Mercedes 1972 250 W114 stalls on takeoff

1968 - 1976: 220, 220D, 230, 230.4, 230.6, 240D, 250, 250C, 250CE, 280, 280E, 280CE, 300D
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gav
Posts: 1
Joined: Tue 09 Jan, 2018 12:19 pm
Model you own: w114

Mercedes 1972 250 W114 stalls on takeoff

Post by gav » Sat 14 Apr, 2018 5:23 pm

Hi OzBenz Folks,

I am looking for some assistance in diagnosing a rather annoying Zenith carby stall-on-takeoff problem, which makes the car almost undrivable, but frustratingly runs incredibly well otherwise.

Hardware:
* Mercedes 1972 250 W114 2.8 litre 130-Engine
* Twin Zenith INAT 35/40 carburettors with
--* Electrically heated automatic choke.
--* Power jet, valve and diaphragm.

Symptoms:
* Stall on takeoff from standing idle at half throttle, every time - not intermittent. (Partially mitigated by pumping throttle a couple of times before takeoff.)
* Extreme hesitation on takeoff from standing idle at full throttle.

Notes:
* Starts and idles well, automatic choke functions as expected when cold.
* Runs well (after takeoff) under various loads and revs, hot or cold.
* Engine’s cylinder compression within spec.

Actions performed so far:
* Carby kits - diaphragms, gaskets, float valve, etc.
* New plugs, leads, ignition coil, points, distributor cap and rotor button.
* Intake air pre-heat flap locked open.
* Throttle pump adjustment to take out any slack on plunger.
* Adjusted fuel nozzle spray contact point up throat from butterfly.
* Fuel return valve sems ok.
* Float chamber vent valves seem ok.
* Float height adjusted.
* Satisfied that tuning has been careful and correct.

I think I have just about exhausted the information found on the many Benz blogs, forums and manuals, so any further ideas on how to diagnose this problem would be greatly appreciated.

Cheers,

gav.

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John Green
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Posts: 6395
Joined: Mon 20 Jun, 2005 4:49 am
Model you own: w112
Region: Australian Capital Territory
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Re: Mercedes 1972 250 W114 stalls on takeoff

Post by John Green » Sun 15 Apr, 2018 10:08 am

Did you machine all the surfaces flat on the Zenith's? All carbys work on either a mechanical operation (accelerator pump) or vacuum. Accelerator pump is easy to check as you can see it. The vacuum required to suck fuel though the jets is less obvious to check. Also check the air bleeds for the float chamber. This is the area of the carb where you don't want a vacuum..

Also check your valve clearances, this is part of a full tune.
M.B Spares & Service 14-16 Lyell St, Fyshwick ACT. Ph 02 6239 1099

http://mbspares.com.au - Supporting Australia's Mercedes-Benz Enthusiasts.

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