Oh woe! The fuel system is acting up again.

I have been experiencing some kangarooing with my 300E, driving at low speeds with a steady throttle. I thought this was due to backlash in the drivetrain but have been advised in this forum that it may well be a weak mixture - thanks Duke2.6 - so I tweaked the mixture and have managed to get the problem under control, not eradicated entirely but at least its now barely noticeable.

Unfortunately, during my investigations, I've discovered that the engine is running rich on cylinders 1 and 6 but seems to be Ok on the other 4 pots. The offending plugs are a sooty black, dry not wet.

I've just put in new plugs, changed the fuel filter and bled the crap out of the fuel lines. The distrib cap and rotor were replaced about a year ago, as were the coil, plug wires, injectors and seals. I've swapped out the FPR with a known good one, all to no effect.

The engine doesn't seem to burn oil between changes at 5,000km and runs fine on open throttle, its just at a steady throttle it starts bucking. As I said, richening up the mixture helps but at starting idle, the engine runs a bit lumpy, just like its not firing cleanly on all cylinders. My mechanic set the mixture by feel, adjusting it until the engine ran smoothly at idle. Hopelessly unscientific, I know, but I don't have the luxury of a lambda system and the attendant electronic feedback. I had to stop and tweak the mixture 1/2 turn richer today after i took the car out for a run.

I'm now reasonably certain that the kangarooing is a mixture issue but not sure why it would be running rich on two cylinders and fine on the others.

So, I'm seeking pointers as to where to start looking next.

Fuel Distributor?

Valve stem seals?

Potentiometer (I have a new one)?

ECU?

Any suggestions most welcome. I've just reviewed the result of my search and a 2001 post from stevebfl (praise be!) suggests the valve stem seals may be the most likely culprit. Anyone care to back this up?

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1987 300e manual 250,000 km (sold)