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  #1  
Old 02-16-2000, 12:07 AM
wjbell
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I've been having a problem with acceleration on my '71 250C (what haven't I been having problems with!). The carbs have been rebuilt, no vacuum leaks, and tuned - idles perfectly with no RPM fluctuations. When I accelerate, I have to be very gentle on the pedal or else the engine chugs and surges and sometimes if I don't let-up, it will finally start to accelerate but with hesitations. It smoothes-out when I back-off the pedal and accelerate slowly again. I suspect the distributor vacuum advance? I looked at the 3-way valve and it clicks open/close when I remove the electrical lead, and am assuming that the lines are connected correctly based on the letters stamped by the connections: S=vacuum, F=advance, and U=retard? At idle, the RPM drops when I remove the advance line, so everything appears right. The timing does advance when I increase the throttle, but not much faster than when I increase the throttle with the advance line detached. With the engine off and I apply vacuum to the advance line, there isn't much resistance from the distributor, and the push rod moves just a little. Am I not applying enough vacuum or could the dist. diaphragm need to be replaced?
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  #2  
Old 02-16-2000, 09:02 AM
LarryBible
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WJBell,

It's been a long time since I drove a carbureted Benz, but I've messed with lots of carbureted cars of all kinds.

Although your vacuam advance may not be working properly, although you didn't say anything to make me believe that it isn't, I believe that your problem is one of two things:

It could be retarded timing, although it sounds as if you have set the timing, or;

The most likely problem is the accelerator pump in the carburetor. When accelerating lightly, the accelerator pump has virtually no effect. The accelerator pump squirts raw fuel in the airstream whenever the throttle is quickly opened. Whenever the throttle is quickly opened, there is a sudden inrush of air. If there is not a large addition of fuel at the same time, the engine will "starve for gas" and stumble.

Also, if everything else is correct, you could disconnect the vacuam advance and during acceleration, you would never know that it is disconnected. This is because the vacuam advance does not add timing unless you are not accelerating. When you are accelerating, there is low, or no vacuam. The vacuam advance is actually an economy device. Whenever you are cruising with a slightly opened throttle, the vacuam increases, which advances the timing. The advanced timing while cruising, provides lower fuel usage.

The easiest way to check the accelerator pump, is to remove the air cleaner, peer down the carburetor with the choke open, and of course, the motor not running. Snap the throttle open quickly, and you should see a stream of raw gas out of the accelerator pump nozzle. A flashlight is handy for this test. Remember that your dealing with gasoline, so it wouldn't be a good time to have a lit cigarette in your mouth.

Good Luck,

------------------
Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in
Mercedes automobiles
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  #3  
Old 02-16-2000, 09:08 AM
LarryBible
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WJBell,

One other subsystem of your carburetor could be contributing to your problems.

In all carburetors there is a system with various names, depending on the carburetor, which enrichens the mixture during acceleration. At curise, with a high vacuam, it will get fuel only from the main jet(s). As you accelerate and the vacuam drops, the enrichment device adds fuel to the mixture to richen it during acceleration. If this is not working properly, the engine can stumble. This would probably have show itself more in a slow opening of the throttle. The accelerator pump problem that I described in my previous message would show up more when "snapping" the throttle open.

Good Luck,

------------------
Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in
Mercedes automobiles
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  #4  
Old 02-16-2000, 12:11 PM
wjbell
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Is that "enrichener" a series of vacuum passages, or is there an actual device? Also, for the first time in a long time today, I also got a few backfires through the carb, so I'm thinking that the intake/exhaust manifold gasket could be the problem? The problem is so bad now that I can't accelerate much past 50mph, and takes about 2 minutes to get there (I can't drive her anymore because I'm tired of the honks and fingers, like I'm doing it on purpose!). I'm almost positive that the carb/manifold mounts are airtight.
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  #5  
Old 02-16-2000, 02:49 PM
LarryBible
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WJ,

As I said before, I'm not familiar with the specific carburetor your car has. The enrichening system, sometimes called a power valve or economizer valve, can be built in several ways. It will probably consist of a diaphragm that is vacuam actuated (to be precise it is actuated by the lack of vacuam causing it to enrichen the mixture). There is at least one carburetor that I know of that uses a piston and spring in a bore instead of a diaphragm.

