Rebuilt engine missfires
 

I have got my rebuilt engine back. It starts, but the engine shakes below 1200rpm. If I give it a bit more throttle, it missfires a lot.
I removed the new NGK BUR9EQ spark plugs, and the rear ones were almost like new. No, or very little, carbon on them. The ones one the front had carbon on them.

I have checked the ignition coils, and all resistanses were ok.

The engine is streetported and uses stock ECU, and the car is imported from Japan, so there's no CEL.