This is a 1989 420 SEL with 280k km. It had a valve job done at 250k km and lots of new parts, prior to that job.

The car starts fine, quick and easy. However, after it warms up, when it accelerates from a dead stop, the acceleration at times is jerky, not a smooth acceleration. Today I noticed, while it was jerking, there was some black smoke coming out the back. I also got this jerky response while accelerating from about 15 kph to 40 kph.

At another stop, I shifted to neutral, gunned the engine to 1200 rpm a couple of times, and then shifted into drive, and it accelerated smoothly.

Any ideas on how to identify the problem and resolve it?

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1989 420 SEL