I recently sold a 1972 Mercedes Benz 350SL to a friend which has some odd ball running issues.

It seems to crank and run well with a slight high idle for about 5 minutes and then very quickly develop a rough idle. The exhaust seems to be spewing a mix of white and black smoke.

The timing is set to spec, the injectors are all firing, the plugs are new and the fuel pump gives plenty of pressure. We’ve tried two different EFI computers and map sensors with no luck and the trigger points are also clean.

I have no idea why it would start smooth and then suddenly run rough. C/O readings from the tail pipe is right at 3%.

Any thoughts would be gratefully appreciated.

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With best regards

Al

Have you measured the fuel pressure?