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Old 06-07-2002, 07:49 PM
Posts: n/a
250C, vacuum problem?

Hi member with my first post. Enjoy the site and have already learned a lot. Purchased a 1971 250C with the Zenith carbs. Put in new plugs, points, wires, rotor ad distributor cap. Changed all fluids, put in fresh gas and started it up. Good compression and oil pressure. Runs well at idle, however when the brake pedal is pushed the idle speed drops and appears to labor. When shifted into reverse or drive the engine dies. I've ordered a couple rebuild kits for the carbs, however I'm wondering if there is some type of vacuum problem as there is a slight pulsing when the brake is applied. Thanks for your thoughts.
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Old 06-07-2002, 10:49 PM
Holson Adi's Avatar
Registered User
Join Date: Jul 2001
Location: Boston, MA
Posts: 2,561

Welcome to mercedesshop

The brake pulsing, I would say is caused by warped rotors. Have you checked the rotors? The 115/114 rotors are cheap (I got them for $28 for the pair)

As for the idle dropping and then causes a stall. Maybe you need to increase the idle a little? I'm sorry not very familiar with gas engines...

Try posting this again in the Vintage forum! You might get more help there!

Good luck,
2008 BMW 335i Coupe
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Old 06-10-2002, 01:17 AM
Senior Member
Join Date: Jun 2001
Location: Northern Calif. (Fairfield Area)
Posts: 2,225
Sounds like a vacuum leak in the brake booster. They can't be rebuilt.Throwing kits in those carbs won't help much unless you know what you are doing. Those 35/40 Zeniths were excellent carburetors, but the problem is finding anyone still around who knows them. Check around to find a professional senior mechanic in you area who knows Zeniths. If someone tells you a carb is a carb, move on.

Good luck,
Auto Zentral Ltd.
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Old 06-10-2002, 03:55 AM
Col Tigwell
Posts: n/a
Owning one of those cars, my advice is to leave the carby's alone, or at least leave to the very last. They can be a real problem to get in tune.

The clue that may help, is the fact that it stalls when in reverse, this is the only band in the transmission that can be adjusted.

These engines do not develope full power until they are at full operating temperature. Does this only do this when the car is first started, and go away when it is hot. So you may need to have the band adjustment checked.

Whats revs is the engine idleing at, if it is too low, then of course it will stall.

DO NOT fidle at this stage with anything other than the idle screw, the one screw can do both carbys together, it is located on the front of the front carby. You will see the throttle lever screw on the lever which opens the throttle. Idle should be 900 RPM with the motor at operating temperature. If you do not have a balancing device for the two carbies, leave it alone and find a shop that does have such a device.

This is not a job for the in experienced, and I will testify that if you play with anything else, the car will run worse, not better.

You need to get a service manual, Haynes make one 63020 346 referance and could be the best $20 you ever spent.


Col Downunder
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