You need a good manual on that specific carburetor and/or do some exploratory surgery. If you have never really messed with carburetors, you may want to enlist a good fuel system mechanic for the job.

I read one time that carburetor was a french word that meant "don't screw with it".

Have you checked to see that the accelerator pump is squirting fuel when snapping the throttle open?

Good Luck,

------------------
Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in
Mercedes automobiles
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  #6  
Old 02-16-2000, 02:53 PM
LarryBible
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WJ,

Well I can continue to have additional thoughts after sending you messages. I happened to think, my wife had a '72 or '73 250 many years ago. If I remember, it had two, two barrel progressive Zenith carburetors. Is this the same arrangement as yours? If so I have a Haynes manual somewhere covering that car. I don't remember if it covers the carburetors very well. I do remember that the carbs were extremely difficult to remove, but reasonably straight forward to overhaul.

Let me know if this the arrangement you have, and I will try to do a little research for you.

Good Luck,
Larry
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  #7  
Old 02-16-2000, 03:15 PM
wjbell
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Thanks Larry, I do have the Haynes and it covers most of the important carb tuning, etc. The engine does have the dual-carb layout and the more I re-read your posts, the less I think the problem is with the advance (thanks for getting me back on track). The vacuum lines were incorrectly attached to the 3-way valve, however (I hope someone knows German to tell me if the F, S, and U stand for what I think they do). I just pulled the rear carb off on Sunday to re-clean all of the passages and bowl and am thinking that I may have not perfectly alligned the throttle housing/bowl housing gasket. You're right, the carbs are a pain to remove because of the awkward mounting bolts, but I guess I have no other choice but to start eliminating possibilities....
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  #8  
Old 02-16-2000, 03:20 PM
wjbell
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Forgot to ask about the enrichener. The only thing I can think of that sounds remotely similar to what you describe is a diaphragm unit on the rear carb that receives vacuum from the 2-way valve. I believe Haynes refers to it as a throttle dampener (or something), though. Could this be what you were referring to?
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  #9  
Old 02-16-2000, 05:30 PM
LarryBible
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WJ,

If the diaphragm is ONLY on the rear carb, it's probably not what you're looking for. It is what the American MFG's called a dashpot to let the idle speed down slowly.

Keep studying the carburetor, there is something somewhere that is actuated somehow by vacuam and richens when there is low vacuam, and leans when there is high vacuam.

The Zeniths are somewhat complex because they have a primary and secondary on each carburetor that work like the primaries and secondaries on an American four barrel.

Keep on searchin' and don't give up, you'll find it.

Good Luck,
Larry
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  #10  
Old 02-16-2000, 05:32 PM
LarryBible
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WJ,

Here I go again with second thoughts. There is a website called babel-fish.com which can interpret text from any language to any language. Pretty handy for interpreting German to English if you know what the words are that correspond with the letters on your valve.

Good Luck,
Larry
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  #11  
Old 02-16-2000, 10:04 PM
wjbell
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Larry, I'm stumped. The accelerator pumps are OK, re-checked the carb/manifold mounts for true, checked gaskets and all openings are aligned, primary/secondary and idle jets are clear, bowls are sparkling clean, air filter is 2 weeks old and no obstructions in the cleaner, vacuum advance OK and apparently working, timing/dwell/idle mixture/plug gap/idle RPM/carb balance all right on the money - so what gives?
Please give input as to the likelihood of the following unchecked areas:
* bad coil
* bad intake/exhaust manifold gasket
* what am I missing?
If a suspect is the intake/exhaust manifold, I've looked at the mounting bolts and they look extremely awkward to reach - any tips to make removal easier? I'm keeping a stiff upper lip here, but my persistence had better start showing some results soon!
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  #12  
Old 02-17-2000, 09:17 AM
LarryBible
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WJ,

I believe that if you had a manifold vacuam leak, the engine would not idle. Since you've tried your accelerator pumps, and you said that your centrifugal advance is okay, it still points toward your power valve(enrichening system). Have you been able to identify the portion of the carb(s) tha provide low vacuam enrichment? With the symptoms you continue to describe and the things that you've checked it still seems that this is where you problem lies.

