Problem with A/F Ratio, etc.
Have you checked all the basic stuff already like the fuel pump relay or swapped in another walbro 255 to make sure that yours is in fact running right? It does sound like an injector issue though if you're getting a bunch of misfires at idle and low load and with the AFR shifting so much. Generally if it was one of the other things I mentioned it would run fine except when switching into open loop under load then would go lean.
Is the car throwing any other codes or just the misfire?
Is the car throwing any other codes or just the misfire?
