You might have to reset your float levels on your carbs. I was setting up two 97's on my wife's Model A Ford (I know iat is only a 4 banger)which has a two port Riley and when I first started it up it was running in a lean condition. The carbs were set up with a float level for a Ford V8. With two carbs the engine vacuum was lower making the carbs run lean. I raised the float level on both carbs about a 1/10th of and inch at a time till the car was running a lot better and the plugs looked OK. The more carbs you run the lower the intake manifold vacuum will be and you will have to raise the float level to compensate for it.

John