Don't get discouraged. I've been through things like this before, and with my personality, they continue to gnaw at me day and night. Hang in there and keep investigating.

I have to run right now for a meeting which will last most all day, when I get a little time, maybe tomorrow, I'll reread all your e-mails regarding this problem and try to think it through some.

HANG IN THERE AND DON'T GIVE UP
Larry
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  #13  
Old 02-17-2000, 09:37 AM
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Join Date: Oct 1999
Location: Tulsa, OK USA
Posts: 139
WJ:
I have a 71 250C also. I replaced my Zeniths with Webbers (eventually) but have rebuilt them (Zeniths) in the past. "Assuming" the car did not exhibit these characteristics before you rebuilt the carbs, and nothing else was changed, then you can pretty much figure the problem is in the carbs/manifold mounts. Did you use new insulating blocks when you put the carbs back on? If not, did you clean the old gasket material from the blocks and make some new ones? Two other areas to check. All the rebuild kits I have seen for these carbs have more than one gasket for each mating surface. They look pretty much identical BUT THEY ARE NOT. If you don't use the proper one you may block an internal passage and/or open an area that needs closed. The top (cover) plate warps due to people over-tightening the aircleaner screws. This causes air leaks. You can surface them by working them over some 300 to 400 wet/dry sandpaper on a FLAT surface (bench). When the surface of the plate is all clean and shiney, they are surfaced. It is also possible that the metering jets and/or air correctors are reversed. Note. the air corrector stamped "4S" is for the PRIMARY side NOT the secondary side ("S" is not for secondary).
One last item. These carbs need to be "flow balanced" to work properly. The amount of air through each carb should be equal at idle. I have had strange things happen when the were far our of sync. Best way to do this is with a flow meter but you can get close by using a short piece of heater hose from your ear to the PRIMARY on the carb(s). Try to get the sounds balanced. If you have one carb rich and one lean nasty things happen.
Hope this helps a little.

BTW. Go buy a cheap 13mm open/box combination wrench and cut it in half. Makes carb removal/replacement much easier for the inboard studs.

Dan

------------------
Dan Taylor/ Tulsa, OK MBCA '71 250C/'81 300TD-T

[This message has been edited by DANTRCAV (edited 02-17-2000).]
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  #14  
Old 02-17-2000, 02:20 PM
LarryBible
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WJ,

Let's hear three cheers for Dan Taylor.

You found someone familiar with the breed.

Good Luck,

------------------
Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in
Mercedes automobiles
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  #15  
Old 02-17-2000, 04:24 PM
wjbell
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Thanks Larry and Dan. Dan, now I know why you switched to Webers! I'm going to do a complete tear-down (again) tonight on the rear carb, which is the one I re-cleaned right before all of this happened. I have a feeling that it's something really dumb and obvious.
Dan, as far as the carb balancing goes, I bought a single balance meter and rigged a plastic margarine tub to fit over the carb. JC Whitney also carries a carb balancer for about $20-30 and they claim really does the trick - any experience with these? Any other tips so that I can rule-out mis-tuned carbs?
Larry, I've been thinking of the enrichening system in this way (since nothing obvious from my re-build memory comes to mind): the system would require a vacuum passage and a fuel passage. The best vacuum source and fuel mixture outlet I would think would be at the throttle body, but I just can't recall a fuel source from the bowl, or even a fuel/air mixture from primary or idle jets, that's connected to the throttle body. There's a passage for the idle fuel cut-off solenoid, but nothing else is jumping out at me. Looks like it's time to go back and map all of the passages because this is one area where Hayne's just doesn't cut it!
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