low speed lean mixture cis-e 500sl

Following rear chassis are fuel line leak my mechanic replaced both fuel filters in my 1990 500sl (119 dohc engine). Shortly thereafter the fuel pumps began making a noticable whining noise, whereas prior to the filter replacement they were quiet. Also prior to the filter replacement the engine was running rough under load for a period of time but then seemed to smooth out just prior to the filter replacement. After replacement of the filters and two new pumps the engine is again runnung very rough at low speed and backfiring throught the intact. It appears to be running lean. The mech indicated that the injection system had compensated for the low pressure from the defective pumps and now that new ones are installed it has somehow leaned itself out again. Any ideas? How do you adjust the mixture on a cis-e system?