really would have greatly assisted my father who has done a fantastic job with the engine (and the Haynes manual). He also agrees that the manual can be very misleading.

We've seen much of the advice and comments on Zeniths as well as the comments on Webers and looked at the Jam Engineering site. We are just exploring options at present given the carby problems. Can anyone advise what cars have had Webers that could be used to replace Zeniths? What model of Weber would suit? What manifold would fit a 130 engine, that would also suit Webers? Any information on conversion would be very welcome. Visits to wreckers yards in Australia may be our only option due to Australian exchange rates. Any views?

In my surfing (not this site), I came across a post from an Australian BMW owner who had Zeniths (with lots of trouble) and had changed them to Webers. He had made a comment that Zeniths were unsuited to the heat we experience in Australia and more suited to colder European climates. He was very impressed by the Webers. That view, in conjunction with some of the feedback we've seen on the Webers (and conversions), makes us think that this may be the way to go.

Any assistance would be most welcome.

Regards, Jennifer & Tom
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  #2  
Old 01-19-2003, 09:54 AM
LarryBible
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I think that you have been misled about the Zeniths. IMHO they work well.

I think your biggest challenge is that in this day and time there are more people that understand electronic fuel injection than there are that understand carburetors.

Every carburetor has basically the same subsystems. If you understand these subsystems, you can reverse engineer most any carburetor.

I have not had my hands on a set of Zeniths in probably fifteen years, but as I recall, the biggest challenge was getting them off and on. I had to make a special wrench to reach underneath.

I think you should try to find an old timer that has rebuilt and successfully tuned a zillion carburetors. It doesn't matter if he has ever seen or heard of a Zenith. If he understands the principles of carburetion he will be able to figure them out.

I still have a pair of carburetor kits for those Zeniths if you can't find them anywhere.

Good luck,
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  #3  
Old 01-19-2003, 02:34 PM
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I might take slight exception to Larry's advice about any tech with zillions of hours on carbs. that person will be able to fix the problem but it will take a MB zenith expert to possibly do it at a price that doesn't resemble medical research.

From your decription you aren't running on the idle system. Look down the throat of the carb. Can you see gas when its running coming from the main venturi (you probably can)? You should not see gas coming from the venturi at idle, absolutely NONE!

Idle is affected by a number of conditions on those carbs. One is they have electrical solenoids shutting off the flow - verify. Two the secondaries are vacuum operated and often stick partly open. Destroys idle. Third, like all carbs the idle passages are the smallest, easiest to plug passages in the carb. And fourth the idle circuit passes through the top cover for an air bleed. The top cover is unvariably warped on these carbs and the top gasket doesn't seal the area around the airbleed. This in effect makes the airbleed vastly bigger and the result is that only air is pulled through the idle passages. This can be fixed by either sanding the top on a piece of flat glass (till the area around the airbleed is cleared) or by double gasketing the real bad ones. To do this I cut the center section out of a second gasket as the center is the area that is raised.

There is every bit as good of a chance of making the zenith run as an old weber. The JAM kit seems only a problem due to money. If you are thinking that other used junk is an alternative you will be faced not only with old carb problems but the problems of re-engineering.
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  #4  
Old 01-19-2003, 07:50 PM
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I have one additional suggestion.

Check to see that the idle stop are correctly adjusted. Normally this is set on the front carb only. There is an adjuster with a lock nut and you may need to pry the cap off the top to turn the adjuster. Once off cold idle, this is what controls idle speed.

You may have float housing vent valves. These need to be adjusted to open the correct amount when the actuating levers are against the idle stops.

You may need to pry the cap off the top of the carb(s) to adjust the idle stop.

I would not give up on your carbs. They fact that you have a good cold start and warm up without backfire means that they are in basically good shape.

The key to keeping them that way is to make sure that the heat risers are not sticking in the open position. This is what causes the warping. MB sells a rebuild kit for the risers. If yours are sticking in the open position, turn them manually so they stick closed.

I have spare parts if you need them. Good luck!
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'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
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  #5  
Old 01-20-2003, 08:08 AM
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Join Date: Jan 2003
Location: Werribee, Victoria AUSTRALIA
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Merc 72 - Zeniths and Webers

Hi,

Thank you all for the responses and the offers of spares. I'm passing the advice onto Dad and we'll keep you posted of our progress; at this stage we won't give up on the Zeniths.

Thanks, Jennifer
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  #6  
Old 01-26-2003, 08:01 PM
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Re: 1972 Merc 250 - Zeniths & Webers

A couple of notes I have after working on my Zeniths is this..
how's your muffler? Is it old and crusty? If so it may cause you problems with idle.

I could not get my car to idle smoothly until I replaced the muffler. Likewise leaks in the vacuum effect air pressue in the carbs as will leaking or blocked exhaust lines and mufflers.

With the engine cold, start the engine and put your hand under the carbs and feel for air movement that is not coming from the fan. Do this quickly has the exhaust manifold warms up pretty fast and will burn you. Then run your hands near the exhaust pipes and feel for leaks.

