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  #1  
Old 09-19-2006, 11:43 AM
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Join Date: Sep 2005
Posts: 343
Stall in gear while weather is cool

hi all,

I had installed a new head on my 72 250, took a while. I rebuilt the carburetor. Fired it up on Sunday, stalled. Tightened the choke and I was on the road.

This morning, weather was a little cool, 65 vs 85 on Sunday, and it took a lot of coaxing to get her running. When I did, putting her in drive or reverse cased her to stall. Let her get to temperature, failed again.

Any idea what may be causing this trouble?

Thanks

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  #2  
Old 09-20-2006, 10:14 PM
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Join Date: Jun 2001
Location: Northern Calif. (Fairfield Area)
Posts: 2,225
I have no clue where in the world you are, but if you were in my area, I'd charge you $500.00 plus parts to ge through those Zeniths and solve your problems. You might just want to put a set of Webbers on.
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  #3  
Old 09-21-2006, 12:49 AM
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Thx autozen.

I'm in Texas. Before I change to webbers I'd like to make sure it is the carbs and not a bad condensor or fuel pump. Any idea what the PSI should be for the pump?
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  #4  
Old 09-21-2006, 07:06 AM
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Join Date: Apr 2000
Location: Falls Church, VA
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I wouldn't write that check just yet.

What do you mean by "tightened the choke"?

There are a lot of posts on these carbs, so you will find some ideas there. Also the Haynes manual has a decent write-up. But in summary.

Let the car sit overnight. Pull the air cleaner off. Hit the gas and make sure the chokes close. Start the car, and watch that the flaps open more or less together. While it is warming up, hit the gas and see if the flaps open a bit as RPM's go up - this will verify that the pull-downs are working.

Be sure that your fast idle is high enough, at least 1200 RPM on the highest step. You need the book to show you how to adjust this and the pull-downs.

You also need to make sure that you timing is correct.

The manual says you are supposed to get a "good squirt" from the pump at the carb connection.
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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
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  #5  
Old 09-23-2006, 06:47 PM
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Location: Northern Calif. (Fairfield Area)
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Chuck, it's not quite that simple. I invested dozens of hours on these carbs in the early 70s and finally perfected the technique. Choke flap closure and pre throttle gap are best set on the bench. You need to do some tweaking on the arm that gives you initial pre throttle gap when vacuum pulls on the little diaphragm on top of the choke housing. You also need to remove the choke housing from the carb and tweak the little spring that advances the fast idle cam. Once this is done the carbs can be installed after all circuits have been chacked for function with a can of carb cleaner with a straw attached. The carbs need to be balanced with a unisyn while isolated from each other. Then they are connected with rod (tension free). With out the connecting rod attached and the engine at operating temp you open the throttle of each carb and advance each choke with a small screwdriver. Start the car without touching the throttle and check the ripums. If memory serves me it is 1800. I'd have to check my old training manuals from Mercedes. Adjust one or the other carbs up or down til the connecting rod fits tension free. I believe when they went from the 35/40s to 32/40s in 72, one screw was removed.
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  #6  
Old 09-23-2006, 10:32 PM
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Well, my answer was geared to checking the basics - choke position, pull down, and fast idle speed.

If these basics are right, and if you follow the instructions in the Haynes manual, you will get a car that runs OK.
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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
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  #7  
Old 09-24-2006, 08:22 PM
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Join Date: Sep 2005
Posts: 343
Gentlemen,

Thank you for your support. I found my problem. The carbs where not tight enough on the manifold and causing a massive air leak.

Autozen, I know what you mean about these carbs being a bear. I had trouble getting them to sync. Though the adjusters are pointing in the same area, the rear carb would not suck in enough. Am I wrong in saying adjusting the idle adjuster to make it "bigger" or wider should make the RPM's go up? I was doing that with the rear and I could not hear the rear carb through a hose. I had to finaly bring it all the way closed and adjust by ear until they sounded the same. Even then I had a hard time adjusting the idle speed.

Also, the 72 came with some emmissions dashpost that don't work and MB does not sell replacements. The chassis says to set the RPM's at 4 ATDC with vacuum on retard.

Since the retard does not work, what RPM's should I set it and what advance?

Thanks
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  #8  
Old 10-03-2006, 11:54 PM
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Join Date: Jun 2001
Location: Northern Calif. (Fairfield Area)
Posts: 2,225
ggenovez,
I will try to answer your questions, because you are a courteous gentleman. I don't post here anymore, because I am burnt out like all the other professionals who have left before me. Instead of earning our just salary, mine happens to be $70.00/hr, we work for free to help MB owners learn about ther cars. I don't mind sharing my 30 years on knowledge with people who want to learn and solve problems on their cars like you on your 72 250, but I'm tired with dealing with ungratefull young punks who want free advice and have no real interest in their car other than the bling bling value it can generate.We professionals doinate our time and get no thanks in return. I have done cars and really don't care about cars. My passion is cooking. I have been an electronics technician, a real estate agent, a professional mechanic, and now I want to learn to be a chef. I'm 3/4 of the way there. I will PM you my phone # so you can call you this weekend. I will totally explain the Zeniths. As a grasshopper, I have learned them well.

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