Carburetor Flooding - '71 250c - Mercedes-Benz Forum

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post #1 of 7 (permalink) Old 10-26-2011, 08:42 PM Thread Starter
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Carburetor Flooding - '71 250c

I am having a time with these carburetors. After a careful rebuild and several subsequent teardowns, I still have flooding in the front carb. So far I've checked and/or renewed the inlet valve, jets and mixing tubes. I also blew compressed air through the return line and heard bubbling in the gas tank. I've adjusted both carbs four ways to Sunday. The car starts and runs, but when I look into the primary bore of the front carburetor, I can see gas blowing out of the feed tube so fast that it puddles on the butterfly. Surely this isn't normal. I'm about ready to go find another carburetor, unless someone can tell me what else to try. Thanks. -G.
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post #2 of 7 (permalink) Old 10-26-2011, 10:46 PM
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Hi Glenn. I could be the float.I hope you cleaned it as well. If to much crud has acumulated on it , it might not float enough to close properly. The butterflys should also be slightly open maybe 1/16 or so. Adjust your choke housing when cold to get some clearance. Did you check your idling jets? The needle opening should be 049 mm. Good luck. Just trying to help.
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post #3 of 7 (permalink) Old 10-28-2011, 08:49 PM
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The carb body sections are prone to warping. Check them with a machinist flat.

Consider upgrading to EFI, such as Megasquirt.

Near Manassas Va. '65 220S, 2006 Ford Crown Victoria Police Interceptor, '99 Volvo V70, '72 350SL 4 speed

Not part of the in-crowd since 1952. It's BRAKES not breaks. You break a bone, use brakes to stop your car. /rant
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post #4 of 7 (permalink) Old 10-28-2011, 09:29 PM
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If the float is brass is may need to be re-solidered.

If you can take it out, tape a bunch of pennies to it, when it sink's watch for bubbles.

Bubbles are bad.
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post #5 of 7 (permalink) Old 10-29-2011, 02:09 PM Thread Starter
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Got it!

I'd been beating myself up over this stupid carburetor for weeks now, but I finally got it fixed. I checked everything mentioned in this thread, and everything was fine. Well, except I didn't realize that the manual on has the jet sizes listed under "Data and Adjusting Values." Not that it helped very much, since I'm not sure which if any of my four carburetors came with the car when new.

Among the things I did not disassemble on the carbs were the "pre-atomizers". These are the small crossbars with a jet (feed tube) in the middle that allow the fuel-air mix into each bore of the carb. When I looked closely at the pre-atomizer in the bore in question, I could not see the small rubber seal on the end where one is required. I had not seen these seals in the carburetor kits, so I did not remove the pre-atomizers. Now that I needed a seal, I took another look in the kits and found some seals that looked too small. Decided to try one anyway and sure enough it fit with some stretching. And that fixed the problem.

Although frustrating, I have to admit that this experience has made me learn practically everything about how these carbs work. In addition to the jamiekop site, I can recommend a BMW site for a good explanation of the inner workings of these carbs. Links below. Thanks for the help -- much appreciated. -G.

Adjustments and Specs:
Zenith Carb Manual TOC

How They Work:
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post #6 of 7 (permalink) Old 11-04-2011, 05:34 PM Thread Starter
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Really Got It!

The day after I wrote the above, I went out and started the car and it was dripping gas in the front carb again. I found a bulletin board post elsewhere that suggested this would result from the carburetors being out of sync, so I decided to synchronize them. I had an old Uni-Syn lying around and I made a hood from a piece of Tupperware. The carbs were way out of sync -- the front carb was pulling more air. After synchronizing, the dripping stopped. I will of course check it again next time I start the car, but as of today it appears to be fixed. -G.
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post #7 of 7 (permalink) Old 11-05-2011, 08:55 AM
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To do it properly you need TWO Uni-sync's plus two vacuum gauges AND a set of dial calipers.

When you have the carbs sync'ed correctly there is no better set-up. They have to be sync'ed all the way through or you'll have, flat spots, sputtering, rich or lean areas, etc.

I know guys who claim they can do it using a bit of hose to hear the sound but they just get it, "close enough".

After dealing with those carbs I'm sure you realize now why M-B went to fuel injection on most models.
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