Within the cylinder head there is (on the left side of the engine) a recessed combustion chamber (with a flat piston crown), in which the exhaust valve and the spark plug are located while the remaining area (on the right side) forms a squish area that is intended to swirl the fuel-air-mixture in the combustion chamber.

The M123’s relatively quiet operation was marred by high gasoline consumption, although the engine actually reached its most important development goal of being more economical than the carbureted 280 models.

But especially after the launch of the new four-cylinder M102 in the 200 and 230 E models, the customer interest fell sharply since the M102 did almost everything better while its petrol consumption was significantly lower.

The M123’s direct replacement, the 2.6 liter variant of the M103 (starting 1984), was better in every way: that engine not only was cheaper to produce, lighter and much more fuel-efficient, it was running smoother and also had more power and increased revving vivacity compared with the more sedate M123.

Some specific weakness left aside, any double-register carburetor might be regarded as the most complicated device to generate fuel-air mixture widely used in passenger cars.

With prevalence of the three-way catalytic converter (needing specific air fuel ratio) double-register carburetors became obsolete and were replaced temporarily by electronically controlled simpler carburetors and then finally by fuel injection systems that are more reliable in the long term and can be easily maintained.