I may be incorrect but I think the test to see if your carbs do have leaks is that if you put a folded shop rag over the top of the car then the idle should go down and if leave it on it should stall the engine. If there is no change in the idle then the carb is still getting air from somewhere.

I have also found that using premium gas really does nothing for your car except burn money.

Since you are in a hot climate then the heat risers in the exhaust manifold that are supposed to warm up your carbs should be permanently closed otherwise they will warp your carbs or warp them again. If the rear carb feels much warmer than the front after the car has been running for awhile then the heat riser is probably stuck open and has probably warped the rear carb. Neither carb should be so hot that it might burn you but if it is then it is probably warped.
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  #7  
Old 02-01-2003, 06:04 PM
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Thanks Gmask,

I'll pass your advice onto Dad. The Merc had a new muffler approx. 8 years ago, and in that time probably travelled 20,000 miles - but we'll check for leaks.

The removal of leaded petrol from the marketplace has been a challenge to owners of older cars particularly where the cars have engines which don't have harded valve seats; in these instances you must buy an additive to add to each tank of fuel to protect the valve seats. Lead replacement petrol (LRP) came into the marketplace (as an alternative for cars which run on leaded petrol and can't/don't want to run on unleaded) but it will be removed from sale in a few years. People who use LRP pay extra for their gas - equivalent to about 20 US cents per gallon.

When my Merc was on the road, it ran on leaded petrol and it hasn't been on the road for a couple of years so I've no experience since then with the new fuel.

Thanks for your assistance, Jennifer
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  #8  
Old 02-01-2003, 10:33 PM
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Quote:
Originally posted by tom&jen
When my Merc was on the road, it ran on leaded petrol and it hasn't been on the road for a couple of years so I've no experience since then with the new fuel.

Hmm.. well I'm under the impression that this is not an issue with this car. Somebody lese here may beable to verify that but they haven't sold leaded gas nation wide for some time here and I don't recall it ever being areccomneded to add it for cars from the 70's.
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Old 02-20-2003, 07:29 AM
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Hi gmask

Apologies for the delay in responding, have had computer problems at this end.

The Shell Petroleum website (www.shell.com.au) states that no pre-1976 Mercedes can run on unleaded petrol. Furthermore, if a car uses their Optimax fuel, and it doesn't have hardened valve seats one application of their 'Valvemaster' product is required per tank of petrol.

But, have you (or anyone out there) had experience in using the JC Whitney Carburetor Synchronizer (SKU 88ZX2792N) - this is the direct reading, dial type one on their web site? We intend purchasing one from the USA because very few people here understand what a carby synchronizer is (believe it or not) let alone have them for sale. The only thing available in Australia appears to be a poor quality English plastic unit. We'd be happy to hear of any tips in the using the Whitney product which appears to be of high quality. The unit appears to have a rubber skirt and fits a variety of carby shapes.

Dad checked the carby and we have no leaks on the carby so I'm still hoping that it's a simple adjustment.

Regards, Jennifer
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  #10  
Old 02-20-2003, 08:29 AM
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The Whitney unit has been very highly recommended on the mercedes-veterans.com board.

You can also get the carbs very close with a length of heater hose or an automotive stethoscope, listening to the noise at the base of each carb and adjusting the idle accordingly.

E-mail me if you need the adjustment procedure off the CD.

I have run unleaded in my coupes for years with no problems.
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'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
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  #11  
Old 02-20-2003, 08:35 AM
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You can also stick a ball point pen into the idle air bleed hole to get a real good measure of both balance and idle strength. Finds the warped tops right away.
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  #12  
Old 02-20-2003, 01:19 PM
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>>The Shell Petroleum website (www.shell.com.au) states that no pre-1976 Mercedes can run on unleaded petrol. Furthermore, if a car uses their Optimax fuel, and it doesn't have hardened valve seats one application of their 'Valvemaster' product is required per tank of petrol.

Hmmm that's news to me?..I would rather trust a source from Mercedes as obviously Shell wants to sell their products. I dunno?

Can anybody else confirm this?

>>>But, have you (or anyone out there) had experience in using the JC Whitney Carburetor Synchronizer (SKU 88ZX2792N) - this is the direct reading, dial type one on their web site?
The unit appears to have a rubber skirt and fits a variety of carby shapes.

I have two of these and they are good but you also need the hood adapter and offhand I can't remember where I got that but it wasn't from Whitney.
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  #13  
Old 02-24-2003, 08:20 AM
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Thank you all for your advice. I've just ordered the JC Whitney carby synchronizer and passed on the posts to Dad. He's located the idle air bleed hole on the carby but hasn't yet checked the balance and idle strength.

I re-visited the Vintage Mercedes forum and gmasks' thread "Diagnose Dual Zenith Carbs 72 250 engine" which identified Baum Tools (www.baumtools.com) as the supplier of the carburetor hood but also provided details about adapting a Rubbermaid food storage container to do the same job.

Thanks, Jennifer
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