It appears that tuning and adjusting aren't the problem.

The archives are full of theory of operation of your K-jet system. That is if you have a K-jet system. I have never seen a 280TE with an O2 sensor (from the factory - there were no 280TE made in a US model, but I owned a 1980 280TE for over ten years), but it could be possible. If it has one it will then have to be decided whether you have K-jet w/Lambda or KE-jet fuel systems to start your diagnostics.

In 1985 US cars they would be K-jet w/Lambda, but the 124 car had KE in 1986. Things often happened a year earlier in Europe so I'm can't say what you have although my guess would be no O2 sensor.

Till we know the system hard to give advise. (make that usefull advise)

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician