W126 560SEC runs rich

Hi all, i just put a 560 engine in my 420SEC, complete with ECU - the car starts, idles revs but wont drive - no power - like you are pulling away in top. plugs show it is running really rich (cold start injector disabled)
swinging the mixture screw either way only makes it worse. the engine and car were both fine before the swap.
Anybody any ideas

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