The same exact thing happened to my 74 240D.
The problem. The key in the governor shaft had plowed the key-way almost half way around the shaft. I remover the shaft and had a machine shop cut a new key-way 180deg from the original. Never could find out what had placed such a tremendous force on the shaft. The difference in timing came on slowly over a few weeks,not all at once. After pulling the rad the job was fairly easy.

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1984 300SD turbo 126
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