Changing idle on 420 SEL 1989

[carlitos]

I bought this car in California a few months ao, and had an immissions test, which passed. But when he was finished, the inspector/mechanis told me my idle was to fast, and the timing was off one deree. He then changed my idle from 950Rpms to 650 Rpms.Since then, after I have driven the car for more then 10 minutes, and shut it off, it is really hard to start. After 2 or sometimes 3 or 4 times it will get up to 650 rpms. It usually stays around 200 to 300 rpms the first several times. I have taken it to several MB dealers, I live in Mazatlan Mexico, and have been told that there is no way to change idle speed. Also, they don't work on this model car here, only diesels.
Anybody help know what the mechanic did?I asked the mechanic what he did and he wouldn't tell, nor would he change it back
Carlitos