W123 with Stromberg or Pierberg carburetor use 115 engine, 200, 220 and 230 model variants. US models have idle fuel shutoff solenoid for main mixture adjustment. Non-US models have 10 mm hex screw for main mixture adjustment. Both versions have a triangle shaped fuel-return diaphragm where the two fuel hoses connect to carburetor. This diaphragm eventually leaks and causes very rich mixture because fuel is sucked through the vacuum hose (located on bottom of carburetor). Difficult starting, blackened exhaust (unburned fuel) and poor mileage are typical symptoms of failed vacuum diaphragm. Complete flooding can occur as well, which may explain engine stopping after driving a while as well as not restarting when parked briefly. Blackened exhaust after warm restart indicates flooding, long cranking accompanied by stumbling and rough running for a few moments may indicate lack of fuel delivery. Three screws hold the triangle shaped diaphragm in place. Fuel starvation and insufficient delivery point to a weak fuel pump or air leaking into the suction line. Crank the engine with carburetor fuel delivery line disconnected, pointed into a container. Strong steady stream of fuel should be visible. If not, fuel pump and suction line need attention.
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